Describe the general trends in ionization energies down a group and across a period.

As we go down a group  the valence electrons are less strongly attracted  by the nucleus of the atom due to the shielding effect of the inner electrons. Therefore the ionization energies decrease down a group.

As we go across a group the positive charge in the nucleus increases as the number of protons in the nucleus increases ,and the attractive force on the valence electrons increases, so the ionization energies also  increase.

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