First ionisation energy: The energy required to remove 1 mole of electrons from 1 mole of atoms in their gaseous state
E (g) ——> E+ (g) + e-
Second ionisation energy: The energy required to remove 1 mole of electrons from 1 mole of positively charged ions in their gaseous state
E+ (g) ——> E2+ (g) + e-
Across a period - ionisation energy increases
Down a group - ionisation energy decreases
If ionisation energy increases, nuclear attraction increases also as there are more protons which makes it harder to remove electrons. The atoms will also want to gain electrons rather than lose as they are wanting to be like the nearest noble gas.
If ionisation energy decreases, screening increases as the electrons are further away from the nucleus. Nuclear attraction decreases also, making it easier to remove an electron.
The second ionisation energy is always larger than the first as it is trying to remove an electron from a positively charged ion.
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