Key concepts in chemistry Study Guide

Every atom contains a nucleus, within which are subatomic particles called protons and neutrons. Orbiting this nucleus are electrons. The number of protons within an atom's nucleus is known as its atomic number. This specific quantity of protons is fundamental to an atom's identity because it uniquely defines which element it belongs to. For example, all atoms of carbon invariably possess six protons, and any atom with exactly six protons is, by definition, carbon. Changes in the number of neutrons or electrons do not alter an atom's elemental identity, though they can result in isotopes or ions of that element, respectively.

When atoms form covalent bonds, they share pairs of electrons, resulting in the creation of molecules. Molecules are discrete groups of atoms held together by shared electrons. This is distinct from ionic bonding, where ions are formed. Covalent bonding commonly occurs between non-metal atoms, such as in water (H₂O) or oxygen (O₂). Each molecule formed by covalent bonding consists of a specific number and type of atoms joined together through these shared electron pairs.