Key concepts in chemistry Study Guide

The relative atomic mass (Ar) of an element is calculated by finding the weighted average of the masses of its isotopes, taking their relative abundances into account. Isotopes are atoms of the same element with identical numbers of protons but different numbers of neutrons, resulting in distinct mass numbers. To find Ar, multiply the mass number of each isotope by its percentage abundance, add these results, and then divide by 100 (if using percentages). This approach ensures the calculated Ar reflects the typical sample found in nature, providing an averaged value rather than the mass of any single isotope. The final answer is usually not a whole number, because an element's isotopes typically have different masses and occur in different proportions.

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.