Periodic Properties of the Elements

Ionization Energy

The ionization energy of an atom measures how strongly an atom holds its electrons

Note that this does not mean the energy required to remove an electron from the n=1 shell (i.e the ground state orbital), the ground state here refers to the lowest energy electron configuration for the element in question

The first ionization energy, I₁, is the energy needed to remove the first electron from the atom:

Na(g) -> Na⁺(g) + 1e^-

The second ionization energy, I₂, is the energy needed to remove the next (i.e. the second) electron from the atom

Na⁺(g) -> Na²⁺(g) + 1e^-

The higher the value of the ionization energy, the more difficult it is to remove the electron

As electrons are removed, the positive charge from the nucleus remains unchanged, however, there is less repulsion between the remaining electrons

• Z_eff increases with removal of electrons
• Greater energy is needed to remove remaining electrons (i.e. the ionization energy is higher for each subsequent electron)

• There is also a big increase in ionization energy for removal of an electron from an inner shell (lower n value).
  
  This is due to the fact that when you move to an orbital with a lower principle quantum number, you are removing an electron which is much closer to the nucleus (and has a higher attraction for the nucleus)
  
  The inner shell electrons are too tightly bound to be ionized or shared with another atom, and thus do not participate in chemical bonding

First ionization energies as a function of atomic number

• Within each period (row) the ionization energy typically increases with atomic number
• Within each group (column) the ionization energy typically decreases with increasing atomic number

The basis for these observations:

• As the effective charge increases, or as the distance of the electron from the nucleus decreases, the greater the attraction between the nucleus and the electron. The effective charge increases across a period, in addition, the atomic radius decreases
• As we move down a group the distance from the nucleus increases and the attraction of the electrons for the nucleus decreases

Which of the following elements has the lowest ionization energy? B, Al, C and Si

Probably Al. Its valence electrons have a higher principle quantum number, and are therefore further away from the nucleus, than C or B. Furthermore, its nucleus would have a lower effective nuclear charge than Si.

1996 Michael Blaber