Key concepts in chemistry Study Guide

An ion is defined as an atom or a group of atoms that carries an electrical charge. This charge arises when an atom, or a group of atoms covalently bonded together, either gains or loses one or more electrons. If electrons are lost, the ion becomes positively charged, as there are now more protons than electrons. These are called cations. If electrons are gained, the ion becomes negatively charged, as there are now more electrons than protons. These are called anions. The number of protons in an atom's nucleus determines its atomic number and elemental identity, and remains unchanged during ion formation.

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.