 # Atomic Equations and Isotopes

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## Atomic Equations and Isotopes

#### Atomic notation and equations

To help us to describe atoms and nuclei, we use numbers and letters. Here is an example: The atom in the diagram is described by the numbers and letters shown next to it. All atoms of a certain element will have the same number of protons in the nucleus.

The top number is called the mass number or the nucleon number. It tells you how many particles are in the nucleus, i.e. how many protons and neutrons.

The bottom number is called the proton number or the atomic number. It tells you how many protons there are in the nucleus.

The letters give you a clue as to the name of the atom. This is an atom of 'helium', He.

The number of electrons in an atom is the same as the number of protons. That makes the atom neutral overall (neither negative nor positive).

If the numbers are not equal, the atom becomes a charged particle. We call these charged particles ions.

Now it's your turn to have a go. Using the diagrams, you should be able to tell what the missing numbers are.

Try typing them in the correct place and then check your answer.

Example 1: A Lithium Atom You can also write equations for nuclear reactions. In these equations you will need to make sure that:

1. The total number of protons is the same before and after the reaction.
2. The total number of nucleons is the same before and after the reaction.

In this reaction, carbon and helium is combined to form oxygen. Notice that if you add up the numbers at the top of each atom (the nucleon numbers) on the left hand side you get the same as the total for the numbers on the right hand side.

The same is true for the proton numbers.

Have a go at this one. These reactions will not happen in real life, but they should give you an idea of how the system works. We will come back to this idea again.

#### Isotopes

The number of protons is the thing that decides how an atom is going to behave and therefore what element the atom belongs to. If you change the number of protons in an atom, you change the type of atom (it becomes an atom of another element.)

However, you can change the number of neutrons in an atom without changing the type of atom.

For example, hydrogen is an atom that contains only 1 proton. Atoms with more or less neutrons in them are called isotopes. The picture below shows three isotopes of hydrogen. So all the atoms below are hydrogen, except one. Which one?

Click on the atom that is not an isotope of hydrogen and find out if you are correct:  