WJEC Chemistry for AS: Student Bk

Basic structure An understanding of the general trends and behaviour gives us great predictive power. The chemistry of the elements is governed largely by their outer electrons so that arranging elements in groups according to their outer structure simplifies our study of their behaviour. Ionisation energy (I.E.) and electronegativity ( x ) increase diagonally across the table (i.e. across a period and up a group). For example, the the I.E.s for Cs and F are 376 and 1680kJ respectively. Electrons are thus readily lost in the s block giving cations and ionic componds; entering the p block in Group 3 I.E.s become too high so that electron sharing (covalency) is usual, but the more electronegative elements in Groups 6 and 7 can accept electrons to form anions in ionic compounds. Valency normally rises with group number up to a maximum of four and then falls (8 minus the group number) to one in Group 7. Elements are generally metals when IEs are low in the left and lower regions of the table and the d block transition elements; non-metals occur in the high I.E., upper right, portion and semiconductor elements, such as silicon, are found between these two regions. Melting temperature trends are more complex, depending on atomic mass, bond type and the type of solid structure but decrease down Group 1, rise down Group 7, increase across a period up to Group 4 (carbon melts above 3500°) and the drop sharply as elements form diatomic molecules held in the solid by weak intermolecular forces. s block d block p block Group 1 2 transition metals 3 4 5 6 7 8 inert gas Oxidn no 1 2 −2 −1 Redox reducing oxidising Ions cations anions Oxides basic acidic MT decrease down increase to Group 4 inc. down Element type metals non-metals I.E. and x increase diagonally across and up ▲ Trends in the periodic table Knowledge check (i) In the periodic table the values of ionisation energy and electronegativity down a group and across a period. (ii) The solubility of Group II hydroxides down the group and the solubility of Group II sulfates down the group. 37 Group 1 all salts soluble Group 2 M(OH) 2 solubility increases down group – Mg(OH) 2 insoluble MSO 4 solubility decreases down group – BaSO 4 insoluble MCO 3 all insoluble ▲ Solubility in water HOW SCIENCE WORKS The development of scientific knowledge and understanding of chemistry over many years in the 19th century led to the periodic table of Mendeleev that has given a great increase in understanding and predictive power in the subject. Gaps left in the table directed the search to then–unknown elements, such as the important semiconductor germanium, which when found had exactly the properties predicted by Mendeleev. The regular trends in properties down and across the groups led to a great simplification in the chemistry of the elements and the table served to guide the development of the electronic structure advances at the beginning of the 20th century. 1.6 The periodic table 57