When we look in the periodic table, for each element we find two numbers – one above the atom and one below it. The number above is the atomic number. It is the atomic number that defines the element. This number is the number of protons in the nucleus. There can be any number of protons, from one upwards. Since the atom overall has to be neutral and the proton an electron have equal and opposite charges - +1 and -1 respectively, the number of protons and electrons are equal. There is another particle in the atom – the neutron, which is, as it's name suggests, neutral, so it has no charge. Neutrons and protons exist together in the nucleus. They have virtually the same mass, which we take as +1 for both. The electron is much lighter and we take the mass of an electron as zero. We may summarise these properties of protons, electrons and neutrons in a table.
|
Mass |
Charge |
|
|
Proton |
1 |
1 |
|
Electron |
0 |
-1 |
|
Neutron |
1 |
0 |
Suppose we have the symbol for an atom, example,
The number at the top is the mass number and at the bottom is the atomic number. They are the other way around from they way they appear in the periodic table.
In this example, the number of protons=number of electrons=33. The mass number is the total number of protons + neutrons since protons and neutrons both have a mass of one. Hence the difference between the top and bottom numbers gives the number of neutrons =70-33=37.