Structure of an atom

We are learning how to:

• name subatomic particles and state their charges

• state the number of protons, neutrons and electrons in a given atom

• explain why an atom has no charge.

Atoms

An atom consists of three types of subatomic particle: protons, neutrons and electrons.

At the centre of every atom is its nucleus, containing the protons and neutrons.

The electrons move around the nucleus in shells or orbits.

Protons and electrons are electrically charged. Neutrons are neutral – they have no electric charge. Table 8.3 summarises these charges.

An atom carries an equal number of positive and negative particles and is, therefore, neutral.

Mass number

Sometimes, a Periodic Table shows two numbers for each element. The smaller number is the atomic number, and the larger number is the mass number.

The mass number (M) is the sum of the number of protons and neutrons (n) that are in the nucleus of an atom of the element; that is:

M = number of p + number of n

Activity 8.4

Here is what you should do:

Copy and complete the following table.

Check your understanding

1. State the value of the charge of each of the following particles: a) electron b) proton c) neutron. a) What particles does the atom have? b) Is an atom of hydrogen neutral?

2. Hydrogen has Z = 1 and M = 1.

Key terms

proton the subatomic particle carrying a positive electrical charge neutron the subatomic particle carrying no electrical charge electron the subatomic particle carrying a negative electrical charge nucleus the centre of an atom, which may contain protons and neutrons shell a ring-like structure formed by the arrangement of electrons around the nucleus orbit a ring-like structure around the nucleus, containing electrons mass number the total number of protons and neutrons that are in the nucleus of an atom of an element