Health Education Systems Inc (HESI) A2 Exam Quiz: Atomic Structure And Ions
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Atomic Structure And IonsQuestion 1 of 19

If a neutral atom of potassium (K, atomic number 19) loses one electron, what is the result?

An anion with a charge of -1.
A cation with a charge of +1.
A neutral atom of argon (Ar, atomic number 18).
An isotope of potassium.
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Health Education Systems Inc (HESI) A2 Exam Quiz

Health Education Systems Inc (HESI) A2 Exam Quiz: Atomic Structure And Ions

Practice Atomic Structure And Ions in Health Education Systems Inc (HESI) A2 Exam with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Atomic Structure And Ions, giving you a quick way to practice the rules, question types, and explanations that matter most for Health Education Systems Inc (HESI) A2 Exam.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

If a neutral atom of potassium (K, atomic number 19) loses one electron, what is the result?

  1. An anion with a charge of -1.
  2. A cation with a charge of +1. (correct answer)
  3. A neutral atom of argon (Ar, atomic number 18).
  4. An isotope of potassium.
Explanation: When you encounter questions about atoms gaining or losing electrons, you're dealing with ion formation—a fundamental concept in chemistry that appears regularly on the HESI exam. Let's trace what happens to potassium. A neutral potassium atom has 19 protons (which give it its atomic number) and 19 electrons (which balance the positive charge). When it loses one electron, you now have 19 protons but only 18 electrons. Since protons carry positive charge and electrons carry negative charge, the atom now has a net positive charge of +1. Any atom that gains or loses electrons becomes an ion, and specifically, an atom that loses electrons becomes a positively charged ion called a cation. Looking at the wrong answers: Choice A describes an anion with -1 charge, but anions form when atoms gain electrons, not lose them. Choice C suggests the atom becomes argon, but losing an electron doesn't change the number of protons—the atom is still potassium, just ionized. While the ion does have 18 electrons like argon, it still has 19 protons, making it K⁺, not Ar. Choice D claims it becomes an isotope, but isotopes differ in neutron number, not electron number. The correct answer is B: losing an electron creates a cation with +1 charge. Remember this pattern: losing electrons = cations (positive), gaining electrons = anions (negative). The mnemonic "cats are positive" can help you remember that cations are positively charged.

Question 2

An ion with a +3 charge contains 23 electrons. The nucleus of this ion also contains 30 neutrons. What is the mass number of this ion?

  1. 26
  2. 53
  3. 56 (correct answer)
  4. 59
Explanation: When you encounter atomic structure questions, focus on three key numbers: protons, neutrons, and electrons. The mass number always equals protons plus neutrons, while charge tells you about the electron-proton relationship. Start by finding the number of protons. Since this ion has a +3 charge and contains 23 electrons, it must have lost 3 electrons from its neutral state. A neutral atom has equal protons and electrons, so this ion originally had 23 + 3 = 26 electrons, meaning it has 26 protons. Now you can calculate the mass number: protons + neutrons = 26 + 30 = 56. This confirms answer choice C is correct. Let's examine why the other options are wrong. Choice A (26) represents only the number of protons—this would be the atomic number, not the mass number. Choice B (53) appears to add neutrons and electrons (30 + 23), but electrons have negligible mass and don't contribute to mass number. Choice D (59) likely comes from incorrectly adding all three values (26 + 30 + 3), treating the charge as an additional component rather than understanding it represents missing electrons. Remember this key relationship: for any atom or ion, mass number = protons + neutrons. The charge only tells you about electron gain or loss, never affects the nuclear composition. When solving these problems, always determine the proton number first by considering the charge, then add the given neutrons.

Question 3

Which particle's number must be changed to convert an atom of one element into an atom of a different element?

  1. Proton (correct answer)
  2. Neutron
  3. Electron
  4. Quark
Explanation: This question tests your understanding of atomic structure and what defines an element's identity. When you encounter questions about converting one element to another, focus on what fundamentally distinguishes different elements on the periodic table. The number of protons in an atom's nucleus determines the element's identity. This is called the atomic number, and it's unique for each element. Hydrogen has 1 proton, carbon has 6 protons, oxygen has 8 protons, and so on. To convert one element into another, you must change the number of protons in the nucleus. Answer choice (A) is correct because only changing the proton count transforms the fundamental identity of the atom. Let's examine why the other options don't work. Choice (B), neutrons, is incorrect because changing neutron number only creates different isotopes of the same element. For example, carbon-12 and carbon-14 both have 6 protons but different numbers of neutrons—they're still both carbon. Choice (C), electrons, is wrong because gaining or losing electrons creates ions (charged atoms) but doesn't change the element's identity. A sodium atom and a sodium ion are both still sodium. Choice (D), quarks, is incorrect because quarks are subatomic particles that make up protons and neutrons, but changing quark arrangements would alter the protons themselves, which brings us back to option (A). Remember this key principle: protons = identity. On chemistry questions involving element conversion or transmutation, always think about the atomic number first. The number of protons is the atomic "fingerprint" that never changes unless you're dealing with nuclear reactions.

Question 4

An atom's identity as a specific element is determined by the number of which subatomic particle in its nucleus?

  1. Neutrons, because they contribute to the atomic mass and stability.
  2. Electrons, because their configuration determines chemical reactivity.
  3. Protons, because this number is unique for each element. (correct answer)
  4. Protons and neutrons combined, as this gives the mass number.
Explanation: When you encounter questions about atomic structure, remember that atoms have three main subatomic particles, each with a distinct role in determining an element's properties. The number of protons in an atom's nucleus is called the atomic number, and this is what definitively identifies which element you're looking at. Every carbon atom has exactly 6 protons, every oxygen atom has 8 protons, and every hydrogen atom has 1 proton. This relationship never changes—it's like each element's unique fingerprint. If you change the number of protons, you literally change the element itself. Option A is incorrect because neutrons can vary within the same element (creating isotopes), so they don't determine identity. Carbon-12 and Carbon-14 both have 6 protons but different numbers of neutrons—they're still both carbon. Option B is wrong because electrons orbit outside the nucleus and can be gained, lost, or shared during chemical reactions. When this happens, you get ions, but the element's identity remains unchanged because the protons stay the same. Option D describes the mass number (protons + neutrons), which tells you about an atom's mass but not its elemental identity. Different isotopes of the same element have different mass numbers but are still the same element. For HESI chemistry questions, always remember this fundamental rule: protons determine identity, neutrons affect mass and stability, and electrons determine chemical behavior. The atomic number (proton count) is your key to identifying any element on the periodic table.

Question 5

An unknown particle has an atomic number of 12 and a mass number of 25. If this particle has 10 electrons, which statement is true?

  1. The particle is a neutral atom with 13 neutrons.
  2. The particle is a cation with a charge of +2 and 13 neutrons. (correct answer)
  3. The particle is an anion with a charge of -2 and 12 neutrons.
  4. The particle is a cation with a charge of +2 and 12 neutrons.
Explanation: When you encounter atomic structure problems, you need to identify three key components: protons (atomic number), neutrons (mass number minus atomic number), and electrons, then determine the particle's charge and identity. Given information: atomic number = 12, mass number = 25, electrons = 10. The atomic number tells you this particle has 12 protons. To find neutrons, subtract: 25 - 12 = 13 neutrons. Since a neutral atom has equal protons and electrons, but this particle has 12 protons and only 10 electrons, it has lost 2 electrons, making it a cation with a +2 charge. Choice B correctly identifies this as a cation with +2 charge and 13 neutrons. Choice A is wrong because it describes a neutral atom, but neutral atoms have equal numbers of protons and electrons (12 each), not 12 protons and 10 electrons. Choice C incorrectly calls this an anion with -2 charge, but anions have more electrons than protons—this particle has fewer electrons than protons. Additionally, C states 12 neutrons when we calculated 13. Choice D correctly identifies the +2 cation but incorrectly states 12 neutrons instead of 13. For HESI atomic structure questions, always follow this systematic approach: identify protons from atomic number, calculate neutrons by subtracting atomic number from mass number, then compare protons to electrons to determine charge. Remember that losing electrons creates positive cations, while gaining electrons creates negative anions.

Question 6

A patient is administered a medical tracer containing a radioactive isotope, Technetium-99 (⁹⁹Tc). A standard atom of Technetium (Tc) has an atomic number of 43. How many neutrons are in the nucleus of a Technetium-99 atom?

  1. 43
  2. 56 (correct answer)
  3. 99
  4. 142
Explanation: When you encounter questions about atomic structure and isotopes, you need to understand the relationship between mass number, atomic number, and neutrons. The key formula is: Mass Number = Atomic Number + Number of Neutrons. In Technetium-99, the "99" represents the mass number (total protons and neutrons), while the atomic number of 43 tells you the number of protons. To find the neutrons, rearrange the formula: Number of Neutrons = Mass Number - Atomic Number. So: 99−43=5699 - 43 = 56 neutrons. Let's examine why the other options are incorrect: Choice A (43) represents the atomic number itself - the number of protons, not neutrons. This is a common trap where students confuse the two subatomic particles. Choice C (99) is the mass number, representing the total of protons AND neutrons combined. Students sometimes select this thinking it asks for the total nuclear particles rather than just neutrons. Choice D (142) results from incorrectly adding the mass number and atomic number (99+43=14299 + 43 = 142), which has no meaning in atomic structure and suggests fundamental confusion about the relationship between these values. Study Strategy: For HESI questions involving isotopes, always identify what each number represents first. The mass number (superscript or number after the element name) minus the atomic number (subscript or given separately) always equals the number of neutrons. Practice this subtraction with different isotopes to build automaticity, as medical tracers and radioisotopes appear frequently in healthcare contexts.

Question 7

An ion has 53 protons and a charge of -1. The most common isotope of this element has 74 neutrons. What is the mass number of this common isotope?

  1. 54
  2. 126
  3. 127 (correct answer)
  4. 128
Explanation: When you encounter atomic structure questions, remember that mass number equals the total number of protons plus neutrons in an atom's nucleus. The charge tells you about electrons, but mass number depends only on the heavy particles in the nucleus. Given that this ion has 53 protons, you can identify it as iodine (element 53 on the periodic table). The -1 charge means it has gained one electron (54 electrons total), but this doesn't affect the mass number calculation. The key information is that the most common isotope has 74 neutrons. To find the mass number, simply add protons and neutrons: 53+74=12753 + 74 = 127. This makes C) 127 the correct answer. Looking at the wrong answers: A) 54 represents a common mistake where students add the extra electron to the proton count, forgetting that electrons have negligible mass. B) 126 occurs when students subtract one from the correct answer, possibly confused by the -1 charge. D) 128 happens when students add the extra electron to the correct mass number, again forgetting that electron mass is insignificant compared to protons and neutrons. For HESI chemistry questions, remember this pattern: mass number always equals protons plus neutrons, regardless of the ion's charge. The charge only tells you about electron gain or loss, which doesn't meaningfully change the atom's mass. Focus on the nucleus when calculating mass numbers, and ignore electron count for these calculations.

Question 8

A sample of an element consists of atoms containing 8 protons, 8 neutrons, and 8 electrons, as well as atoms containing 8 protons, 10 neutrons, and 8 electrons. These two types of atoms are best described as:

  1. cations and anions of the same element.
  2. isobars of different elements.
  3. isotopes of the same element. (correct answer)
  4. a neutral element and its corresponding ion.
Explanation: When you encounter atomic structure problems, focus on what the proton, neutron, and electron counts tell you about the identity and relationship between atoms. Both atoms described have 8 protons, which means they're both oxygen atoms—the number of protons defines an element's identity. The key difference is their neutron count: one has 8 neutrons, the other has 10 neutrons. When atoms of the same element have different numbers of neutrons, they're called isotopes. Both atoms also have 8 electrons, making them electrically neutral. This confirms answer C is correct. Let's examine why the other options are wrong. Choice A suggests cations and anions, but both atoms have equal numbers of protons and electrons (8 each), so neither carries a charge. Choice B mentions isobars, which are atoms with the same mass number but different atomic numbers—these atoms have the same atomic number (8) but different mass numbers (16 and 18). Choice D suggests one neutral atom and one ion, but again, both atoms are neutral since their proton and electron counts match. For HESI chemistry questions, remember this pattern: protons determine the element's identity, electrons determine charge, and neutrons create isotopes. When you see atoms with the same proton count but different neutron counts, you're looking at isotopes. This concept appears frequently, so practice identifying the relationship between atomic particles and their effect on an atom's properties.

Question 9

Consider the following two particles: Particle X has 26 protons and 23 electrons. Particle Y has 27 protons and 27 electrons. Which statement correctly compares them?

  1. X is an anion of iron and Y is a neutral atom of cobalt.
  2. X is a cation of iron and Y is a neutral atom of cobalt. (correct answer)
  3. X and Y are isotopes of the same element.
  4. X is a neutral atom of iron and Y is a cation of cobalt.
Explanation: When you encounter questions about atomic particles, focus on three key numbers: protons (which determine the element), electrons (which determine charge), and the relationship between them. Let's analyze each particle systematically. Particle X has 26 protons, which means it's iron (Fe) since the number of protons defines the element. With 26 protons but only 23 electrons, it has lost 3 electrons, giving it a +3 charge — making it a cation (positively charged ion). Particle Y has 27 protons, identifying it as cobalt (Co). Since it has equal numbers of protons and electrons (27 each), it's electrically neutral. Now let's examine each answer choice. Choice A incorrectly calls X an anion — remember, anions are negatively charged ions formed when atoms gain electrons. Since X has fewer electrons than protons, it's positively charged. Choice B correctly identifies X as a cation of iron and Y as a neutral cobalt atom. Choice C suggests they're isotopes, but isotopes are different forms of the same element with varying neutron numbers. These particles are completely different elements (iron vs. cobalt). Choice D incorrectly calls X neutral when it clearly has unequal protons and electrons. The correct answer is B. Remember this pattern: protons = element identity, electron deficit = cation, electron excess = anion, equal protons and electrons = neutral atom. Always count carefully and don't confuse anions with cations — this is a common trap on chemistry questions.

Question 10

What is the overall charge of an atomic nucleus, and which particles are responsible for that charge?

  1. Neutral, due to the presence of neutrons.
  2. Negative, due to the presence of electrons.
  3. Positive, due to the presence of protons. (correct answer)
  4. Variable, depending on the number of neutrons.
Explanation: When you encounter questions about atomic structure, focus on the three fundamental particles and their locations: protons and neutrons in the nucleus, electrons orbiting outside. The atomic nucleus carries a positive charge because it contains protons, which each carry a +1 charge. Since the nucleus contains only protons and neutrons (no electrons), and neutrons are electrically neutral, the overall nuclear charge equals the number of protons present. This positive charge is what attracts and holds the negatively charged electrons in their orbital paths around the nucleus. Let's examine why each incorrect answer misses the mark. Choice A incorrectly states the nucleus is neutral due to neutrons. While neutrons are indeed neutral particles, their presence doesn't make the nucleus neutral—they simply don't contribute to the charge either way. Choice B claims the nucleus is negative due to electrons, but electrons don't reside in the nucleus; they orbit around it in electron shells. Choice D suggests the charge varies with neutron number, but since neutrons carry no charge, adding or removing them (creating isotopes) doesn't change the nuclear charge. For HESI science questions, remember that atomic structure follows consistent rules: protons determine both the element's identity and the nuclear charge, neutrons affect atomic mass but not charge, and electrons determine chemical behavior but exist outside the nucleus. Always associate "nucleus" with "protons" when thinking about charge—this connection appears frequently in chemistry and physics contexts on health science exams.

Question 11

The nucleus of an atom contains nearly all of its mass. Which two particles account for this mass?

  1. Protons and electrons
  2. Neutrons and electrons
  3. Protons and neutrons (correct answer)
  4. Only protons
Explanation: When you encounter atomic structure questions, focus on the three fundamental particles and their locations: protons and neutrons in the nucleus, electrons in the surrounding electron cloud. The nucleus contains nearly all of an atom's mass because it houses both protons and neutrons, which are approximately 1,836 times more massive than electrons. Protons carry a positive charge and determine the element's identity, while neutrons are electrically neutral but contribute equally to the nucleus's mass. Together, these nucleons (nuclear particles) account for over 99.9% of an atom's total mass. Let's examine why the other options miss the mark. Choice A (protons and electrons) incorrectly includes electrons, which have negligible mass compared to nuclear particles. Choice B (neutrons and electrons) makes the same error by including lightweight electrons while excluding the massive protons. Choice D (only protons) overlooks neutrons entirely, which would eliminate roughly half the nuclear mass in most atoms. The key misconception these distractors exploit is underestimating how dramatically protons and neutrons outweigh electrons. Think of it this way: if a proton were the size of a bowling ball, an electron would be smaller than a marble, yet they occupy vastly different spaces within the atom. For HESI chemistry questions, remember that mass concentrates in the nucleus (protons + neutrons), while volume comes from the electron cloud. When you see questions about atomic mass or nuclear composition, immediately think "protons and neutrons" as the heavy hitters.

Question 12

An ion of phosphorus (P, atomic number 15) has a mass number of 31 and a charge of -3. How many protons, neutrons, and electrons does this ion contain?

  1. 15 protons, 16 neutrons, 12 electrons
  2. 15 protons, 16 neutrons, 18 electrons (correct answer)
  3. 16 protons, 15 neutrons, 15 electrons
  4. 15 protons, 31 neutrons, 15 electrons
Explanation: When you encounter atomic structure problems, remember that three key pieces of information define any atom or ion: the atomic number (protons), mass number (protons + neutrons), and charge (difference between protons and electrons). For this phosphorus ion, start with what never changes: the number of protons. The atomic number 15 tells you phosphorus always has 15 protons, regardless of whether it's neutral or ionized. Next, find the neutrons using the mass number: neutrons = mass number - protons = 31 - 15 = 16 neutrons. The tricky part is electrons. In a neutral atom, electrons equal protons (15). But this ion has a -3 charge, meaning it gained 3 extra electrons. So: 15 + 3 = 18 electrons. The negative charge indicates more electrons than protons. Choice A incorrectly calculates 12 electrons, which would create a +3 charge (15 protons - 12 electrons = +3), not -3. This represents a common sign error. Choice C mistakenly uses 16 protons instead of 15, confusing the number of neutrons with protons. The atomic number, not the neutron count, determines the element's identity. Choice D shows 31 neutrons, incorrectly using the entire mass number instead of subtracting protons. It also shows 15 electrons, ignoring the -3 charge entirely. Choice B correctly identifies 15 protons, 16 neutrons, and 18 electrons. Study tip: For ion problems, use this sequence: atomic number = protons (always), mass number - protons = neutrons, then adjust electrons based on charge (negative charge = extra electrons, positive charge = fewer electrons).

Question 13

The two most common isotopes of chlorine are Chlorine-35 and Chlorine-37. What is the fundamental difference between these two isotopes?

  1. Chlorine-37 has two more protons than Chlorine-35.
  2. Chlorine-37 has two more neutrons than Chlorine-35. (correct answer)
  3. Chlorine-37 has two more electrons than Chlorine-35 in their neutral state.
  4. Chlorine-37 has a different chemical reactivity than Chlorine-35.
Explanation: When you encounter questions about isotopes, focus on what makes isotopes unique: they are atoms of the same element with different numbers of neutrons. The mass number (the number after the element name) tells you the total number of protons plus neutrons in the nucleus. All chlorine atoms have 17 protons—this defines them as chlorine. Chlorine-35 has a mass number of 35, meaning it contains 35 total protons and neutrons. Since there are 17 protons, it must have 18 neutrons (35 - 17 = 18). Chlorine-37 has a mass number of 37, so it contains 20 neutrons (37 - 17 = 20). The difference is exactly two neutrons, making answer B correct. Let's examine why the other options are wrong. Answer A incorrectly suggests Chlorine-37 has more protons. If this were true, it wouldn't be chlorine anymore—it would be a different element entirely, since the number of protons determines an element's identity. Answer C claims Chlorine-37 has more electrons in its neutral state. However, neutral atoms always have equal numbers of protons and electrons, so both isotopes would have 17 electrons. Answer D suggests different chemical reactivity, but isotopes of the same element have nearly identical chemical properties because they have the same electron configuration. Remember this key pattern for the HESI: isotopes questions test whether you understand that isotopes differ only in neutron number. The mass number minus the atomic number always gives you the neutron count—use this formula to solve isotope problems quickly.

Question 14

Which of the following describes the process of forming an Al³⁺ ion from a neutral aluminum (Al, atomic number 13) atom?

  1. The aluminum atom gains three electrons to its outermost shell.
  2. The aluminum atom loses three protons from its nucleus.
  3. The aluminum atom loses three electrons from its outermost shell. (correct answer)
  4. The aluminum atom gains three neutrons in its nucleus.
Explanation: When you encounter questions about ion formation, focus on what happens to electrons—this is the key to understanding how atoms become charged particles. To form an Al³⁺ ion, a neutral aluminum atom must become positively charged by 3 units. Since aluminum has atomic number 13, it normally has 13 protons (positive) and 13 electrons (negative), making it electrically neutral. The ³⁺ charge indicates the atom now has 3 more protons than electrons. The correct answer is C because aluminum achieves this +3 charge by losing 3 electrons from its outermost shell. After losing these electrons, aluminum has 13 protons but only 10 electrons, giving it a net charge of +3. This electron loss also gives aluminum the stable electron configuration of neon, which explains why this ion forms so readily. Here's why the other options are wrong: A is incorrect because gaining electrons would make the ion negatively charged (Al³⁻), not positively charged. B is wrong because atoms cannot lose protons during normal chemical processes—proton number defines the element itself, so losing protons would change aluminum into a different element entirely. D is incorrect because gaining or losing neutrons changes the atom's mass but doesn't affect its electrical charge, since neutrons are neutral particles. Remember this pattern: positive ions (cations) form when atoms lose electrons, while negative ions (anions) form when atoms gain electrons. The charge number tells you exactly how many electrons were gained or lost.

Question 15

Which statement accurately compares a neutral atom of fluorine (F, atomic number 9) with a fluoride ion (F⁻)?

  1. The fluoride ion has one more proton than the neutral fluorine atom.
  2. The neutral fluorine atom has one fewer electron than the fluoride ion. (correct answer)
  3. Both the atom and the ion have the same number of electrons.
  4. The neutral fluorine atom and the fluoride ion have different numbers of neutrons.
Explanation: When you encounter questions about atoms versus ions, focus on understanding what changes when an atom gains or loses electrons to form an ion. A neutral fluorine atom has 9 protons (its atomic number) and 9 electrons, giving it no overall charge. When fluorine forms a fluoride ion (F⁻), it gains one electron to achieve a stable electron configuration. This means the fluoride ion has 9 protons and 10 electrons, resulting in a net negative charge of -1. Option B correctly states that the neutral fluorine atom has one fewer electron than the fluoride ion. The neutral atom has 9 electrons while the ion has 10 electrons—a difference of one electron. Option A is incorrect because the number of protons never changes when forming ions. Both the neutral atom and the fluoride ion have exactly 9 protons. Changing the number of protons would create a different element entirely. Option C is wrong because the atom and ion have different numbers of electrons (9 versus 10). The whole point of ion formation is the gain or loss of electrons. Option D is incorrect because neutron numbers don't change during ion formation. Both the atom and ion can have the same number of neutrons unless you're dealing with different isotopes, which isn't the case here. Remember this key principle: ion formation only involves the gain or loss of electrons. The number of protons (which determines the element's identity) and neutrons remains constant. When you see negative ions, think "gained electrons"; for positive ions, think "lost electrons."

Question 16

Which of the following statements correctly describes the relationship between an atom's protons and electrons in a cation?

  1. A cation has more electrons than protons.
  2. A cation has fewer electrons than protons. (correct answer)
  3. A cation has an equal number of electrons and protons.
  4. A cation has lost protons from its nucleus.
Explanation: When you encounter questions about ions and atomic structure, focus on what happens to electrons versus protons during ion formation. The nucleus of an atom (containing protons) remains unchanged during chemical processes - only electrons are gained or lost. A cation is a positively charged ion that forms when an atom loses one or more electrons. Since protons carry positive charge and electrons carry negative charge, removing electrons leaves behind more positive charges than negative charges, creating the overall positive charge characteristic of cations. Let's examine why answer B is correct: when an atom loses electrons to become a cation, the number of protons stays the same while the number of electrons decreases. This creates an imbalance where protons outnumber electrons, resulting in the positive charge. Answer A describes an anion, not a cation. Anions form when atoms gain electrons, giving them more electrons than protons and creating a negative charge. Answer C describes a neutral atom where positive and negative charges are balanced. Answer D reflects a fundamental misconception - protons cannot be lost from the nucleus during ordinary chemical processes. Nuclear changes require much more energy than chemical reactions provide and would change the element's identity entirely. Remember this pattern: cations are positive because they have fewer electrons than protons (think "cat-ion" = "positive ion"), while anions are negative because they have more electrons than protons. The number of protons never changes during ion formation - only electron count changes.

Question 17

The atomic mass of lithium (Li) on the periodic table is 6.94 amu. This value is not a whole number primarily because...

  1. protons and neutrons do not have a mass of exactly 1 amu.
  2. the mass of electrons is included in the total.
  3. it is a weighted average of the masses of its naturally occurring isotopes. (correct answer)
  4. lithium atoms easily lose an electron, which changes their mass.
Explanation: When you encounter atomic masses on the periodic table that aren't whole numbers, you're seeing the result of isotopic averaging. Elements exist as multiple isotopes—atoms with the same number of protons but different numbers of neutrons—and these isotopes occur in different abundances in nature. The correct answer is C because atomic mass represents a weighted average of all naturally occurring isotopes of an element. Lithium has two stable isotopes: lithium-6 (mass ≈ 6 amu, about 7.5% abundance) and lithium-7 (mass ≈ 7 amu, about 92.5% abundance). The weighted average calculation gives us approximately 6.94 amu, which matches the periodic table value. Let's examine why the other options are incorrect. Option A suggests that individual protons and neutrons don't have masses of exactly 1 amu, which is true but doesn't explain why we see fractional values on the periodic table—individual atoms still have whole number masses. Option B incorrectly states that electron mass is included; however, electrons are so light (about 1/1836 the mass of a proton) that they're essentially negligible in atomic mass calculations. Option D confuses chemical behavior with atomic mass—losing electrons creates ions but doesn't change the atom's fundamental mass as listed on the periodic table. Remember this pattern: whenever you see non-whole number atomic masses, think isotopes and weighted averages. The HESI often tests whether you understand that periodic table values represent natural mixtures of isotopes, not individual atoms.

Question 18

Which subatomic particle has a negligible mass compared to the others and is located outside the nucleus?

  1. Proton
  2. Neutron
  3. Electron (correct answer)
  4. Nucleon
Explanation: When you encounter questions about atomic structure, focus on the three fundamental subatomic particles and their key properties: location, charge, and mass. The electron is the particle that fits both criteria in this question. Electrons have an extremely small mass (approximately 1/1836 the mass of a proton) and are found in electron shells or orbitals outside the nucleus. This makes them essentially negligible in terms of atomic mass calculations, which is why atomic mass units focus primarily on protons and neutrons. Looking at the incorrect options: Choice A (proton) is wrong because protons have significant mass (approximately 1 atomic mass unit) and are located inside the nucleus. Choice B (neutron) is also incorrect since neutrons have nearly the same mass as protons and are found within the nucleus alongside protons. Choice D (nucleon) is wrong because this term refers to particles found in the nucleus - specifically protons and neutrons collectively - so by definition, nucleons are located inside the nucleus and have substantial mass. The key distinction here is that while protons and neutrons each contribute about 1 atomic mass unit to an atom's total mass, electrons contribute virtually nothing to the overall mass despite being crucial for chemical bonding and atomic behavior. For HESI success, remember that atomic structure questions often test your understanding of the mass-location relationship: heavy particles (protons and neutrons) are in the nucleus, while the light particle (electron) orbits outside it.

Question 19

An ion of an unknown element has a charge of -2 and contains 36 electrons. Which element is it?

  1. Argon (Ar, atomic number 18)
  2. Sulfur (S, atomic number 16)
  3. Selenium (Se, atomic number 34) (correct answer)
  4. Krypton (Kr, atomic number 36)
Explanation: When you encounter questions about ions and electron counts, you need to connect three key pieces: the number of electrons, the charge, and the atomic number of the neutral element. Start with what you know: this ion has 36 electrons and a -2 charge. The negative charge tells you this atom gained electrons. Since the ion has a -2 charge, the neutral atom must have had 2 fewer electrons than the ion currently has. Therefore, the neutral element originally had 34 electrons. In a neutral atom, the number of electrons equals the number of protons, which equals the atomic number. So you're looking for an element with atomic number 34. This is selenium (Se), making C correct. Let's examine why the other options don't work: A) Argon has atomic number 18, meaning 18 protons. An Ar²⁻ ion would have 20 electrons, not 36. B) Sulfur has atomic number 16. An S²⁻ ion would have 18 electrons, not 36. D) Krypton has atomic number 36, which might seem tempting since the ion has 36 electrons. However, if Kr had a -2 charge, it would have 38 electrons (36 + 2), not 36. The trap in option D is assuming the number of electrons in the ion equals the atomic number of the element. Remember that negative ions have gained electrons, so they have more electrons than their neutral atoms. For ion problems on the HESI, always work backwards: electrons in ion → charge → electrons in neutral atom → atomic number → element identity.