Atoms are the building blocks of chemistry. Each atom contains negatively and positively charged particles (electrons and protons). When there are the same number of electrons as protons, these charges cancel each other out – but if there’s an imbalance the atom will carry an electrical charge.

The difference between the number of electrons and protons in an atom determines how strong the charge is, and what that atom can bond with. Atoms with a positive charge will be attracted to negatively charged atoms to form a molecule. This bonding between atoms is the key to how molecules interact with each other. The positioning of atoms in a molecule may give it polarity. If two positively charged atoms are near each other in a molecule, that area may carry a slighty positive charge, while elsewhere the charge may be slightly negative.

An example of this is water.

H₂O is one of the most common molecules we encounter, and the position of the positively charged hydrogen atoms is critical in how it changes state, from solid (as ice) to liquid to gas (as steam).

Other molecules interact with water, combining with the atoms to create new soluble liquids. Fizzy drinks are carbonated, an example of a chemical reaction between carbon dioxide and water that only occurs under pressure.