It was once believed that the atom was the smallest unit of matter. We now know that the atom can be broken down into protons, neutrons, and electrons. In this article, we will cover how to use your periodic table to determine the number of protons, neutrons, and electrons in the atom.

## Background Knowledge

In order to determine the number of protons, neutrons, and electrons in an atom, you need to remember some background knowledge from other videos: What is an Atom and Element Notation. If you do not remember this information or need a refresher, please check out our videos on all of these topics!

### Using the Periodic Table

The periodic table gives you a lot of information. We have a whole video that lays out all the information you can get from the periodic table so please check that out! Here we are going to look at just one square on the periodic table to understand what information it tells us. Here we have a square of the element oxygen. In the middle, you see the element symbol. Under the symbol, you see the element name. This number here at the top is the atomic number. And the number here at the bottom is the average atomic mass. This information will be important to help us determine the number of protons, neutrons, and electrons in an atom.

### Determining the Number of Protons

To determine the number of protons in an atom, you need to remember that the number of protons in an atom is equal to the atomic number. So as we see here, the atomic number of oxygen is 8, so that means it has 8 protons. The atomic number of Mg is 12 which means it has 12 protons.

### Determining the Number of Electrons

In an atom, the number of protons is equal to the number of electrons. Remember that protons are positive and electrons are negative. Atoms are neutral, which means they have no charge. This is because they have the same # of positive and negative charges. So using our previous examples, oxygen has 8 protons which means an atom of oxygen also has 8 electrons. Magnesium has 12 protons which means an atom of magnesium also has 12 electrons. Protons and electrons will always be equal in an atom.

### Determining the Number of Neutrons

The last particle is the neutron. The number of neutrons is usually found using the mass number formula. The formula for mass number is mass number equals the number of protons plus the number of neutrons. So if we know the mass number, we can subtract the number of protons from that to find the number of neutrons. In order to know the mass # we need either the symbolic or isotope notation. If you need a refresher on element notation, check out our video!

### Examples

Using the symbolic notation shown here, we can see that the mass number is 15. Since the symbol is oxygen, we know there are 8 protons. To find the neutrons, we just have to subtract 8 from 15 to get 7. The isotope notation here of magnesium shows a mass number of 24. Magnesium has an atomic number of 12 so we know is has 12 protons. In order for the mass number to be 24, you would need to add 12 neutrons to the 12 protons.

Here we see the isotope notation calcium-41. Calcium will have 20 protons because that is the atomic number from the periodic table. This will also have 20 electrons because the number of protons and the number of electrons will be the same because it’s an atom.