The distance from the center of an ion’s nucleus to its outermost electron

 Cation – ion that has lost electrons (POSITIVE
CHARGE)
 Anion – ion that has gained electrons (NEGATIVE
CHARGE)

1.
Cations will always be smaller than their parent atoms

There are two reasons:
 Many times when electrons are lost by an atom it loses enough to empty the outermost energy level
 If the valence electrons are still on the same energy level, losing electrons decreases the electron-electron repulsion allowing the ion to get smaller

+1
Na 1s 2 2s 2 2p 6 3s 1
Na +1 1s 2 2s 2 2p 6
You lost the only electron on the 3 rd energy level so the valence energy level is now the 2 nd level so it is smaller

+1
Mg 1s 2 2s 2 2p 6 3s 2
Mg +1 1s 2 2s 2 2p 6 3s 1
The valence electrons are still on the
3 rd EL but Mg +1 will be smaller because there are fewer valence electrons so they are not repelling as much

2. Anions will be larger than their parent atoms

 When electrons are added to an atom, there is an increase in the electronelectron repulsion causing the atoms’ radius to expand

-1
Cl [Ne]3s 2 3p 5
Cl -1 [Ne]3s 2 3p 6
Cl -1 has more electrons shoved on the valence energy level so there is more electron-electron repulsion hence a bigger size

Practice Problem:
Which is smaller?
1.
2.
Ca or Ca +2
S or S -2
1.
Ca +2
2.
S

Easier Way???
1. Look at valence energy level first
Higher the energy level, the bigger the atom
2. Look at # of protons second (only if needed)
More protons = smaller atom
3. Look at # of electrons last (only if needed)
More electrons = larger atom

Which is larger?
 Rb +1 or S -2
 Na +1 or O -2
 F -1 or Ne
 Pb +1 or Pb +2
 Rb +1
 O -2
 F -1
 Pb +1