Question 1

Which of the following statements best describes isotopes?

Accepted Answer

Atoms with the same number of protons but different numbers of neutrons.

Question 2

Given the following data for iron isotopes: $$^{54}\mathrm{Fe} (6.85$$%, 53.939 amu), $$^{56}\mathrm{Fe} (91.75$$%, 55.935 amu), $$^{57}\mathrm{Fe} (2.12$$%, 56.935 amu), and $$^{58}\mathrm{Fe} (0.28$$%, 57.933 amu), calculate the atomic mass of iron to two decimal places.

Accepted Answer

$$55.85\ \mathrm{amu}$$

Question 3

Which isotope of hydrogen is most abundant, given that the atomic mass of hydrogen is $$1.00794\ \mathrm{amu}$$?

Accepted Answer

Protium ($$^1\mathrm{H})$$

Question 4

Bromine exists as two isotopes: Br-79 (50.7% abundance) and Br-81 (49.3% abundance). What is the approximate atomic weight of bromine?

Accepted Answer

$$80$$

Question 5

Which of the following best defines atomic mass (atomic weight) as used in the periodic table?

Accepted Answer

The weighted average mass of all naturally occurring isotopes of an element.

Question 6

A sample of chlorine contains 75.77% $$^{35}\mathrm{Cl} (34.969 $$amu) and 24.23% $$^{37}\mathrm{Cl} (36.966 $$amu). What is the atomic mass of chlorine?

Accepted Answer

$$35.45\ \mathrm{amu}$$

Question 7

Which of the following is a correct application of the formula for atomic weight: $$	ext{Atomic Weight} = \sum (	ext{Isotope Abundance} 	imes 	ext{Isotope Mass})$$?

Accepted Answer

Multiplying each isotope's mass by its fractional abundance and summing the results.

Question 8

A patient receives a diagnostic scan using a radioactive isotope of iodine, $$^{131}\mathrm{I}. $$Compared to $$^{127}\mathrm{I}$$, $$^{131}\mathrm{I}$$ has:

Accepted Answer

The same number of protons but more neutrons.

Question 9

Which of the following would NOT change if you compared two isotopes of the same element?

Accepted Answer

The number of protons.

Question 10

A naturally occurring element has two isotopes: 60% isotope A (mass = 10 amu) and 40% isotope B (mass = 12 amu). What is the atomic mass of the element?

Accepted Answer

$$10.8\ \mathrm{amu}$$

Question 11

Which of the following real-world scenarios best illustrates the importance of isotopic abundance in medicine?

Accepted Answer

Using $$^{99m}\mathrm{Tc} in $$imaging because its abundance and decay properties make it ideal for diagnostic scans.

Question 12

If the atomic mass of magnesium is $$24.305\ \mathrm{amu}$$, which isotope is most abundant: $$^{24}\mathrm{Mg}$$, $$^{25}\mathrm{Mg}$$, or $$^{26}\mathrm{Mg}$$?

Accepted Answer

$$^{24}\mathrm{Mg}$$

Question 13

Which of the following is true about the atomic number (Z) of isotopes?

Accepted Answer

It is the same for all isotopes of a given element.

Question 14

A sample of lead contains 1.4% $$^{204}\mathrm{Pb} (203.973 $$amu), 24.1% $$^{206}\mathrm{Pb} (205.974 $$amu), 22.1% $$^{207}\mathrm{Pb} (206.976 $$amu), and 52.4% $$^{208}\mathrm{Pb} (207.977 $$amu). What is the atomic mass of lead?

Accepted Answer

$$207.2\ \mathrm{amu}$$

Question 15

Which of the following best explains why the atomic mass of an element is rarely a whole number?

Accepted Answer

It is a weighted average of the masses of all naturally occurring isotopes.

Question 16

Which isotope of carbon is used as the standard for defining the atomic mass unit (amu)?

Accepted Answer

$$^{12}\mathrm{C}$$

Question 17

If a sample of an unknown element contains two isotopes, X-100 (mass = 100 amu, 70% abundance) and X-102 (mass = 102 amu, 30% abundance), what is the atomic mass of the element?

Accepted Answer

$$100.6\ \mathrm{amu}$$

Question 18

Which of the following would be the best method to distinguish between two isotopes of the same element in a laboratory?

Accepted Answer

Measuring their masses using mass spectrometry.

Question 19

Which of the following is a correct statement about the relationship between isotopic mass and atomic mass?

Accepted Answer

Atomic mass is the weighted average of the isotopic masses based on their natural abundances.

Isotopes and Atomic Mass: Fundamentals for GOB Chemistry

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