- Element Z has two naturally occurring isotopes. The lighter isotope, ⁸⁵Z, has a natural abundance of 72.0%. The relative atomic mass of Z is 85.56. What is the mass number of the other isotope?
Answer: 87
Let the mass number of the heavier isotope be M. Its abundance is 100% - 72.0% = 28.0%. The relative atomic mass is the weighted average: (85 × 0.720) + (M × 0.280) = 85.56. This gives 61.20 + 0.280M = 85.56. Rearranging, 0.280M = 85.56 - 61.20 = 24.36. Solving for M: M = 24.36 / 0.280 = 87.
- A sample contains atoms of the same element with mass numbers 79 and 81. If the average atomic mass of this sample is 79.6 u, and the atomic number is 35, what is the ratio of neutrons in the heavier isotope to neutrons in the lighter isotope?
Answer: 46:44, representing a neutron difference that accounts for the mass variation between isotopes
For atomic number 35: lighter isotope (mass 79) has 79-35=44 neutrons; heavier isotope (mass 81) has 81-35=46 neutrons. Ratio of heavier to lighter is 46:44. Choice B reverses the ratio. Choice C uses mass numbers instead of neutron counts. Choice D incorrectly calculates some other relationship.
- Based on the information in the passage, what is the difference in charge between atoms N and M?
Answer: Atom N has a charge that is 5 units more positive than atom M's charge
Let atom M have p protons originally, so atom N has p+2 protons. From their neutral states: M gains 1 electron (charge = p - (p+1) = -1), while N loses 2 electrons (charge = (p+2) - (p+2-2) = (p+2) - p = +2). The difference in charge is: N's charge - M's charge = (+2) - (-1) = +3. However, we must also account for N having 2 more protons than M originally. Total difference: 2 (extra protons) + 3 (electron changes) = 5 units more positive for N.
- Which statement provides the primary reason why the relative atomic masses listed in the periodic table are not integers?
Answer: Elements naturally exist as a mixture of isotopes with different masses and relative abundances.
The primary reason for non-integer relative atomic masses is that they are the weighted average of the masses of the naturally occurring isotopes of an element. While mass defect and the precise masses of nucleons (A and D) do cause the mass of a single isotope to deviate slightly from an integer, the major reason for values like 35.45 for chlorine is the averaging of different isotopes (e.g., ³⁵Cl and ³⁷Cl). The mass of electrons (B) is negligible.
- Two particles, P and Q, are isoelectronic. Particle P is an ion of element X with a charge of 2− and a mass number of 34. Particle Q is a neutral atom of a noble gas. If particle P has two more neutrons than protons, what is element X?
Answer: Sulfur
For particle P, let p be the number of protons and n be the number of neutrons. We are given: mass number = p + n = 34, and n = p + 2. Substituting the second equation into the first: p + (p + 2) = 34, which gives 2p = 32, so p = 16. The element with 16 protons is Sulfur (S). The information about particle Q (a noble gas with 18 electrons, Argon) is consistent, as S²⁻ also has 16+2=18 electrons, but the question asks for element X.
- The element neon (Ne) has a relative atomic mass of approximately 20.2 and consists of isotopes with mass numbers 20, 21, and 22. What can be deduced from this information?
Answer: The isotope ²⁰Ne is the most abundant.
The relative atomic mass is a weighted average of the isotopic masses. Since the relative atomic mass (20.2) is very close to 20, the isotope with mass number 20 must be the most abundant, heavily weighting the average towards its own mass. If the isotopes were in equal abundance (A), the average would be near 21. If ²²Ne were most abundant (C), the average would be closer to 22. Chemical reactivity (D) is the same for all isotopes.
- The relative atomic mass of boron is 10.81. It has two stable isotopes, ¹⁰B and ¹¹B. What can be deduced about a random sample of 1000 boron atoms?
Answer: The sample will contain approximately 190 ¹⁰B atoms and 810 ¹¹B atoms.
The relative atomic mass (10.81) is a weighted average. Since 10.81 is much closer to 11 than to 10, the isotope ¹¹B must be significantly more abundant. Let abundance of ¹¹B be x. 11x + 10(1-x) = 10.81 => x = 0.81. So, ¹¹B is ~81% abundant and ¹⁰B is ~19% abundant. In a 1000-atom sample, one would expect about 810 ¹¹B atoms and 190 ¹⁰B atoms.
- An ion X³⁺ has a mass number of 52 and contains 21 electrons. What are the numbers of protons and neutrons in a neutral atom of this element?
Answer: 24 protons and 28 neutrons
The ion has a 3+ charge and 21 electrons. This means it has lost 3 electrons from its neutral state. Therefore, a neutral atom of element X has 21 + 3 = 24 electrons. In a neutral atom, the number of protons equals the number of electrons, so there are 24 protons. The number of neutrons is the mass number minus the atomic number (number of protons), which is 52 - 24 = 28 neutrons.
- Rubidium has a relative atomic mass of 85.47 and two major isotopes, ⁸⁵Rb and ⁸⁷Rb. In a mass spectrometer, the relative abundance of ions is proportional to the height of the peak. Which statement best describes the expected mass spectrum of rubidium?
Answer: The peak at m/z = 85 will be significantly taller than the peak at m/z = 87.
A mass spectrometer separates ions based on their mass-to-charge ratio. It does not measure the average mass. Since rubidium has two isotopes, two peaks will be seen at m/z values corresponding to their mass numbers (85 and 87). The relative atomic mass, 85.47, is much closer to 85 than to 87. This indicates that the ⁸⁵Rb isotope is much more abundant. Therefore, the peak corresponding to ⁸⁵Rb will be significantly taller.
- Which of the following species contains the same number of neutrons as an atom of ⁸⁰₃₅Br?
Answer: ⁸¹₃₆Kr
First, calculate the number of neutrons in ⁸⁰₃₅Br: Neutrons = Mass Number (A) - Atomic Number (Z) = 80 - 35 = 45. Now, calculate the number of neutrons for each option:
A) ⁸⁰₃₆Kr: 80 - 36 = 44 neutrons.
B) ⁷⁹₃₅Br: 79 - 35 = 44 neutrons.
C) ⁸¹₃₆Kr: 81 - 36 = 45 neutrons.
D) ⁸⁰₃₄Se: 80 - 34 = 46 neutrons.
Therefore, ⁸¹₃₆Kr has the same number of neutrons as ⁸⁰₃₅Br.
- A neutral atom of an element has an atomic number Z and a mass number A. Which statement is always correct for any neutral isotope of this element?
Answer: The atom has the same chemical reactivity.
Isotopes of an element have the same atomic number (Z), meaning they have the same number of protons. For a neutral atom, the number of electrons also equals Z. Since chemical reactivity is determined by the electron configuration, all isotopes of an element have identical chemical reactivity. A is incorrect; it has Z electrons. B is incorrect; it has Z protons. D is incorrect by definition.
- How many protons, neutrons, and electrons are present in the selenide ion, ⁷⁹₃₄Se²⁻?
Answer: 34 protons, 45 neutrons, 36 electrons
The notation ⁷⁹₃₄Se²⁻ provides all the necessary information. The atomic number (bottom number, Z) is 34, which is the number of protons. The mass number (top number, A) is 79. The number of neutrons is A - Z = 79 - 34 = 45. The charge is 2-, meaning the neutral atom has gained 2 electrons. A neutral Se atom has 34 electrons, so the ion has 34 + 2 = 36 electrons.
- An atom has 33 protons, 42 neutrons, and 33 electrons. Which of the following symbols correctly represents an isotope of this atom?
Answer: ⁷⁷₃₃As
The original atom has 33 protons, which identifies it as Arsenic (As). Isotopes of an element must have the same number of protons (atomic number) but a different number of neutrons (and thus a different mass number). The symbol ⁷⁷₃₃As represents an atom with 33 protons (so it is As) and a mass number of 77. This means it has 77-33 = 44 neutrons, which is different from the original 42. Therefore, it is an isotope.
- Which statement best explains why the isotopes Chlorine-35 and Chlorine-37 have identical chemical properties?
Answer: They possess the same electron configuration, which dictates how they form chemical bonds.
Chemical properties of an element are determined by its electron configuration, specifically the valence electrons, as these are involved in bonding. Isotopes of an element have the same number of protons and therefore the same number of electrons in their neutral state, resulting in identical electron configurations and chemical properties. The number of neutrons (distractor A) affects mass but not chemical reactivity. Nuclear charge (distractor B) is determined by protons, which is correct, but the link to reactivity is the resulting electron configuration. Distractor D incorrectly describes the reason for their similar properties.
- The relative atomic mass of chlorine is 35.45. Which is a valid conclusion that can be drawn from this fact alone?
Answer: No individual chlorine atom has a mass of 35.45 u.
The relative atomic mass is a weighted average of the masses of the isotopes. Since it is not a whole number, it cannot represent the mass of a single atom (which must have an integer number of protons and neutrons). Therefore, no single chlorine atom can have a mass of 35.45 u. B is incorrect because the ratio is closer to 3:1, not 1:1. C and D are incorrect because mass numbers and neutron counts must be integers.
- The mass of one atom of a specific isotope is 12.000 u by definition. What is this isotope?
Answer: Carbon-12
The unified atomic mass unit (u) is defined as exactly one-twelfth of the mass of a single, neutral atom of carbon-12 in its nuclear and electronic ground state. Therefore, the mass of one carbon-12 atom is exactly 12.000 u by definition. This serves as the standard for all other atomic mass measurements.
- A beam containing three species, P (²⁰Ne), Q (²²Ne), and R (²⁰Ne⁺), is passed through a uniform electric field. Which statement correctly describes their behaviour?
Answer: Only R will be deflected, while P and Q pass through undeflected.
An electric field exerts a force on charged particles. Species P (²⁰Ne) and Q (²²Ne) are neutral atoms and will not be affected by the electric field, so they will pass through undeflected. Species R (²⁰Ne⁺) has a positive charge, so it will be deflected by the electric field. Therefore, only R is deflected.
- Two atoms have the same number of electrons but different numbers of neutrons. One atom has 18 protons and the other has 17 protons. What is the difference in their charges?
Answer: The atom with 18 protons has a charge that is 1 more positive than the other atom
Charge = protons - electrons. Since both atoms have the same number of electrons but one has 18 protons while the other has 17, the difference in charge is 18 - 17 = 1. The atom with 18 protons is more positive by 1. Choice B states the relationship backwards. Choice C ignores the proton difference. Choice D incorrectly suggests neutrons affect charge.
- The nucleus of an atom of element Y contains p protons and n neutrons. The atom loses two electrons. What is the composition of the resulting ion?
Answer: p protons, n neutrons, p-2 electrons
The nucleus is unaffected by the loss of electrons, so the number of protons (p) and neutrons (n) remains the same. A neutral atom has the same number of electrons as protons, which is p. When the atom loses two electrons, the number of electrons becomes p-2. The resulting ion has p protons, n neutrons, and p-2 electrons.
- A sample of naturally occurring gallium contains two isotopes, ⁶⁹Ga and ⁷¹Ga. The relative atomic mass of gallium is 69.72. What is the percentage abundance of the ⁶⁹Ga isotope?
Answer: 64.0%
Let the fractional abundance of ⁶⁹Ga be x and that of ⁷¹Ga be (1-x). The relative atomic mass is the weighted average: (69 x) + (71 (1-x)) = 69.72. Solving for x: 69x + 71 - 71x = 69.72. This simplifies to -2x = -1.28. Therefore, x = 0.64. The percentage abundance is 0.64 * 100 = 64.0%.
