WHAT IS An Atom?

The StRucture of An Atom

Atoms are the smallest parts that make up matter. They are so small that they can't even be seen with a powerful microscope. Each element of the Periodic Table is made up of particular type of atom. For example: the element magnesium is made up of magnesium atoms, the element oxygen is made up of oxygen atoms, and so on.

Atoms are made of three types of subatomic particles: protons, electrons and neutrons.

Protons are positively charged particles found in the nucleus of atoms. Each proton has a relative mass of 1. The number of protons that an atom has is called the atomic number.

Number of protons = atomic number

Neutrons are neutral and have no charge. They are also found in the nucleus of atoms. Each neutron also has a relative mass of 1. The number of neutrons can be found by subtracting the atomic number from the mass number.

Number of neutrons = mass number - atomic number

Electrons are negatively charged particles found moving rapidly in orbits/energy levels in the space surrounding the nucleus. Each electron has a very tiny mass, about 1/1840 the mass of a proton or neutron, so their mass is almost negligible.

Number of electrons = atomic number

Atoms are neutral (have no charge), as they have the same number of positively charge protons in the nucleus and negatively charged electrons surrounding the nucleus in orbits/energy levels.

Atomic NUmber and Mass number

The atomic number of an element tells you how many positively charged protons there are in an atom's nucleus and it also tells you how many negatively charged electrons there are orbiting the atom's nucleus in energy levels or shells.

The mass number of an element tells you the total number of particles in the nucleus (the total number of protons and neutrons in the nucleus). For example, a potassium, K, molecule, with a mass number of 39, has a total of 39 protons and neutrons, 39 particles, in the atom's nucleus.

If we know the atomic number and mass number of an element then we can work out how many protons, electrons and neutrons the element has.

Atomic number = number of protons = number of electrons

Mass number = number of protons + number of neutrons

Example 1 - Fluorine