How to Find the Number of Neutrons in an Atom

The atomic number is the number of protons in an atom, and the number of electrons in an atom equals the number of protons. Finding the number of neutrons requires a bit of math. Here's a quick guide to finding the number of neutrons in an atom.

Difficulty: Easy

Instructions

Things You'll Need:

• Periodic table

1. Search for an element or atom on the periodic table. Note the atomic number in the upper-left corner of the box and the atomic mass on the bottom. These are the only values you'll need to find the number of neutrons in the atom.

2. Round the atomic mass of the atom to its nearest whole number. For example, Calcium's atomic mass is 40.078, so it rounds down to 40. Nickel's atomic mass is 58.6934, so it rounds up to 59.

3. Subtract the atomic number from the rounded atomic mass. The difference is the number of neutrons in the atom. For Calcium, subtract 20 from 40 and obtain a difference of 20. Subtract 28 from 59 to find that Nickel has 31 neutrons.

Tips & Warnings

• The atomic number is equal to both the number of protons and the number of electrons in an atom.

Resources

• More on Protons, Electrons and Neutrons
• Atom by Atom Neutron Exercise
• Online Periodic Table