Mass Number Atomic Number



Atomic number is used to define the type of element a material or substance is. It is the number of protons present in an element’s nucleus. The atomic mass, on the other hand, is the number of both the protons and neutrons present in the nucleus of the element. The mass number is the sum of the number of protons and neutrons in an atom. It is a whole number. The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.

Atomic number and mass number are always whole numbers because they are obtained by counting whole objects (protons, neutrons, and electrons). The sum of the mass number and the atomic number for an atom (A-Z) corresponds to the total number of subatomic particles present in the atom.

How to Find the Number of Protons, Neutrons, and Electrons. Step by Step Explanation with Examples

In this post, we’ll be going over how to determine the number of protons, neutrons, and electrons in an atom or ion.

We’ll first start by discussing what each of the components in the nuclide notation means. Then we’ll go through two examples together.

The Nuclide Notation

Atomic Number Examples

The letter(s) in the middle is the symbol of the element.

The number on the bottom left corner is the atomic number, which tells you the number of protons.

The number on the upper left corner is the mass number, which is equal to the neutrons and protons added together.

Lastly, the charge is on the upper right corner. If there isn’t any number or signs, then it means that atom has no charge and is neutral.

Mass Number Atomic Number Protons Neutrons Electrons Chart

Rules to Finding Number of Protons, Neutrons, and Electrons

# of protons = atomic number

# of neutrons = mass number – atomic number

# of electrons = atomic number – charge

That’s it!

Examples

Great, lets apply the rules to some examples.

Mass number atomic number

# of protons = 17

# of neutrons = 37 – 17 = 20

# of electrons = 17 – 0 = 17

# of protons = 16 (the atomic number is not given, but can be found on the periodic table)

# of neutrons = 32 – 16 = 16

# of electrons = 16 – (-2) = 18

Additional Practice

Try these on your own and check the answer below

  1. 78Se2-
  2. 39K+

ANSWERS

Mass Number Atomic Number
  1. 34 protons, 44 neutrons, 36 electron
  2. 19 protons, 20 neutrons, 18 electron
Mass Number Atomic Number

Additional Resources

If you have any questions, leave a comment below.