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Describe Atomic Structure - Chemistry

  1. Carbon-14 and carbon-12 are isotopes of carbon. Which statement correctly compares them?
    Answer: They have the same number of protons but different numbers of neutrons. This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all carbon atoms have exactly 6 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, which gives them different mass numbers. Carbon-14 and carbon-12 are both carbon (atomic number 6), so both have 6 protons, but carbon-14 has mass number 14 (so 14 - 6 = 8 neutrons) while carbon-12 has mass number 12 (so 12 - 6 = 6 neutrons). Choice B correctly states they have the same number of protons but different numbers of neutrons—this is the definition of isotopes. Choice D incorrectly claims they have different atomic numbers, but isotopes of the same element must have the same atomic number (same proton count) by definition—if they had different atomic numbers, they'd be different elements entirely! The key concept for isotopes: (1) SAME element means SAME atomic number (proton count). (2) DIFFERENT isotopes means DIFFERENT mass numbers (different neutron counts). (3) The notation shows this: ¹²C and ¹⁴C both have the C symbol (carbon, 6 protons) but different mass numbers (12 vs 14). Quick check: Carbon-12 has 6 protons and 6 neutrons (mass = 12), carbon-14 has 6 protons and 8 neutrons (mass = 14)—same protons, different neutrons!
  2. For the isotope potassium-39 (39^{39}K), how many protons, neutrons, and electrons are in a neutral atom?
    Answer: 19 protons, 20 neutrons, 19 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all potassium atoms have exactly 19 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For the neutral potassium-39 atom, the atomic number Z is 19 (protons = 19), mass number A is 39 (neutrons = 39 - 19 = 20), and since it's neutral, electrons = 19. Choice A correctly determines the number of protons, neutrons, and electrons by properly using atomic number, mass number, and the neutral charge relationship. A common distractor like Choice C fails by mistakenly using the mass number as neutrons directly without subtracting Z, leading to an incorrect 39 neutrons instead of 20. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For potassium-39: 39 - 19 = 20 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons. (3) Mass number should equal protons plus neutrons (check: 19 + 20 = 39, correct!). These verification steps catch most errors before you finalize your answer!
  3. A neutral atom has 9 electrons. What is its atomic number (ZZ)?
    Answer: 9 This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number ZZ and defines which element it is), neutrons (neutral/no charge, also in nucleus, number equals mass number AA minus atomic number ZZ, so neutrons = A−ZA - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). The key formulas: protons = atomic number ZZ, neutrons = mass number - atomic number, electrons = protons for neutral atoms! For this neutral atom with 9 electrons, since it's neutral, protons = electrons = 9, so Z=9Z=9 (the element is fluorine). Choice A correctly determines the atomic number as 9 by equating it to the electron count in a neutral atom. Choice C fails by perhaps doubling it or confusing with mass, giving 18 instead of 9. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A−ZA - Z). (3) ELECTRONS: For neutral atoms, electrons = protons, so ZZ = electrons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons (check: 9 protons = 9 electrons, correct!). These verification steps catch most errors before you finalize your answer!
  4. An atom is written as  14N\,^{14}\mathrm{N} (nitrogen-14) and is neutral. How many protons, neutrons, and electrons are in this atom?
    Answer: 7 protons, 7 neutrons, 7 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all nitrogen atoms have exactly 7 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ¹⁴N (nitrogen-14) which is neutral: nitrogen has atomic number 7 (7 protons), mass number 14 (the superscript), so neutrons = 14 - 7 = 7 neutrons, and since it's neutral, electrons = protons = 7 electrons. Choice A correctly identifies 7 protons, 7 neutrons, and 7 electrons using these relationships. Choice C incorrectly states 14 neutrons, confusing the mass number (14) with neutron count—remember, mass number includes both protons AND neutrons, so neutrons = 14 - 7 = 7. The particle counting recipe: (1) PROTONS: Nitrogen's atomic number = 7. (2) NEUTRONS: Mass number minus atomic number: 14 - 7 = 7 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons = 7. Verification: mass number = protons + neutrons = 7 + 7 = 14, matching ¹⁴N perfectly!
  5. A chlorine ion is written as  35Cl−\,^{35}\mathrm{Cl}^{-} (chlorine-35 with a −1 charge). How many protons, neutrons, and electrons does it have?
    Answer: 17 protons, 18 neutrons, 18 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all chlorine atoms have exactly 17 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: negative ions (anions) have more electrons than protons because they've gained electrons (Cl⁻ has 17 protons but 18 electrons, having gained 1). For ³⁵Cl⁻: chlorine has atomic number 17 (17 protons), mass number 35, so neutrons = 35 - 17 = 18 neutrons, and with -1 charge, electrons = 17 - (-1) = 17 + 1 = 18 electrons. Choice C correctly identifies 17 protons, 18 neutrons, and 18 electrons by properly accounting for the negative charge. Choice A incorrectly keeps 17 electrons, missing that the -1 charge means one electron was gained. The particle counting recipe: (1) PROTONS: Chlorine always has 17 protons (atomic number). (2) NEUTRONS: 35 - 17 = 18 neutrons. (3) ELECTRONS: For Cl⁻, electrons = protons - charge = 17 - (-1) = 18 electrons (the negative sign in the charge means we add!). Verification: charge = protons - electrons = 17 - 18 = -1, confirming our Cl⁻ ion!
  6. A potassium ion is written as ³⁹K⁺ (potassium-39 with a 1+1+ charge). How many electrons does ³⁹K⁺ have?
    Answer: 18 This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all potassium atoms have exactly 19 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions like K⁺ have fewer electrons (19 protons but 18 electrons, lost 1). For ³⁹K⁺, electrons = 19 - (+1) = 18. Choice A correctly determines the electrons by subtracting the positive charge from the proton count. A distractor like choice B might use the atomic number without adjusting for charge, but always apply electrons = protons - charge for ions! The particle counting recipe: (1) PROTONS: 19. (2) ELECTRONS: 19 - 1 = 18. Quick checks: Charge = protons - electrons (19 - 18 = +1, correct). Great job verifying!
  7. For the neutral isotope ²³Na (sodium-23), how many protons, neutrons, and electrons are in the atom?
    Answer: 11 protons, 12 neutrons, 11 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all sodium atoms have exactly 11 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For the neutral isotope ²³Na, the atomic number Z is 11 (protons = 11), mass number A is 23 (neutrons = 23 - 11 = 12), and since it's neutral, electrons = 11. Choice A correctly determines the number of protons, neutrons, and electrons by properly using atomic number, mass number, and neutrality for charge balance. A common distractor like choice B fails by mistakenly using the mass number as the proton count, ignoring that protons equal the atomic number, not the mass. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For sodium-23: 23 - 11 = 12 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons. (3) Mass number should equal protons plus neutrons (11 + 12 = 23, correct!). These verification steps catch most errors before you finalize your answer!
  8. A magnesium atom loses 2 electrons to form Mg2+\mathrm{Mg}^{2+}. If the magnesium isotope is magnesium-24 (Mg-24), how many protons, neutrons, and electrons are in Mg2+\mathrm{Mg}^{2+}?
    Answer: 12 protons, 12 neutrons, 10 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all magnesium atoms have exactly 12 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions (cations) have fewer electrons than protons because they've lost electrons (Mg²⁺ has 12 protons but 10 electrons, having lost 2). For Mg²⁺ from magnesium-24: magnesium has atomic number 12 (12 protons), mass number 24, so neutrons = 24 - 12 = 12 neutrons, and with +2 charge, electrons = 12 - 2 = 10 electrons. Choice B correctly determines 12 protons, 12 neutrons, and 10 electrons by properly accounting for the 2+ charge. Choice A incorrectly keeps 12 electrons, forgetting that forming Mg²⁺ means losing 2 electrons. The particle counting recipe: (1) PROTONS: Magnesium always has 12 protons. (2) NEUTRONS: 24 - 12 = 12 neutrons. (3) ELECTRONS: For Mg²⁺, electrons = protons - charge = 12 - (+2) = 10 electrons. Verification: charge = protons - electrons = 12 - 10 = +2, confirming our Mg²⁺ ion!
  9. Oxygen-16 can exist as a neutral atom  16O\,^{16}\mathrm{O} or as the oxide ion O2−\mathrm{O}^{2-}. Compared to the neutral oxygen atom, how does the number of electrons change when forming O2−\mathrm{O}^{2-}?
    Answer: It increases by 2 electrons. This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all oxygen atoms have exactly 8 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: negative ions (anions) have more electrons than protons because they've gained electrons. Neutral ¹⁶O has 8 protons and 8 electrons; O²⁻ has 8 protons but 8 - (-2) = 8 + 2 = 10 electrons, having gained 2 electrons. Choice B correctly states the number of electrons increases by 2 when forming O²⁻ from neutral oxygen. Choice A incorrectly suggests electrons decrease, which would create a positive ion, not the negative O²⁻ we're forming. The key concept: forming negative ions means gaining electrons—the 2- charge on O²⁻ tells us oxygen gained 2 electrons. Think of it this way: to get a 2- charge, we need 2 more negative particles (electrons) than positive particles (protons). Verification: O²⁻ has charge = protons - electrons = 8 - 10 = -2, confirming our oxide ion!
  10. Carbon-14 (¹⁴C) is an isotope of carbon. How many protons and neutrons are in the nucleus of ¹⁴C?
    Answer: 6 protons and 8 neutrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all carbon atoms have exactly 6 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For carbon-14 (¹⁴C), protons = 6 (Z for carbon), neutrons = 14 - 6 = 8. Choice B correctly determines the protons and neutrons by using carbon's atomic number and subtracting from the mass number. A distractor like choice A might assume equal protons and neutrons like in carbon-12, but isotopes vary in neutrons—always calculate A - Z! The particle counting recipe: (1) PROTONS: Atomic number (6). (2) NEUTRONS: A - Z (8). Quick checks: Mass = protons + neutrons (6 + 8 = 14, correct). These verifications make it easy to spot mistakes!
  11. Magnesium-24 is written as 24Mg^{24}\text{Mg}. A magnesium ion Mg2+\text{Mg}^{2+} forms when magnesium loses 2 electrons. How many protons, neutrons, and electrons does 24Mg2+^{24}\text{Mg}^{2+} have?
    Answer: 12 protons, 12 neutrons, 10 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all magnesium atoms have exactly 12 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions (cations) have fewer electrons than protons because they've lost electrons (Mg²⁺ has 12 protons but 10 electrons, having lost 2), and negative ions (anions) have more electrons than protons because they've gained electrons. For ²⁴Mg²⁺: magnesium's atomic number is 12 (protons = 12), mass number is 24, so neutrons = 24 - 12 = 12, and with a +2 charge, electrons = 12 - 2 = 10 electrons (lost 2 electrons to form the 2+ ion). Choice A correctly identifies 12 protons, 12 neutrons, and 10 electrons by properly accounting for the 2+ charge. Choice B incorrectly keeps 12 electrons, forgetting that the 2+ charge means the ion has lost 2 electrons—this would be neutral Mg, not Mg²⁺. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For magnesium-24: mass 24, atomic number 12, so 24 - 12 = 12 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. For ions, electrons = protons - charge (if charge is +2, subtract 2 electrons; if charge is -1, add 1 electron—the negative sign in subtraction handles this!). Example: Mg²⁺ has 12 protons (atomic number), so 12 - (+2) = 10 electrons. Quick checks to verify your answer: the charge should equal protons minus electrons (check: 12 protons - 10 electrons = +2 charge for Mg²⁺, correct!).
  12. A neutral atom has 9 protons and 10 neutrons. Which element is it?
    Answer: Fluorine (F) This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—the proton count identifies the element from the periodic table), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For a neutral atom with 9 protons, it must be fluorine (F), as atomic number 9 corresponds to F, and neutrons=10 is additional info but doesn't change the element. Choice B correctly identifies the element based on the proton count, which is the defining feature. A distractor like choice A might miscount protons or confuse with mass (9+10=19, close to O's isotopes but wrong), but remember, element is always by protons! The particle counting recipe: (1) PROTONS: Given as 9, matches F. (2) NEUTRONS: 10 (given). Quick checks: For neutral, protons = electrons (9=9, assumed correct). Keep up the good work!
  13. A magnesium atom (Mg) loses 2 electrons to form Mg2+\text{Mg}^{2+}. Magnesium has atomic number 12 and the isotope is magnesium-24 (24Mg^{24}\text{Mg}). How many protons, neutrons, and electrons are in 24Mg2+^{24}\text{Mg}^{2+}?
    Answer: 12 protons, 12 neutrons, 10 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all magnesium atoms have exactly 12 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions (cations) have fewer electrons than protons because they've lost electrons (Mg²⁺ has 12 protons but 10 electrons, having lost 2), and negative ions (anions) have more electrons than protons because they've gained electrons. The key formulas: protons = atomic number, neutrons = mass number - atomic number, electrons = protons for neutral atoms or protons - charge for ions! For ^{24}Mg^{2+}, Z=12 so protons=12; A=24 so neutrons=24-12=12; charge +2 so electrons=12 - (+2)=10. Choice A correctly determines the number of protons, neutrons, and electrons by properly using atomic number, mass number, and charge relationships. A distractor like Choice D might add instead of subtract for electrons, but for positive ions, subtract the charge from protons to find electrons—keep practicing, you're improving! The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For magnesium-24: mass 24, atomic number 12, so 24 - 12 = 12 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. For ions, electrons = protons - charge (if charge is +2, subtract 2 electrons; if charge is -1, add 1 electron—the negative sign in subtraction handles this!). Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons. (3) For ions, the charge should equal protons minus electrons (check: 12 - 10 = +2 for Mg²⁺, correct!). (4) Mass number should equal protons plus neutrons (check: 12 + 12 = 24, correct!). These verification steps catch most errors before you finalize your answer!
  14. A nitrogen-14 atom is written as 14N^{14}\text{N}. How many protons, neutrons, and electrons are in a neutral 14N^{14}\text{N} atom?
    Answer: 7 protons, 7 neutrons, 7 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all nitrogen atoms have exactly 7 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For nitrogen-14 (¹⁴N), we know nitrogen's atomic number is 7 (this defines it as nitrogen), the mass number is 14 (given in the notation), and it's neutral (no charge indicated). Using our formulas: protons = 7 (atomic number), neutrons = 14 - 7 = 7, and electrons = 7 (equals protons for neutral atoms). Choice A correctly identifies 7 protons, 7 neutrons, and 7 electrons by properly applying these relationships. Choice B incorrectly states 14 neutrons, which would give a mass number of 7 + 14 = 21, not 14—this is a common error of confusing mass number (14) with neutron count. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For nitrogen-14: mass 14, atomic number 7, so 14 - 7 = 7 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly (7 for nitrogen). (2) For neutral atoms, protons should equal electrons (7 = 7). (3) Mass number should equal protons plus neutrons (check: 7 + 7 = 14 for nitrogen-14, correct!). These verification steps confirm all particle counts!
  15. A neutral atom of carbon-12 (C-12) is given. How many protons, neutrons, and electrons does it have?
    Answer: 6 protons, 6 neutrons, 6 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all carbon atoms have exactly 6 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For carbon-12, the atomic number is 6 (all carbon atoms have 6 protons), the mass number is 12 (the '12' in carbon-12), so neutrons = 12 - 6 = 6 neutrons, and since it's neutral, electrons = protons = 6 electrons. Choice A correctly identifies 6 protons, 6 neutrons, and 6 electrons by properly using these relationships. Choice B incorrectly assumes all 12 particles are protons with no neutrons, misunderstanding that mass number includes both protons and neutrons. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For carbon-12: mass 12, atomic number 6, so 12 - 6 = 6 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly (6 for carbon). (2) For neutral atoms, protons should equal electrons (6 = 6, check!). (3) Mass number should equal protons plus neutrons (6 + 6 = 12, correct!).
  16. An element has atomic number Z=8Z = 8 and mass number A=16A = 16 and is neutral. How many neutrons does this atom have?
    Answer: 8 This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—an element with atomic number 8 is oxygen, having exactly 8 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For this atom with atomic number Z = 8 and mass number A = 16, we can calculate: protons = 8 (given as atomic number), neutrons = 16 - 8 = 8, and electrons = 8 (equals protons since it's neutral). The question specifically asks for the number of neutrons, which is 8. Choice A correctly identifies 8 neutrons by using the formula neutrons = mass number - atomic number = 16 - 8 = 8. Choice B incorrectly states 16 neutrons, which is a common error of confusing mass number (16) with neutron count—if there were 16 neutrons, the mass number would be 8 + 16 = 24, not 16. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For this oxygen-16: mass 16, atomic number 8, so 16 - 8 = 8 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: Mass number should equal protons plus neutrons (check: 8 + 8 = 16, correct!). This verification step ensures you've correctly calculated the neutron count!
  17. A neutral atom has 9 protons and a mass number of 19. What element is it, and how many neutrons does it have?
    Answer: Fluorine, 10 neutrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions (cations) have fewer electrons than protons because they've lost electrons, and negative ions (anions) have more electrons than protons because they've gained electrons. The key formulas: protons = atomic number, neutrons = mass number - atomic number, electrons = protons for neutral atoms or protons - charge for ions! With 9 protons, the element is fluorine (Z=9); mass number A=19, so neutrons=19-9=10. Choice B correctly identifies the element as fluorine with 10 neutrons by using atomic number and mass number properly. A distractor like Choice A might miscount neutrons or confuse elements, but remember Z=8 is oxygen, and neutrons=A-Z=19-9=10 for Z=9—nice work! The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For this atom: mass 19, atomic number 9, so 19 - 9 = 10 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. For ions, electrons = protons - charge (if charge is +2, subtract 2 electrons; if charge is -1, add 1 electron—the negative sign in subtraction handles this!). Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons. (3) For ions, the charge should equal protons minus electrons. (4) Mass number should equal protons plus neutrons (check: 9 + 10 = 19, correct!). These verification steps catch most errors before you finalize your answer!
  18. A neutral atom of sodium-23 is written as 23Na^{23}\text{Na}. How many protons, neutrons, and electrons does this atom have?
    Answer: 11 protons, 12 neutrons, 11 electrons This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all sodium atoms have exactly 11 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For sodium-23 (²³Na), we know sodium's atomic number is 11 (this defines it as sodium), the mass number is 23 (given in the notation), and it's neutral (no charge indicated). Using our formulas: protons = 11 (atomic number), neutrons = 23 - 11 = 12, and electrons = 11 (equals protons for neutral atoms). Choice A correctly identifies 11 protons, 12 neutrons, and 11 electrons by properly applying these relationships. Choice C incorrectly states 23 neutrons, which would give a mass number of 11 + 23 = 34, not 23—this is a common error of confusing mass number with neutron count. The particle counting recipe: (1) PROTONS: Always equal atomic number (look it up or given). This never changes—element identity depends on proton count. (2) NEUTRONS: Subtract atomic number from mass number (A - Z). For sodium-23: mass 23, atomic number 11, so 23 - 11 = 12 neutrons. (3) ELECTRONS: For neutral atoms, electrons = protons. Quick checks to verify your answer: (1) Proton count should match atomic number exactly. (2) For neutral atoms, protons should equal electrons. (3) Mass number should equal protons plus neutrons (check: 11 + 12 = 23 for sodium-23, correct!). These verification steps catch most errors before you finalize your answer!
  19. An atom has atomic number Z=12Z=12 and mass number A=24A=24, and it is neutral. How many neutrons does it have?
    Answer: 12 This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—an atom with Z=12 has exactly 12 protons, making it magnesium), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For this atom: atomic number Z = 12 means 12 protons; mass number A = 24; therefore neutrons = A - Z = 24 - 12 = 12 neutrons; since it's neutral, electrons = protons = 12 electrons. Choice A correctly identifies 12 neutrons using the formula neutrons = mass number - atomic number. Choice B incorrectly gives 24 neutrons (confusing mass number with neutron count), choice C gives 36 (adding instead of subtracting), and choice D gives 10 (subtracting in the wrong direction). The neutron calculation is straightforward: neutrons = mass number - atomic number = 24 - 12 = 12. Quick verification: mass number should equal protons plus neutrons (12 + 12 = 24, correct!). This formula works for all atoms and isotopes—just subtract atomic number from mass number to find neutrons!
  20. A magnesium atom (Mg) loses 2 electrons to form Mg2+\mathrm{Mg}^{2+}. How many electrons does Mg2+\mathrm{Mg}^{2+} have? (Magnesium has atomic number 12.)
    Answer: 10 This question tests your understanding of atomic structure—how protons, neutrons, and electrons are arranged in atoms and ions, and how atomic number and mass number relate to these subatomic particles. Every atom consists of three types of subatomic particles: protons (positive charge, in the nucleus, number equals atomic number Z and defines which element it is—all magnesium atoms have exactly 12 protons), neutrons (neutral/no charge, also in nucleus, number equals mass number A minus atomic number Z, so neutrons = A - Z), and electrons (negative charge, in electron cloud around nucleus, equal to protons in neutral atoms for charge balance). For ions, electrons change while protons stay the same: positive ions (cations) have fewer electrons than protons because they've lost electrons (Mg²⁺ has 12 protons but 10 electrons, having lost 2). For Mg²⁺: magnesium's atomic number is 12, so it always has 12 protons; neutral Mg would have 12 electrons; losing 2 electrons to form Mg²⁺ means electrons = 12 - 2 = 10 electrons. Choice C correctly identifies 10 electrons by subtracting the 2 lost electrons from the original 12. Choice A incorrectly gives 14 electrons (adding instead of subtracting), choice B gives 12 electrons (forgetting about the charge), and choice D confuses this with some other calculation. The key insight: when atoms lose electrons, they become positive ions—for Mg²⁺, the 2+ charge tells us exactly how many electrons were lost (2), so we subtract: 12 - 2 = 10 electrons. Quick verification: charge equals protons minus electrons (12 - 10 = +2, correct!).

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