Ionization Energy Trend

At the end of the article, you will able to describe Ionization energy trend. We know that the cations or positive ions are formed when the neutral atoms (generally metal atoms) lose electrons. They can do so only in the gaseous state since they are isolated. Now energy is needed to overcome the force of attraction between the nucleus and electrons so that the latter (electrons) may be released. This is known as ionization energy.

It may be defined as :

The minimum amount of energy which is needed to remove the most loosely bound electron from a neutral isolated gaseous atom in its ground state to form a cation.

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Ionization Energy Trend

Ionization energy trend varies with the periods and groups.  Along with a period ionization enthalpies (energy) are expected to increase in moving across from left to the right, because the nuclear charge increases and the atomic size decreases. While the ionization enthalpies of the elements decrease from the top to the bottom of any group (size of atom increases).Ionization Energy Trend

Energy Trend in Group

The decrease in ionization energy trend down any group is because of the following factors:

  • There is an increase in the number of the main energy shells (n) in moving from one element to other.
  • There is also an increase in the magnitude of the screening effect due to the gradual increase in the number of inner electrons.
  • Although the nuclear charge also increases down the group which is likely to result in increased ionization enthalpy but its effect is less pronounced than the two factors listed above.

The net result is the decrease in ionization enthalpies in a group in the periodic table.

Energy Trend in Period

Generally, on moving from left to right along a period, the energy trend increases with increase in atomic number but with certain exceptions. The order breaks, where an electron of a slightly higher energy is removed from the same shell.

For example,

Element Li Be B C N O F Ne
I.E. 500 900 800 1086 1403 1314 1681 2081

Thus the break occurs at ‘B’ and ‘O’. It is all because of Higher stability of half and fully filled orbitals.

Transition elements

Increase in nuclear charge helps in holding the valence electrons in ns orbitals more firmly while the shielding effect of (n-1) d-electrons on ns electron acts in opposite direction. The net result is that there is a slight change in Ionization energy.


This is all about the basics of Ionization energy trend.