Just as we use words to name objects, in chemistr y we use chemica l nomenclature, that is, formu las and names to represent and name compounds. There a re r u les for w r it ing formu las and r u les for naming compounds. 1.1. Valence The va lence is the capacit y that an element has to combine w ith another. The hyd rogen atom has a va lence of 1 and is used as a reference. Va lence is def ined as the number of H atoms that can combine w ith an atom of any other element . For example, the formu la of water is H2O and the formu la of ammon ia is NH3. The va lence of O is 2 because it is combined w ith 2 H atoms; and the va lence of N is 3 because it is combined w ith 3 H atoms. 1.2. Oxidation number The concept of valence has some limitations and does not explain the formulas of many compounds. To overcome this, we use ox idation numbers. The ox idation number is the number of electrons that an atom ga ins or loses (completely or pa r t ia l ly) when it forms a compound . The ox idat ion number is negative i f the atom ga ins electrons. The ox idat ion number is positive i f the atom loses electrons. The table below shows the most common ox idat ion numbers for some elements. 1 -1 +1 H 2 13 14 15 16 17 +1 Li +2 Be -3 +3 B -4 +2 +4 C -3 +1 +3 +5 +2 N -2 -1 O -1 F +1 Na +2 Mg 3 4 5 6 7 8 9 10 11 12 +3 Aℓ -4 +2 +4 Si -3 +3 +5 P -2 +2 +4 +6 S -1 +1 +3 +5 +7 Cℓ +1 K +2 Ca +2 +3 Cr +2 +3 +4 Mn +2 +3 Fe +2 +3 Co +2 +3 Ni +1 +2 Cu +2 Zn +3 Ga -4 +2 +4 Ge -3 +3 +5 As -2 +2 +4 +6 Se -1 +1 +3 +5 +7 Br +1 Rb +2 Sr +2 +4 Pd +1 Ag +2 Cd +3 In +2 +4 Sn -3 +3 +5 Sb -2 +2 +4 +6 Te -1 +1 +3 +5 +7 I +1 Cs +2 Ba +2 +4 Pt +1 +3 Au +1 +2 Hg +1 +3 Tℓ +2 +4 Pb +3 +5 Bi Metals have positive oxidation numbers. They are found on the left and in the centre of the periodic table. Non-metals can have positive and negative oxidation numbers. They are on the right of the periodic table. 1. Names and formulas of chemical compounds In genera l , the ox idat ion numbers of an element depend on the g roup the element is in. For example: A l l of the elements in g roup 1 have the ox idat ion number +1, and a l l of the elements in g roup 17, -1. The elements in g roup 17 that have posit ive ox idat ion numbers have +1, +3 , +5 and +7. These a re the odd numbers up to the un it of the g roup number. Look at other simi la r relat ionsh ips bet ween the ox idat ion numbers of the elements in g roups 2 and 16, and those of g roups 13 and 15. 290
RkJQdWJsaXNoZXIy