Number the elements sodium, magnesium, phosphorus, and chlorine in the predicted order ionization energies from the highest to the smallest. !

Chlorine              1
Phosphorus        2
Magnesium         3
Sodium               4

The order  of  ionization energies  from  the highest  to smallest is as below

Chlorine -1 ( highest)

phosphorus -2

magnesium-3

sodium-4 (smallest)

Explanation

sodium, magnesium,phosphorus  and chlorine are in period  three in periodic table.
The trend of ionization energy across period 3 is that it increases across the period ( from the left to the right).
The ionization energy increases  across the period 3 because  there are more protons  in each nucleus  which led to increase in nuclear  charge across the period. ( from left to the right)
This led to increase  in force  of attraction between nucleus and outer  electron.( from the left to the right)
 among the four element  given above from the left  of period 3  , Sodium is 1st element  followed  by magnesium, then phosphorus and then  chlorine.
Therefore since sodium is in left side it has the smallest ionization energy while chlorine has the highest ionization energy  since is is located at the fur right  of period  3.

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