Practice Mass Number in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

The total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify specific isotopes of an element.

How heavy the nucleus is — each proton and neutron contributes about 1 atomic mass unit.

Showing a random 20 of 50 problems.

Example 1

easy
An atom has 17 protons and 18 neutrons. What is its mass number and what element is it?

Example 2

medium
In the symbol 1123Na^{23}_{11}\text{Na}, identify the mass number and the number of neutrons.

Example 3

easy
Hydrogen-3 (tritium) has 11 proton. Find its mass number and neutron count.

Example 4

easy
Can mass number be a decimal?

Example 5

challenge
After beta-minus decay (an electron is emitted from the nucleus), how does the mass number change?

Example 6

hard
After alpha decay (24He^4_2\text{He} ejected), the parent's mass number decreases by how much, and atomic number by how much?

Example 7

medium
A 2963Cu{}^{63}_{29}\text{Cu} atom: how many neutrons does it contain?

Example 8

medium
Why are electrons safely ignored in A=Z+NA = Z + N?

Example 9

easy
Do different isotopes of one element have the same mass number?

Example 10

easy
An atom has mass number 3232 and 1616 protons. How many neutrons?

Example 11

easy
An atom of nitrogen has 7 protons and 7 neutrons. State its mass number.

Example 12

medium
An atom has A=65A = 65 and 3535 neutrons. Identify the element.

Example 13

medium
Two ions 2040Ca2+^{40}_{20}\text{Ca}^{2+} and 2040Ca^{40}_{20}\text{Ca}: which has the larger mass number?

Example 14

easy
Find the mass number of an atom with 26 protons and 30 neutrons.

Example 15

easy
Are electrons counted in the mass number?

Example 16

challenge
Magnesium has isotopes Mg-24 (79%), Mg-25 (10%), and Mg-26 (11%). Estimate the average atomic mass.

Example 17

medium
An atom has mass number 31 and 16 neutrons. Find its atomic number and element.

Example 18

medium
Why is chlorine's atomic mass 35.5 even though every chlorine atom has a whole-number mass number?

Example 19

medium
An atom has mass number 56 and 30 neutrons. What is its atomic number and element?

Example 20

medium
Carbon-13 has mass number 13. How many neutrons does it have, and how does that compare to carbon-12?
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