Learning Outcomes Atomic radii (covalent radii only). Explanations for general trends in values: (i) down a group (ii) across a period (covalent radii of main group elements only). First ionisation energies. Explanations for general trends in values: (i) down a group (ii) across a period (main group elements) and for exceptions to the general trends across a period. Second and successive ionisation energies. Evidence for energy levels provided by successive ionisation energy values.

Atomic radii

Atomic radii trends In general, the atomic radii values decrease across the period and increase down the group

Atomic radius Half the distance Between the nuclei of 2 atoms of the same element Joined by a single covalent bond

trends

Reasons for increase down a group The additional electrons are going into a new shell which is further from the nucleus Screening effect of inner electrons

Screening effect

Reasons for decrease across a period Increasing nuclear charge. No increase in the screening effect

IONISATION ENERGY Some elements lose electrons very easily, e.g. sodium and potassium Silver and gold have very little tendency to lose their electrons and hence are very unreactive

definition The first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

Na loses an electron

equation Na  Na + + e -

I. E. in groups I and II

Chlorine increases in size

Ionisation Energy decreases going down a group

Reasons for decrease down Increasing atomic radius. Screening effect of inner electrons

Ionisation Energy increases across a period

Reasons increase across Increasing nuclear charge. Decreasing atomic radius

IE and periodic table.

Exceptions to the general trends Beryllium and nitrogen have higher values than expected Reasons: Be  1S 2 2S 2 (Full orbitals give greater stability) N  1S 2 2S 2 2Sx 1 2Py 1 2Pz 1 (3 half filled orbitals give greater stability)

EVIDENCE FOR EXISTENCE OF ENERCY LEVELS Suppose we measure the first, second, third, etc. up to the nineteenth ionisation energy of potassium K = 1S 2 2S 2 2P 6 3S 2 3P 6 4S 1 K = 2,8,8,1 First ionisation energy has the lowest ionisation energy value. Electron in the 4s sublevel is easiest to remove

Potassium ionisation 1st ionisation energy: K(g) → K+(g) + e – n=1 2nd ionisation energy: K + (g) → K 2+( g) + e – n=2

Potassium IE’s n ionisation energy (kJ mol–1)

Potassium IE’s A, one electron has been removed from potassium The second electron is much more difficult to remove since this electron is being removed from the K+ ion

Potassium IE’s K + This ion has eight electrons in the outer shell ( B,C) The full outer sublevel (3p 6 ) has extra stability and therefore will require more energy to remove electrons from it. (B,C) C

Potassium IE’s B to C we are removing eight more electrons (point C on the graph), there is another sudden jump D is being removed from a shell which is closer to the nucleus