Q: Which of the following best explains why the mass of an electron is considered negligible compared to the mass of a proton or neutron?

The electron has a mass of $$9.11 \times 10$$^{-28}$$ g, which is about 1/1836 the mass of a proton.

Question 1

Which of the following statements best describes the Law of Conservation of Mass as demonstrated in a chemical reaction such as the combustion of methane ($$\mathrm{CH_4$}) $$shown below?

$$\mathrm{CH_4$ + 2O_2 \rightarrow CO_2$ + 2H_2O}$$

Accepted Answer

The total mass of reactants equals the total mass of products.

Question 2

Dalton’s Atomic Theory states that all atoms of a given element are identical. Which modern discovery contradicts this postulate?

Accepted Answer

The existence of isotopes.

Question 3

Which experiment provided evidence for the existence of a small, dense, positively charged nucleus in the atom?

Accepted Answer

Rutherford’s gold foil experiment

Question 4

A neutral atom of $$^{125}\mathrm{I}$$ contains how many protons, neutrons, and electrons?

Accepted Answer

53 protons, 72 neutrons, 53 electrons

Question 5

Which of the following best explains why the mass of an electron is considered negligible compared to the mass of a proton or neutron?

Accepted Answer

The electron has a mass of $$9.11 	imes 10$$^{-28}$$ g, which is about 1/1836 the mass of a proton.

Question 6

Which of the following is a correct representation of the atomic symbol for an ion with 34 protons, 44 neutrons, and 36 electrons?

Accepted Answer

$$^{78}\mathrm{Se}^{2-}$$

Question 7

Given the following isotopic abundances for hydrogen:

- $$^1\mathrm{H}$$: 99.9885% (1.0078 amu)
- $$^2\mathrm{H}$$: 0.0115% (2.0141 amu)
- $$^3\mathrm{H}$$: negligible (3.0160 amu)

Calculate the average atomic mass of hydrogen.

Accepted Answer

$$1.0079$$ amu

Question 8

Which of the following best describes the result of Thomson’s cathode ray tube experiment?

Accepted Answer

It demonstrated that atoms contain negatively charged particles called electrons.

Question 9

A sample contains two isotopes of element X: $$^{151}\mathrm{X} (150.99 $$amu) and $$^{153}\mathrm{X} (153.03 $$amu). If the average atomic mass is 151.69 amu, what is the percent abundance of $$^{153}\mathrm{X}$$?

Use the formula:

$$\mathrm{Atomic\ Mass} = f_1$ 	imes m_1$ + f_2$ 	imes m_2$

Accepted Answer

34%

Question 10

Which of the following statements about isotopes is correct?

Accepted Answer

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

Question 11

Which of the following elements is most likely to be the unknown element with isotopes of atomic masses 35.9675 (0.337%), 37.9627 (0.063%), and 39.9624 (99.600%)?

Accepted Answer

K (Potassium)

Question 12

Which of the following best describes the difference between carbon monoxide ($$\mathrm{CO}) $$and carbon dioxide ($$\mathrm{CO_2$}) in $$terms of the law of multiple proportions?

Accepted Answer

The ratio of the mass of oxygen that combines with a fixed mass of carbon is a simple whole number.

Question 13

Which subatomic particle is responsible for the chemical identity of an element?

Accepted Answer

Proton

Question 14

Which of the following best explains why $$\beta$$ particles are deflected in the opposite direction to $$\alpha$$ particles in a magnetic field?

Accepted Answer

$$\beta$$ particles are negatively charged and $$\alpha$$ particles are positively charged.

Question 15

A neutral atom has 20 protons, 20 neutrons, and 18 electrons. What is the correct atomic symbol for this ion?

Accepted Answer

$$^{40}\mathrm{Ca}^{2+}$$

Question 16

Which of the following ions has the greatest number of electrons?

Accepted Answer

$$^{31}\mathrm{P}^{3-}$$

Question 17

Which scientist is credited with first observing radioactivity?

Accepted Answer

Henri Becquerel

Question 18

Which of the following best describes the nuclear model of the atom proposed by Rutherford?

Accepted Answer

A dense nucleus containing protons and neutrons, with electrons occupying the surrounding space.

Question 19

Calculate the average atomic mass of iron using the following data:

- $$^{54}\mathrm{Fe}$$: 53.94 amu (5.845%)
- $$^{56}\mathrm{Fe}$$: 55.93 amu (91.75%)
- $$^{57}\mathrm{Fe}$$: 56.94 amu (2.119%)

Use the formula:

$$\mathrm{Atomic\ Mass} = f_1$ 	imes m_1$ + f_2$ 	imes m_2$ + f_3$ 	imes m_3$

Accepted Answer

55.68 amu

chapter 2 (gen chem, n) Atomic Theory and Structure: Foundations of Modern Chemistry

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