An atom is the smallest particle that still characterises an element; they are complex associations of many particles of matter including neutrons, protons and electrons.

You can think of atoms as ‘building blocks’ that come together to make ever more complex bodies.

Particles of matter – Atoms – Elements – Molecules – Macromolecules – Cell organelles – Cells – Tissues – Organs – Systems – Organisms – Populations – Ecosystems – Biospheres – Planets – Planetary Systems with Stars – Galaxies – The Universe

There are over 100 elements in the periodic table and what makes each of the them different is the number of neutrons, protons and electrons.

Protons and neutrons are always found in the nucleus (except in hydrogen which only has an electron and proton).

Electrons move around the nucleus on discreet orbitals or electron shells.

  • Neutrons are large and have no charge
  • Protons are large and have a positive charge
  • Electrons are small and have a negative charge. The mass of an electron is almost 1,000 times smaller than the mass of a proton.

Atomic number – is the number of protons found in an atom.
Atomic mass – is the total mass of an atom (including neutrons, protons and electrons).

It’s the number of protons that determines what element an atom is.

  1. A shell is sometimes called an orbital or energy level.
  2. Shells are areas that surround the center of an atom.
  3. The center of the atom is called the nucleus.
  4. Electrons live in something called shells – distinct orbits around the nucleus.
  5. Each of those shells has a name.
  6. The maximum number of electrons that each shell can accommodate, going from the middle to the outside, is 2, 8, 8, 18.

Atomic shells like to be full.

Some atoms have too many electrons (one or two extra). These atoms like to give up their electrons, like sodium and magnesium.

An atom of sodium

A Sodium atom
An atom of magnesium

A magnesium atom


Some atoms are really close to having a full shell. Those atoms go around looking for other atoms who want to give up an electron, like oxygen and flourine.

An atom of oxygen

An oxygen atom
an atom of flourine

A flourine atom


The rules for the first 18 elements are very straight-forward.

  1. Electrons fit nicely into three shells.
  2. These elements make up most of the matter in the universe.

Things get more complicated as you advance further through the elements of the periodic table.

Ions are atoms with either extra electrons or missing electrons.

Isotopes have different numbers of neutrons.

Compounds are groups of two or more elements that are bonded together. There are two main types of bonds that hold those atoms together, covalent and electrovalent/ionic bonds. Covalent compounds happen when the atoms share the electrons, and ionic compounds happen when electrons are donated from one atom to another.

Find out more about Chemistry: Atoms, Carbon, Acids & Alkalis, Solid, Liquid, Gas & Plasma


  1. These diagrams are incorrect.
    The first orbitals only have space for two electrons in total.
    These diagrams show 8 electrons in the first orbitals.
    If you count them you can see that you’ve shown 17 electrons in the sodium atom yet you say there are only 11.

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