Ionisation Energy

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Ionisation Energy
Defined as the energy required to remove the most loosely held electron from one mole of gaseous
atoms to produce one mole of gaseous ions each with a single positive ion.
1 Mole: 6.0023 x 10 to the power of 23
Formula: X (g) -------------> X + (g) + E-
Ionisation energy is measured in Kjmol-1 (KJ per mole.)
Factors
Charge on nucleus
The more protons there are in a nucleus, the more positively charged the nucleus is and the more
strongly electrons are attracted to it, thus this would involve an increase in ionisation energy.
The distance between electron and nucleus
Attraction falls rapidly with distance, electron close to nucleus will be more strongly attracted then
one a few shells away. So a higher ionisation energy is required.
Number of electrons between the outer electrons and the nucleus
Electrons in the inner levels repel the outer electrons. This lessens the pull of the nucleus by inner
electrons and is known as shielding.
Paired or not
Two electrons in the same orbital experience a bit of repulsion from each other. This offsets the
attraction of the nucleus, so paired electrons are easier to remove.
Trends
Period 1
With He and H neither have inner shells so there is no shielding present. He has two protons in the
nucleus whereas H only has one, hence the He electrons are more strongly attracted to the nucleus,
so they are more difficult to remove. So He has the highest ionisation energy out of all the elements
as only He and H are without shielding.
Period 2
In the case of Li (1s22s1) and Be (1s22s2), Be has one more proton than Li and no extra inner
electrons so nuclear charge on Be is higher and so Be has a higher first ionisation energy.
With Be (1s22s2) and B (1s22s22p1) B has one more proton and also two extra inner electrons, these
cancel out the proton charge to just +1 which is less than Be's +2 so the ionisation energy of Be is
higher than that of B
In general, the first ionisation energy increases across a period because the nuclear charge
increases but the shielding remains the same.

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From B (1s22s22p1) to N (1s22s22p3) the proton number increases, but the number of electrons
shielding the nuclear charge remains the same at 4. Thus the effective nuclear charge increases from
B to N and the electrons become progressively harder to remove.
The first ionisation energy thus increases from B to N.
So far we have used the concepts of nuclear charge and shielding to explain the difference in
ionisation energies, but these cannot explain the fall between N and O.…read more

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