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for example. Li and Na...... whch one got greater ionisation enrgy?
the further away from the nucleus the outer electron is, the less attraction from the nucleus it feels, therefore it is easier to remove, so the ionisation energy decreases.
in this case Na would have a lower ionisation energy, because it's lower down the group, so it's outer electrons are further away from the nucleus, so they're easier to remove than in Li.