The nucleus is at the centre of the atom and contains the protons and neutrons. Protons and neutrons are collectively known as nucleons.

The nucleus

The nucleus is at the centre of the atom and contains the protons and neutrons. Protons and neutrons are collectively known as nucleons.

Virtually all the mass of the atom is concentrated in the nucleus, because the electrons weigh so little.

Working out the numbers of protons and neutrons

No of protons = ATOMIC NUMBER of the atom

The atomic number is also given the more descriptive name of proton number.

No of protons + no of neutrons = MASS NUMBER of the atom

The mass number is also called the nucleon number.

This information can be given simply in the form:

How many protons and neutrons has this atom got?

The atomic number counts the number of protons (9); the mass number counts protons + neutrons (19). If there are 9 protons, there must be 10 neutrons for the total to add up to 19

.

The atomic number is tied to the position of the element in the Periodic Table and therefore the number of protons defines what sort of element you are talking about. So if an atom has 8 protons (atomic number = 8), it must be oxygen. If an atom has 12 protons (atomic number = 12), it must be magnesium.

Similarly, every chlorine atom (atomic number = 17) has 17 protons; every uranium atom (atomic number = 92) has 92 protons.