ANOMALIES OF IONIZATION ENERGY IN THE PERIODIC TABLE

Ionization energy which is also known as ionization potential is the energy needed or required to remove an electron from a gaseous atom in its ground state.it is measured in electron volt (eV) or in joules i.e.

    Atom + IP (energy)   →  unipositive ion  +  electron

For example

    Na(g)  + 5.138eV  →     Na⁺_(g)   +   e-

Ionization energy is inversely proportional to the atomic radius as:

I.P  ⋉  1⧸A.R        Where A.R is atomic radius

In this section we are only concerned about the period in the periodic table, because there is no anomaly in terms of ionization energy down the group.
The basic factors that affects ionization energy of an atom includes;

• The distance of the outermost electron from the nucleus
• The size of the positive nuclear charge; and 
• the screening effect of the inner electron

             VARIATION OF IONIZATION ENERGY ACROSS THE PERIOD

variation in ionization energy with atomic number for the first twenty elements

It has been observed that atomic radius affects ionization energy of atoms in the periodic table.

Ionization energy increases across a period due to decrease in atomic radius in going across the period. This decrease in atomic radius is as a result of addition of electron to the same shell as you go across the period, this addition increases the size of the nuclear charge and thereby increases the force of attraction between the nucleus and outermost electron, which will then cause a decrease in the size of the atom.

therefore, a very large amount of energy is needed to remove electron from elements as you go across the period. However, there are some anomalies. The explanation of the observed anomalies in the trend of ionization energy within the period are as follow; 

• Berylium and Boron 

Berylium



           

 Boron

If you look more closely at the figure above, we notice that there is a decrease in ionization energy as you go from berylium to boron. originally, the ionization energy of boron is suppose to be higher than berylium, but reverse is the case. Observing the electronic configuration of berylium and boron above, you will observe that all the subshell in berylium are filled, but the 2p subshell of boron contains only one electron. just as filled electron shell are associated with extra stability,there is also some extra energetic stability associated with completely filled subshel. This means that the electronic configuration of berylium is more stable than that of boron and thereby has a higher ionization energy.

• Nitrogen and Oxygen

Nitrogen

                                                 

 Oxygen

      

a similar case also occur as we move from nitrogen to oxygen. In nitrogen, the p-orbital are half filled and this conferred more stability on the electrons, hence more energy (i.e. high ionization energy) will be required to remove one of them. hence, the ionization energy of nitrogen is higher than that of oxygen as shown in the electronic configuration above.

Other anomalies are between; 

• Magnesium and Aluminium
• phosphorus and sulphur

NOTE: Ionization energy decreases down the group due to increase in atomic radius. because as you go down the group more shells are added and the distance of the nucleus and the outer electron increases. This increase reduces the force of attraction, so little amount of energy is needed to remove one of the electrons.