Trends across a period
Ionisation energy shows a general increase across each period.
Across each period the number of protons increase, so there is more attraction acting on the electrons. Electrons are added to the same shell, so the outer shell is drawn inwards slightly. There is the same number of inner shells, so electron shielding will hardly change.
Across a period, the attraction between the nucleus and outer electrons increases, so more energy is needed to remove an electron. This means that the first ionisation energy increases across a period. There is also a decrease in atomic radius across a period, because the increased nuclear charge pulls the electrons in towards it.
Starting the next period, there is a sharp decrease in first ionisation energy between the end of one period and the start of the next period. This reflects the addition of a new shell, further from the nucleus, which leads to increased distance of the outermost shell from the nucleus and increased electron shielding of the outermost shell by inner shells.
ACROSS A PERIOD, INCREASED NUCLEAR CHARGE IS THE MOST IMPORTANT FACTOR.
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