Learning Outcomes Atomic radii (covalent radii only). Explanations for general trends in values: (i)...

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

definitionThe 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 1S2 2S2 (Full orbitals give greater stability)• N 1S2 2S2 2Sx1 2Py1 2Pz1 (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 = 1S2 2S2 2P6 3S2 3P6 4S1

• 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) → K2+(g) + e– n=2

Potassium IE’sn ionisation energy

(kJ mol–1)

1 419 2 3051 3 4412 4 5877 5 7975 6 9649 7 11343 8 14942 9 16964 10 48577 11 54433 12 60701 13 68896 14 75950 15 83152 16 93403 17 99771 18 444911 19 476075

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Sucessive Ionisation of Potassium

Series1

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

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Sucessive Ionisation of Po-tassium

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A

C

Potassium IE’s• K+ This ion has

eight electrons in the outer shell ( B,C)

• The full outer sublevel (3p6) has extra stability and therefore will require more energy to remove electrons from it. (B,C)

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Sucessive Ionisation of Po-tassium

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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

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Sucessive Ionisation of Po-tassium

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