What is the theory associated with the radius of an atom as it (a) gains an electron (b) loses an electron?
Electron shielding. The atom has it gains electrons increases in radius as the protons have less pull on the electrons, The atom decreases in radius has it loses electrons.
take Lithium as one example when Lithium become
Take Oxygen as another example when Oxygen become
The theory says that the radius of an atom (a) increases as the atom gains an electron and (b) decreases as the atom loses an electron.
The effect is caused by electron shielding.
It considers that the electrons are attracted to the nucleus and are also repelled by each other.
An outer electron is "shielded" from the attraction of the nucleus by the inner electrons and a bit by the other outer electrons.
The greater the shielding, the less the attraction experienced by the outermost electrons.
(a) Gaining an electron
Consider a fluorine gaining an electron to form a fluoride ion.
A fluorine atom has nine electrons and nine protons.
An outer electron is partially screened by the other eight electrons.
It "sees" only an effective charge of +5.2, and this determines its atomic radius.
If we add a tenth electron, it is now partially screened by the other nine electrons.
It sees an effective charge of +4.85.
The attraction for the nucleus is less, so the electrons spend more of their time further from the nucleus.
The atomic radius increases.
(b) Removing an electron
Consider a sodium atom gaining an electron to form a sodium ion.
A sodium atom has 11 electrons and 11 protons.
The outer electron "sees" only an effective charge of +2.2, and this determines its atomic radius.
If we remove this electron, we remove some of the screening effect.
Each outer electron now sees an effective charge of +6.85.
The attraction for the nucleus is greater, so the electrons spend more of their time closer to the nucleus.
The atomic radius decreases.