Chemistry (Instant Revisions) by Jayne De courcy

By Jayne De courcy

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3p heat energy required ∆E e– (electron to be promoted) energy released in the form of light 3s Chemical reactions The elements become more reactive down groups 1 and 2. The atoms of the elements lower down the groups lose electrons more easily than those higher up, as illustrated by the trends in ionisation energies of the elements. All group 1 metals burn in oxygen. Example Lithium forms lithium oxide 4Li(s) + O2(g) 2Li2O(s) The oxide anion is O2–. Sodium produces a mixture of sodium oxide, Na2O, and sodium peroxide, Na2O2.

It is a trigonal planar molecule. Cl 120° B Cl Cl Four bond pairs, CH4 The electron pairs adopt a tetrahedral arrangement. 5° 35 S TRUCTURE AND B ONDING (10) Five bond pairs, PCl5 The electron pairs are arranged in a trigonal bipyramidal shape. Cl Cl 120° P Cl Six bond pairs, SF6 The electron pairs are arranged in an octahedral shape. 90° Cl Cl F F F 90° S F F F Species with lone pairs Examples Four electron pairs as three bond pairs and one lone pair, NH3 or PCl3 H Four electron pairs as two bond pairs and two lone pairs, H2O The shape is V-shaped (bent).

Example Molecules consisting of two identical atoms, such as H2 and Cl2 The electron cloud is equally distributed between the two nuclei. In molecules where two different atoms are joined by a covalent bond, the bond pair of electrons is not shared equally. Example Hydrogen chloride molecule δ+ δ+ δ– δ– H Cl Because the Cl atom is more electronegative than x the H atom, the bond pair is pulled towards the Cl atom. This results in a covalent bond with some partial ionic character; the small charges present are indicated by a δ.

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