Question 1

Which of the following statements best describes the law of definite proportions as demonstrated by Joseph Proust?

Accepted Answer

A compound always contains the same elements in the same ratio by mass.

Question 2

A forensic scientist is analyzing tooth enamel to determine a person's year of birth. Which isotope is most relevant for this analysis, and why?

Accepted Answer

Carbon-14, because its atmospheric levels changed due to nuclear testing and can be traced in biological samples.

Question 3

Which of the following changes would result in a different element?

Accepted Answer

Changing the number of protons in the nucleus.

Question 4

A sample of silicon contains three naturally occurring isotopes: Si-28 (92.2%, mass = 27.98 u), Si-29 (4.7%, mass = 28.98 u), and Si-30 (3.1%, mass = 29.97 u). What is the average atomic mass of silicon? (Round to two decimal places.)

Accepted Answer

28.09 u

Question 5

Which statement about Dalton’s atomic theory is NOT consistent with modern atomic theory?

Accepted Answer

Atoms are indivisible and indestructible.

Question 6

A neutral atom of element X has 17 protons and 18 neutrons. What is its mass number and what is the symbol for this isotope?

Accepted Answer

Mass number = 35; symbol: 35Cl

Question 7

Which of the following best explains why the atomic mass of chlorine listed on the periodic table is not a whole number?

Accepted Answer

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

Question 8

Which experiment or observation provided the strongest evidence that most of the mass of an atom is concentrated in a small, dense nucleus?

Accepted Answer

Rutherford’s gold foil experiment, which showed that most alpha particles passed through but some were deflected at large angles.

Question 9

A mass spectrometer is used to analyze a sample of magnesium. The resulting spectrum shows three peaks corresponding to masses of 24, 25, and 26. What does this indicate about the sample?

Accepted Answer

Magnesium has three naturally occurring isotopes with different numbers of neutrons.

Question 10

Which of the following best describes an isotope?

Accepted Answer

Atoms of the same element with different numbers of neutrons.

Question 11

A neutral atom has 12 protons, 12 neutrons, and 10 electrons. What is the charge of this atom, and what is its correct symbol?

Accepted Answer

Charge = +2; symbol: 24Mg2+

Question 12

Which of the following real-world applications relies on the law of conservation of mass?

Accepted Answer

Calculating the amount of reactants needed to produce a desired amount of product in industrial chemical synthesis.

Question 13

Which statement about the structure of the atom is correct?

Accepted Answer

Most of the atom’s volume is empty space, with a dense nucleus containing protons and neutrons.

Question 14

A sample contains two isotopes of element Q: Q-100 (60% abundance, mass = 100 u) and Q-102 (40% abundance, mass = 102 u). What is the average atomic mass of element Q?

Accepted Answer

100.8 u

Question 15

Which of the following is a direct application of isotope ratios in geology?

Accepted Answer

Determining the age of rocks using uranium-lead dating.

Question 16

Which of the following best explains why atoms are electrically neutral under normal conditions?

Accepted Answer

The number of protons equals the number of electrons.

Question 17

Which of the following statements about mass spectrometry is correct?

Accepted Answer

It separates isotopes based on their mass and provides their relative abundances.

Question 18

Which of the following scenarios would NOT result in the formation of an isotope?

Accepted Answer

An atom gains or loses electrons to form an ion.

Atomic Structure and Nuclear Chemistry: Key Concepts

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