[Explained] Trend of Ionization Energy in the Periodic Table

 Ionization energies of elements vary in a regular fashion across a period as well as on going down a group. 

Trend of Ionization Energy for first 60 elements

The variation of the first ionization energies for elements with atomic numbers 1-60 in the figure above.

For more detail on Ionization Energy and Successive Ionization Energy.

The graph consists of several maxima and minima. The maxima are found at noble gases which possess stable electronic configuration. Due to very high ionization energies, these gases are almost inert and show extremely low chemical reactivity. The minima occur at alkali metals. Due to very low ionization energies these elements get easily ionized and are highly reactive.

From the graph it is apparent that the ionization energies of the elements how a periodic trend.

In general, they increase across a period and decrease in going down a group. Let us see the trends in more detail down below.

Trend of Ionization Energy Across a Period

In general, the ionization energy increases with increase in atomic number across a period.

The gradual increase in the values of ionization energy on moving from left to right in a period is due to the combined effect of the following factors:

The nuclear charge increases regularly with increase in atomic number.

Number of shells remain the same and the differentiating electrons enter into the same shell.

Size of the atom.

Due to the combined effect of the above factors, the attractive forces between the nucleus and the valence electron increase as one moves from left to right in a period. 

Thus, more and more energy is required to remove the electron from the atom and consequently, the ionization energy increases with increase in the atomic number in a period.

For more detail on the Factors that affect Ionization Energy

The variation in the values of ionization energies of elements across the second period is shown in the graph below. This figure shows that, in general, the values of ionization energies increase in going from Li to Ne (Lithium to Neon). 

Trend of Ionization Energy for second period elements

What are the exceptions to the periodic trends in ionization energy why do they occur?

However, there are certain exceptions also. For example, boron and oxygen have lower ionization energies as compared to Be and N respectively. These discrepancies can be explained as follows:

The electronic configuration of Be and B are

Be (Z = 4) = 1s² 2s²

B (Z=5) = 1s² 2s² 2p¹

Although Boron possesses a higher nuclear charge, yet its ionization energy is less as compared to that of beryllium. This may be attributed to the following reasons.

2s electrons are more penetrating than 2p electrons 

2s electrons in beryllium are not shielded by the inner electrons as efficiently as 2p electrons in boron.

Beryllium possesses a stable electronic configuration as all the orbitals are completely filled in it.

Due to these factors , beryllium possesses much higher ionization energy as compared to the value it should have otherwise. 

The value of ionization energy of boron is in accordance to the regular tend but due to a sudden increase in the value for beryllium, the ionization energy of boron appears to be low.

Similarly, a decrease in ionization energy going from N to O is due to the presence of half filled shell in nitrogen which makes its configuration relatively more stable and gives a boost to the ionization energy of nitrogen.

N (Z = 7) = 1s² 2s² 2p_x¹ 2p_y¹ 2p_z¹

O (Z = 8) = 1s²  2s² 2p_x² 2p_y¹ 2p_z¹

Due to higher ionization energy of nitrogen, the ionization energy of oxygen appears to get lowered.

The exceptionally very high value of ionization energy of neon is due to its stable electronic configuration.

Trend of Ionization Energy In A Group

The value of ionization energy decreases regularly with increase in atomic number in going down the group. The regular decrease in going down the group may be attributed to the following factors.

Number of shells and therefore the size of the atom increases with increase in atomic number in going down a group.

Due to increase in the number of inner electrons, the shielding effect increases and the valence electrons get more and more loosely bound to the nucleus as atomic number increases in going down a group.

Although nuclear charge increases in going down a group, the effect of increased nuclear charge gets cancelled by an increase in atomic size and increase in the shielding effect. Thus the, attractive forces between the nucleus and atomic number. 

Consequently, the ionization energies of the elements decrease regularly in going down a group. The decreasing trend in ionization energies in group 1 is shown in the graph below.

Trend of Ionization Energy in alkali metals

What best accounts for the periodic trends seen in ionization energy?

The trend in effective nuclear charge accounts for the increase in ionization energy across a period.

The trend in number of electrons accounts for the decrease in ionization energy down a group.

These statements summarize the answer to the question.

You may also want to check out some common questions based on Ionization Energy.

Read: [Solved] Arrange These Elements According To First Ionization Energy

           [Solved] Which Reaction Below Represents The First Ionization Of O(Oxygen)?

           [Solved] Write Equations That Show The Process For The First Two Ionization Energies.