Question 1

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

Accepted Answer

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

Question 2

A sample of magnesium contains three naturally occurring isotopes: 24Mg (78.99%, mass = 23.985 amu), 25Mg (10.00%, mass = 24.986 amu), and 26Mg (11.01%, mass = 25.983 amu). What is the average atomic mass of magnesium?

Accepted Answer

24.31 amu

Question 3

Which experiment or observation most directly led to the conclusion that most of the mass of an atom is concentrated in a small, dense nucleus?

Accepted Answer

Rutherford’s gold foil experiment

Question 4

A neutral atom of chlorine has an atomic number of 17 and a mass number of 35. How many neutrons does it contain?

Accepted Answer

18

Question 5

Which of the following best explains why isotopes of the same element have nearly identical chemical properties?

Accepted Answer

They have the same number of electrons and protons.

Question 6

A forensic scientist uses mass spectrometry to analyze a sample and finds two peaks corresponding to masses of 28 and 29 amu for silicon. What does this indicate about the sample?

Accepted Answer

The sample contains at least two isotopes of silicon.

Question 7

Which of the following statements is NOT a postulate of Dalton’s atomic theory?

Accepted Answer

Atoms can be subdivided into protons, neutrons, and electrons.

Question 8

A certain element has two naturally occurring isotopes: 10X (mass = 10.012 amu, abundance = 19.91%) and 11X (mass = 11.009 amu, abundance = 80.09%). What is the atomic mass of element X?

Accepted Answer

10.81 amu

Question 9

Which of the following real-world applications relies on the measurement of isotope ratios?

Accepted Answer

Estimating the year of birth using 14C in tooth enamel.

Question 10

An atom has 15 protons, 16 neutrons, and 18 electrons. What is the charge of this atom?

Accepted Answer

3-

Question 11

Which statement best explains why the atomic mass of an element on the periodic table is rarely a whole number?

Accepted Answer

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

Question 12

Which of the following would be considered a violation of 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 13

Which of the following best describes the difference between protons and neutrons?

Accepted Answer

Protons have a positive charge, while neutrons are neutral.

Question 14

Which scenario best illustrates the law of multiple proportions?

Accepted Answer

Two elements combine to form more than one compound, and the ratios of the masses of one element that combine with a fixed mass of the other are small whole numbers.

Question 15

Which of the following is true about the structure of the atom according to modern understanding?

Accepted Answer

Most of the atom's volume is empty space, with a dense nucleus at the center.

Question 16

A sample contains 100 atoms of element Y. 60 atoms have a mass number of 80, and 40 atoms have a mass number of 82. What is the average mass number of element Y in this sample?

Accepted Answer

80.8

Question 17

Which of the following best describes an isotope?

Accepted Answer

Atoms of the same element with different numbers of neutrons.

Question 18

Which of the following would NOT change if an atom gained or lost a neutron?

Accepted Answer

The atomic number

Question 19

Which of the following is a direct application of Dalton’s atomic theory in modern chemistry?

Accepted Answer

Predicting the ratios in which elements combine to form compounds.

Atomic Structure and Isotopes: Foundations and Applications

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