Q: Which formula correctly calculates the average atomic mass of an element with two isotopes?

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

Q: Which of the following is the correct relationship for mass number?

$\text{Mass number} = \text{protons} + \text{neutrons}$

Q: Which of the following is the correct calculation for the average atomic mass of an element with $60\%$ isotope $A$ (mass $10$) and $40\%$ isotope $B$ (mass $12$)?

$10 \times 0.6 + 12 \times 0.4 = 10.8$

Question 1

Which of the following statements correctly describes the atomic number of an element?

Accepted Answer

It is the number of protons in the nucleus.

Question 2

Given the isotope $^{13}C$, what is its mass number?

Accepted Answer

$13$

Question 3

If a carbon atom has an atomic number of $6$ and a mass number of $13$, how many neutrons does it contain?

Accepted Answer

$7$

Question 4

Which formula correctly calculates the average atomic mass of an element with two isotopes?

Accepted Answer

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

Question 5

A sample of carbon contains $99\%$ $^{12}C$ and $1\%$ $^{13}C$. What is the average atomic mass of carbon in this sample?

Accepted Answer

$12.01$

Question 6

Which of the following best explains why the atomic mass of carbon is not a whole number?

Accepted Answer

Because atomic mass is a weighted average of all naturally occurring isotopes.

Question 7

If an element has an atomic number of $19$ and a mass number of $39$, how many neutrons does it have?

Accepted Answer

$20$

Question 8

Which of the following is true for all atoms of the same element?

Accepted Answer

They have the same number of protons.

Question 9

A medical isotope of carbon is $^{11}C$. How many protons and neutrons does it have?

Accepted Answer

$6$ protons and $5$ 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 sample contains $75\%$ $^{35}Cl$ and $25\%$ $^{37}Cl$. What is the average atomic mass of chlorine?

Accepted Answer

$35.5$

Question 12

Which of the following is the correct relationship for mass number?

Accepted Answer

$	ext{Mass number} = 	ext{protons} + 	ext{neutrons}$

Question 13

A neutral atom of carbon has an atomic number of $6$. How many electrons does it have?

Accepted Answer

$6$

Question 14

Which of the following would change if you added a neutron to a carbon atom?

Accepted Answer

Its mass number

Question 15

A scientist discovers a new isotope of carbon with a mass number of $14$. What is its atomic number?

Accepted Answer

$6$

Question 16

Which of the following is the best definition of atomic mass?

Accepted Answer

The weighted average mass of all naturally occurring isotopes of an element.

Question 17

If the atomic mass of an element is $19.0$ and its atomic number is $9$, how many neutrons does the most common isotope have?

Accepted Answer

$10$

Question 18

Which of the following would NOT affect the atomic mass listed on the periodic table for an element?

Accepted Answer

The number of electrons in a neutral atom

Question 19

A hospital uses $^{13}C$ for metabolic studies. How many neutrons does $^{13}C$ have?

Accepted Answer

$7$

Question 20

Which of the following is the correct calculation for the average atomic mass of an element with $60\%$ isotope $A$ (mass $10$) and $40\%$ isotope $B$ (mass $12$)?

Accepted Answer

$10 	imes 0.6 + 12 	imes 0.4 = 10.8$

Atomic Number, Atomic Mass, and Isotopic Average Mass

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