Mass Number Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Mass Number.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept 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.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Mass Number starts by naming the element, charge, and relevant protons, neutrons, or electrons.

Common stuck point: Students often know a formula related to mass number but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?

Worked Examples

Example 1

easy

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

Answer

A = 35,\text{ Chlorine-35}

First step

Mass number

A = \text{protons} + \text{neutrons} = 17 + 18 = 35

Full solution

2
Atomic number $Z = 17$ corresponds to chlorine (Cl).
3
This isotope is written as ${}^{35}_{17}\text{Cl}$ .

The mass number is the total number of nucleons (protons + neutrons) in the nucleus. It is always a whole number and appears as a superscript in isotope notation.

Example 2

medium

Why is the atomic mass of chlorine listed as

35.45

amu on the periodic table, rather than a whole number like

35

37

Example 3

medium

An isotope has 26 protons and 30 neutrons. What is its mass number? Identify the element.

Example 4

medium

Why are electrons safely ignored in

A = Z + N

Example 5

hard

^{238}_{92}\text{U} \rightarrow {}^{A}_{Z}\text{Th} + \alpha

. Find

A

and

Z

for the thorium product.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

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

Example 2

easy

An atom has 17 protons and 18 neutrons. What is its mass number?

Example 3

easy

What does the mass number (

A

) of an atom count?

Example 4

easy

An atom has 6 protons and 6 neutrons. What is its mass number?

Example 5

easy

Is the mass number always a whole number?

Example 6

easy

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

Example 7

easy

Are electrons counted in the mass number?

Example 8

easy

An atom has mass number 23 and 11 protons. How many neutrons does it have?

Example 9

easy

In the notation

^{16}_{8}\text{O}

, which number is the mass number?

Example 10

easy

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

Example 11

medium

An atom has mass number 40 and 18 protons. Find its neutrons and name the element.

Example 12

medium

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

Example 13

medium

Write the mass number of an atom with 19 protons and 21 neutrons, then give its isotope name.

Example 14

medium

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

Example 15

medium

An ion

^{24}_{12}\text{Mg}^{2+}

— does the

2+

charge change its mass number?

Example 16

medium

Two atoms have mass numbers 35 and 37 and both have 17 protons. How do their neutron counts differ?

Example 17

medium

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

Example 18

medium

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

Example 19

medium

In the symbol

^{23}_{11}\text{Na}

, identify the mass number and the number of neutrons.

Example 20

challenge

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

Example 21

challenge

An atom is written

^{A}_{Z}\text{X}

with

Z=20

, and it has 20 neutrons. Find

A

, the element, and the neutron-to-proton ratio.

Example 22

challenge

Why does mass number, not atomic mass, appear in isotope names like 'uranium-235'?

Example 23

easy

An atom has

8

protons and

10

neutrons. Find its mass number.

Example 24

easy

Carbon-14 has mass number

14

and

6

protons. How many neutrons does it have?

Example 25

easy

An atom has mass number

32

and

16

protons. How many neutrons?

Example 26

easy

Lithium-7 has

3

protons. What is its mass number and how many neutrons does it have?

Example 27

easy

Hydrogen-3 (tritium) has

1

proton. Find its mass number and neutron count.

Example 28

medium

An atom has

A = 65

and

35

neutrons. Identify the element.

Example 29

medium

{}^{63}_{29}\text{Cu}

atom: how many neutrons does it contain?

Example 30

medium

Uranium-238 has

92

protons. Find its neutron count.

Example 31

medium

Two isotopes of carbon:

^{12}\text{C}

and

^{14}\text{C}

. How do their neutron counts differ?

Example 32

medium

An atom has

A = 75

and

Z = 33

. Name the element and give its neutron count.

Example 33

medium

How is mass number (

A

) different from atomic mass (

\bar{A}

) reported on the periodic table?

Example 34

medium

Boron has isotopes

^{10}\text{B}

(19.9%) and

^{11}\text{B}

(80.1%). Estimate the average atomic mass.

Example 35

medium

How many neutrons does

^{59}_{27}\text{Co}

have?

Example 36

medium

An isotope of nickel has

A = 60

. If it has

32

neutrons, what is its atomic number?

Example 37

hard

Chlorine has isotopes

^{35}\text{Cl}

(75.77%) and

^{37}\text{Cl}

(24.23%). Compute its weighted average atomic mass.

Example 38

hard

Element X has only two natural isotopes: X-69 (60.1%) and X-71 (39.9%). Find

\bar{A}

and identify the element.

Example 39

hard

After alpha decay (

^4_2\text{He}

ejected), the parent's mass number decreases by how much, and atomic number by how much?

Example 40

challenge

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

Example 41

challenge

An element has three natural isotopes with mass numbers

24, 25, 26

and abundances

79\%, 10\%, 11\%

. Find

\bar{A}

and identify the element.

← Back to Mass Number Formula Explained Practice Mode

Related Concepts

Proton Neutron Isotope Atomic Mass

Background Knowledge

These ideas may be useful before you work through the harder examples.

protonneutron