Same number of electrons?
The relative sizes of cations and anions can be rationalised based on
For ions from different rows of the periodic table
If they have a different number of electron shells occupied
the number of electron shells occupied
the nuclear charge
the nuclear charge
For ions from different rows of the periodic table
If they have a different number of electron shells occupied
the ion with the larger number of electron shells occupied will be larger.
If they have the same number of electron shells occupied, Ions with higher nuclear charge are smaller due to the greater attractive forces between the outer electrons and the nucleus.
The cations shown that are in the same row of the periodic table have the same number of electrons.
Their radius decreases across the row from left to right because the nuclear charge increases.
Example: Na+ > Mg2+ > Al3+
The anions shown that are in the same row have the same number of electrons.
Their radius decreases across the row from left to right because the nuclear charge increases.
Example: O2– > F–
Note that the effect of an additional proton is much more significant for cations than it is for anions.
For example the difference in radius of sulfide and chloride is small by comparison to the difference in radii of sodium, magnesium and aluminium ions.
The cations shown that are in the same row of the periodic table have the same number of electrons.
Their radius decreases across the row from left to right because the nuclear charge increases.
Example: Na+ > Mg2+ > Al3+
The anions shown that are in the same row have the same number of electrons.
Their radius decreases across the row from left to right because the nuclear charge increases.
Example: O2– > F–
Note that the effect of an additional proton is much more significant for cations than it is for anions.
For example the difference in radius of sulfide and chloride is small by comparison to the difference in radii of sodium, magnesium and aluminium ions.