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 uses 14C isotope ratios in tooth enamel to estimate a person's year of birth. Which property of isotopes makes this possible?

Accepted Answer

Isotopes of the same element have different numbers of neutrons, allowing their ratios to change over time.

Question 3

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 4

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

Accepted Answer

Atoms are indivisible and indestructible.

Question 5

A sample of gallium contains two isotopes: 69Ga (mass = 68.9256 u, abundance = 60.1%) and 71Ga (mass = 70.9247 u, abundance = 39.9%). What is the average atomic mass of gallium? (Round to two decimal places.)

Accepted Answer

69.72 u

Question 6

Which of the following elements is most likely to have only one naturally occurring isotope?

Accepted Answer

Sodium (Na)

Question 7

A neutral atom of phosphorus has an atomic number of 15 and a mass number of 31. How many neutrons does it contain?

Accepted Answer

16

Question 8

Which experiment or observation most directly led to the conclusion that most of the atom is empty space?

Accepted Answer

Rutherford’s gold foil experiment

Question 9

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

Accepted Answer

Magnesium has three naturally occurring isotopes.

Question 10

Which of the following best describes the charge of an atom with 17 protons and 18 electrons?

Accepted Answer

The atom has a net charge of -1.

Question 11

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 manufacturing.

Question 12

Which statement about isotopes is correct?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 13

A sample contains 80% isotope X (mass = 10 u) and 20% isotope Y (mass = 11 u). What is the average atomic mass of the element?

Accepted Answer

10.2 u

Question 14

Which of the following is a direct consequence of Dalton’s postulate that atoms retain their identity in chemical reactions?

Accepted Answer

The total number of atoms of each element is conserved during a chemical reaction.

Question 15

Which of the following best explains why mass spectrometry is useful in determining the isotopic composition of a sample?

Accepted Answer

It separates isotopes based on their mass, allowing measurement of their relative abundances.

Question 16

Which of the following statements about subatomic particles is correct?

Accepted Answer

Protons and neutrons have nearly the same mass, which is much greater than that of electrons.

Question 17

A neutral atom of element X has 12 protons, 12 neutrons, and 12 electrons. Which of the following is true?

Accepted Answer

The atomic number is 12 and the mass number is 24.

Question 18

Which of the following scenarios would violate the law of conservation of mass?

Accepted Answer

A chemical reaction in which the total mass of products is less than the total mass of reactants.

Question 19

Which of the following best describes the difference between atomic number and mass number?

Accepted Answer

Atomic number is the number of protons; mass number is the total number of protons and neutrons.

Atomic Structure and Nuclear Chemistry: Key Concepts

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