Atoms are the building blocks of the universe you see around you. They’re electrically neutral, and that’s a good thing for life-forms like us. If atoms weren’t neutral, they’d be unstable, and we probably wouldn’t be here. Why are atoms neutral electrically? The answer is simple: Their negatively charged components (electrons) are completely balanced with their positively charged components (protons). Understanding this introduces you to key ideas for any budding scientist and also forms a bridge to other topics like the existence of non-neutral ions.

Three important particles make up atoms, and each has a different charge. The nucleus contains the protons and neutrons, and the electrons occupy a “cloud” around the outside. Neutrons possess no electric charge, as their name suggests. Protons and electrons are both charged but oppositely so. Protons have a positive charge of 1.6 × 10⁻¹⁹ coulombs, and electrons have a negative charge of −1.6 × 10⁻¹⁹ coulombs. Each proton carries the same positive charge, and each electron carries the opposite, so in many cases, scientists just say +1 for protons and −1 for electrons.

The chemical elements are defined most simply by the number of protons they have. This is called their atomic number, and the periodic table is a sequential list of elements with increasing atomic numbers. Hydrogen has an atomic number of one (meaning one proton), helium has two, lithium has three and so on. Every element has the same number of electrons orbiting the central nucleus. The negative charge from the electrons cancels the positive charge from the protons, so when you consider the whole thing, these atoms are all electrically neutral.

Neutrons sit with protons in the nuclei of most elements too, but since they aren’t charged, they don’t have a part to play in why atoms are electrically neutral. Some elements exist in more than one form with different numbers of neutrons as different isotopes, but this affects their stability when it comes to radioactive decay rather than their charge.

Although all atoms are ordinarily electrically neutral, there are some important exceptions. If an atom loses an electron, then the protons outnumber the electrons, and there is a net charge of +1. Some elements gain an electron and thereby an excess of negative charge, giving them a net charge of −1.

These are called ions, a “cation” for positively charged ion and an “anion” for negatively charged ions. Although this means they’re less electrically stable and will attract oppositely charged ions, certain elements are prone to behaving this way because their outermost electron shell is either one or two electrons away from being “full” or just one or two electrons into a new shell.