Q: Which of the following is the correct formula to calculate the average atomic mass of an element with two isotopes?

$$\text{Average atomic mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2)$$

Question 1

Which of the following statements best describes isotopes?

Accepted Answer

Isotopes are atoms with the same number of protons but different numbers of neutrons.

Question 2

A neutral atom of iron has 26 protons, 30 neutrons, and 26 electrons. What is the atomic number and mass number of this atom?

Accepted Answer

Atomic number = 26, Mass number = 56

Question 3

Which of the following ions is a cation?

Accepted Answer

$$\mathrm{Na^+}$$

Question 4

Given the isotope notation $$^{22}\mathrm{Ne}$$, how many protons, neutrons, and electrons are present in a neutral atom of this isotope?

Accepted Answer

10 protons, 12 neutrons, 10 electrons

Question 5

A sample of gallium contains two naturally occurring isotopes: $$^{69}\mathrm{Ga} (60.11$$% abundance, mass = 68.9256 amu) and $$^{71}\mathrm{Ga} (39.89$$% abundance, mass = 70.9247 amu). What is the average atomic mass of gallium? (Round to two decimal places)

Accepted Answer

$$69.72\ \mathrm{amu}$$

Question 6

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 7

An atom has 17 protons, 18 neutrons, and 18 electrons. What is the symbol and charge for this ion?

Accepted Answer

$$\mathrm{Cl^-}$$

Question 8

Which of the following statements about the Rutherford nuclear model is correct?

Accepted Answer

Most of the atom's mass is concentrated in a small, dense nucleus.

Question 9

A magnesium ion has 12 protons, 12 neutrons, and 10 electrons. What is its charge?

Accepted Answer

$$+2$$

Question 10

Which of the following best describes the periodic law?

Accepted Answer

When elements are arranged in order of increasing atomic number, certain sets of properties recur periodically.

Question 11

A neutral atom of sulfur has an atomic number of 16 and a mass number of 32. How many neutrons does it have?

Accepted Answer

$$16$$

Question 12

Which of the following is the correct formula to calculate the average atomic mass of an element with two isotopes?

Accepted Answer

$$	ext{Average atomic mass} = (	ext{mass}_1 	imes 	ext{fraction}_1) + (	ext{mass}_2 	imes 	ext{fraction}_2)$$

Question 13

A sample contains 75% $$^{35}\mathrm{Cl} ($$mass = 34.969 amu) and 25% $$^{37}\mathrm{Cl} ($$mass = 36.966 amu). What is the average atomic mass of chlorine?

Accepted Answer

$$35.47\ \mathrm{amu}$$

Question 14

Which subatomic particle determines the identity of an element?

Accepted Answer

Proton

Question 15

A certain ion has 20 protons, 20 neutrons, and 18 electrons. What is the symbol and charge for this ion?

Accepted Answer

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

Question 16

Which of the following best describes an anion?

Accepted Answer

An ion with more electrons than protons.

Question 17

If an atom has an atomic number of 15 and a mass number of 31, how many protons, neutrons, and electrons are present in a neutral atom?

Accepted Answer

15 protons, 16 neutrons, 15 electrons

Question 18

Which of the following is a correct statement about the mass of subatomic particles?

Accepted Answer

The mass of a neutron is slightly greater than that of a proton.

Question 19

A neutral atom of carbon-13 ($$^{13}\mathrm{C}) $$has how many protons, neutrons, and electrons?

Accepted Answer

6 protons, 7 neutrons, 6 electrons

Question 20

Which of the following best explains why the atomic mass listed on the periodic table is rarely a whole number?

Accepted Answer

It is the weighted average of all naturally occurring isotopes of the element.

Atomic Structure: Subatomic Particles, Isotopes, and Atomic Mass 08/25/2026

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