Ionization energy: Definition, units, examples trend in periodic table

Question: What is ionization energy? Give its units. Also explain its trend with reason in periodic table.

Answer

Ionization Energy

Ionization energy may be defined as the minimum amount of energy required to remove the valence electron from an isolated gaseous atom.

M(g) + ionization energy ⎯⎯→ M(g) + + e-

Ionization energy indicates the attraction between nucleus and outermost electron. A higher ionization energy means stronger attraction between the nucleus and the outermost electron. While a lower ionization energy indicates a weaker force of attraction between the nucleus and the outermost electron.

Units Units of ionization energy are kJ/mole-1 or kJ/mole.

Trends in Periodic Table

In period: Generally, ionization energy of elements increases from left to right within the period. Reason is that atomic size decreases and nuclear charge (atomic number) gradually increases by one unit in each element on moving left to right within the period. At the same time, more valence electron adds to the same shell which gradually increases the attractive forces between nucleus and valence electrons. As a result, ionization energy increases from left to right in the period.

For example, Be has higher ionization energy than Li. This is because Be has smaller atomic size and greater nuclear charge than lithium.

In group: Ionization energy decreases from top to bottom within the group. This is because size of atom and shielding effect increase which results in decrease in attractive forces between nucleus and valence electrons. As a result, atomic size increases. For example, Li has higher ionization energy than Na. This is due to smaller size and lesser shielding effect in Li than Na.

Figure: Trend of Ionization energy in periodic table

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