PERIODIC TRENDSIonic Size

Ionic Radius

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)

Ionic Radius Trends: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

Na vs Na+1

Na 1s22s22p63s1

Na+1 1s22s22p6

You lost the only electron on the 3rd energy level so the valence energy level is now the 2nd level so it is smaller

Mg vs Mg+1

Mg 1s22s22p63s2

Mg+1 1s22s22p63s1

The valence electrons are still on the 3rd 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

Why?When electrons are added

to an atom, there is an increase in the electron-electron repulsion causing the atoms’ radius to expand

Cl vs Cl-1

Cl [Ne]3s23p5

Cl-1 [Ne]3s23p6

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. Ca or Ca+2

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 NePb+1 or Pb+2

Rb+1

O-2

F-1

Pb+1