In general, ionisation energies increase across a period.
In the whole of period 2, the outer electrons are in 2-level orbitals (2s or 2p).
These are all the same distances from the nucleus, and are shielded by the same number of electrons.
The difference is the increasing number of protons across the period. It causes a greater attraction between the nucleus and the electrons and so it increases the ionisation energies.
In period 3, the trend remains: the ionisation energy increases across the period. All the electrons being removed are in the third level and are shielded by the same number of electrons. They is still an increasing nuclear charge though, and this is what causes the higher ionisation energies.
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