Elements, compounds and mixtures Study Guide

The relative formula mass (Mr) of a substance is the sum of the relative atomic masses (Ar) of all the atoms present in its chemical formula. It represents the mass of one formula unit of that substance, relative to 1/12th the mass of a carbon-12 atom. For example, to calculate the Mr of water (H₂O), you would add the Ar of two hydrogen atoms and one oxygen atom. If Ar of H = 1 and Ar of O = 16, then Mr(H₂O) = (2 × 1) + 16 = 18. The Mr is a dimensionless quantity because it's a relative value, but it is often expressed in grams per mole (g/mol) when discussing molar mass. For covalently bonded molecules, it's sometimes referred to as relative molecular mass. For ionically bonded compounds, where distinct molecules don't exist, it's always called relative formula mass. It's essential for understanding stoichiometry, which is the quantitative relationship between reactants and products in chemical reactions. Calculating Mr accurately is a fundamental skill in chemistry, allowing chemists to predict reactant quantities, product yields, and concentrations.

Dot and cross diagrams are visual representations used in chemistry to show the bonding between atoms in simple covalent and binary ionic substances. They illustrate the arrangement of valence (outermost shell) electrons around the atoms in a molecule or an ionic compound. For covalent bonds, atoms share electrons to achieve a stable outer shell, typically eight electrons (octet rule), although hydrogen aims for two. In these diagrams, electrons from one atom are represented by 'dots' and electrons from the other atom by 'crosses'. The shared electrons are placed in the overlapping region between the atoms. For ionic bonds, one atom transfers electrons to another to form charged ions. The atom that loses electrons becomes a positively charged cation, while the atom that gains electrons becomes a negatively charged anion. In dot and cross diagrams for ionic substances, the electrons are shown as dots or crosses being transferred, and the resulting ions are drawn with their respective charges and full outer shells in square brackets. It's crucial to correctly identify the number of valence electrons for each element involved, which corresponds to its group number in the periodic table (for main group elements). This method helps to visualise how atoms gain stability through chemical bonding by achieving a full outer electron shell.