Brief Particles Refresher

Here I will write a little refresher on what atoms, molecules, compounds and isotopes are.


Atoms are what make up all matter. They are made up of 3 types of particles: protons, neutrons and electrons. The name comes from the Greek word for indivisible. Which is humorous as we now know that that is not the case and they can be broken up further into quarks, which were discovered with the invention of the particle accelerator.

Protons and neutrons form the nucleus, which is what electrons orbit around. The number of protons and electrons are almost always equal, if that is not the case they are called ions instead.


An element is a substance that is made up entirely of an atom and they can all be found on the periodic table.

Each element has a different atomic number, which refers to the number of protons it has. Atomic number is a bit of a rubbish name and is sometimes called, far more sensibly, the proton number. On the periodic table the elements are ordered by this number.

Elements usually have a typical number of neutrons, however they may vary slightly. These variants of elements are called isotopes.

For example, carbon typically has 6 neutrons. However carbon-13 has 7 neutrons, and carbon-14 has 8. Similarly, the isotope Deuterium (aka heavy hydrogen) has 1 neutron, 1 more than typical hydrogen (yes, 0).

Most elements appear naturally, however some are synthetic and have been made with nuclear reactors, particle accelerators or exploding atomic bombs.


Molecules are when 2 or more atoms are joined together. They are joined with either ionic or covalent bonds.

With a covalent bond, electrons are shared.

With an ionic bond, one atom gains an electron whilst another loses one.


Compounds are molecules that are made up of different elements. Some molecules however are not compounds as some only bond with 2 of the same elements, for example hydrogen gas which is made up of 2 hydrogen elements bonded together.