WJEC Chemistry for AS: Study and Rev Guide

1.4 Bonding Atoms bond together to formmolecules by electrical forces either through covalency with the sharing of electron pairs or by the transfer of electrons from one atom to another to form ionic bonds. The type of bond depends on the difference in electronegativity between the atoms. Bonding between molecules is weak (van der Waals forces), although hydrogen bonds are stronger. Molecular shapes are governed by the number of electron pairs around a central atom following the VSEPR theory. p34–40 1.5 Solid structures Solids may be either giant or simple molecules; ionic and metallic solids always being giants comprising millions of units, and covalent structures either giants as in diamond or simple molecules such as I 2 that are held in the solid by weak van der Waals forces. Physical properties of the solids, such as melting temperature and electrical conductivity, relate to underlying structure of the solid. p41–42 1.6 Periodic Table Arranging the elements according to their electronic structures as in the Periodic Table is a very powerful means of rationalising and understanding chemical behaviour. Systematic trends in both physical and chemical properties emerge, in particular the typical valencies and redox properties of the groups. Detailed examples are shown for the s-block elements – reducing agents and metals – and the halogens of Group 7 – oxidising agents and non-metals. p43–46 1.7 Simple equilibria and acid-base reactions Many reactions are reversible and when the forward and reverse reactions occur at the same rate the reaction has reached equilibrium. The position of equilibrium changes if the conditions of a reaction change. The ratio of concentrations of products to reactants can be expressed mathematically using the equilibrium constant. All acids are proton donors, some are strong, others weak. The strength of acids is measured using the pH scale. Acid-base titrations provide opportunities to link practical work to chemical calculations. p47–54 Basic notes Good grasp Fully revised 7