Key concepts in chemistry Study Guide

The periodic table is a fundamental tool in chemistry, organising elements based on their atomic number and recurring chemical properties. A key organisational feature is the division of elements into metals and non-metals. This division is crucial because it helps predict an element's reactivity and physical characteristics. Broadly, metals are located on the left and centre of the periodic table, while non-metals are found on the right side. This arrangement originates from their atomic structures, specifically the number of valence electrons. Metals typically have 1, 2, or 3 valence electrons, which they tend to lose to form positive ions (cations). Non-metals, conversely, usually have 4, 5, 6, 7, or 8 valence electrons (excluding helium) and tend to gain electrons to form negative ions (anions) or share electrons.

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.