Key concepts in chemistry Study Guide

The atomic number, often symbolised by Z, is a fundamental characteristic of an element. It represents the total count of protons found within the nucleus of an atom. This quantity is unique to each element, meaning no two distinct elements can have the same atomic number. Consequently, the atomic number dictates an element's position on the periodic table; elements are arranged in increasing order of their atomic number. For example, hydrogen has an atomic number of 1 as it has one proton, and it is the first element on the periodic table. Helium, with two protons, has an atomic number of 2 and follows hydrogen. Every proton carries a positive charge, making the atomic number also indicative of the nuclear charge of an atom. In a neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons, ensuring the atom has no net electrical charge.

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.