3.2.2 Describe and explain the trends in atomic radii, ionic radii, first ionization energies, electronegativities and melting points for the alkali metals (Li to Cs) and the halogens (F to I). 3.3.3 Describe and explain the trends in atomic radii, ionic radii, first ionization energies and electronegativities for elements across Period 3.

Effective nuclear charge

• Nuclear charge - Number of protons an atom contains. 
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• The inner electrons reduce the attraction of the nucleus for the outer electrons, therefore the outer electrons are shielded from the nucleus and repelled by the inner electrons. 
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• The effective charge experienced by the outer electrons is less than the full nuclear charge. 

Element	Na	Mg	Al	Si
Nuclear Charge	11	12	13	14
Electron Arrangement	2, 8, 1	2, 8, 2	2, 8, 3	2, 8, 4

From left to right, across a periods, one proton is added to the nucleus, there fore its added to the outer most shell, the effective charge increases with nuclear charge because there is not change in the number of inner electrons.

Element	Nuclear charge	Electron arrangement
Li	3	2, 1
Na	11	2, 8, 1
K	19	2, 8, 8, 1

As we descend the group, the increase in the nuclear charge is largely balanced by the increase in the number of inner electrons, both increase by eight between successive elements. The effective nuclear charge stays the same down the group.