[Explained] 5 Factors affecting Ionization Energy and its Trend

Factors affecting Ionization Energy and its Trend

The 5 factors affecting Ionization energy are:

  1. Size of the atom
  2. Nuclear Charge
  3. Screening effect (also known as Shielding effect)
  4. Penetration effect of electrons
  5. Electron configuration

 What is Ionization Energy?

What is Ionization Energy?

Ionization energy is also known as ionisation energy or ionisation enthalpy. An atom is composed of a positively charged nucleus and negatively charged electrons revolving around it. The nucleus exerts an appreciable force of attraction on the planetary electrons.

Therefore, if we wish to remove an electron from the atom, we will have to supply energy to overcome the force of attraction exerted by the nucleus on the electron. This energy is known as ionization enthalpy or ionization energy and may be defined as follows:

The amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a gaseous ion is called ionization enthalpy or ionization energy of that atom.

In the next section we shall discuss the five factors affecting ionization energy in detail.

The 5 factors affecting Ionization Energy

1. Size of the Atom 

In this section we shall see, how size of the atom affects ionization energy?

The ionization energy of an atom is governed by its size. By size we mean the atomic radii of the atom. In a large atom the valence electrons are far from the nucleus and experience lesser force of attraction as compared to a smaller atom in which valence electrons are nearer to the nucleus and experience greater force of attraction.

The attractive force between the electrons and the nucleus is inversely proportional to the distance between them. Consequently farther away the electron from the nucleus, less would be the force of attraction on it and more easily would it be to remove it. Hence, the ionization energy decreases with increase in atomic size.

We can see the trend in ionization energy when we look into elements across a period. 

When we move across a period the the size of the atoms of the elements tend to decrease. To understand this we shall take the elements of the second row as example. Refer the table given below:

The table below is compiled using the data calculated by Clementi and others. The atomic size is very small, therefore we use a unit known as picometer. Picometer is one-twelfth of a meter. The atomic radius that small!

Trend of Atomic Size in Second Row of Elements in the Periodic Table
Group 1 2 13 14 15 16 17 18
Size in pm
167 pm
112 pm
87 pm
67 pm
56 pm
48 pm
42 pm
32 pm

 From the table below it is apparent that the first ionization energy also gradually decreases as we move across the second row of the periodic table. The only exception we notice is that of Beryllium. The reason for this can be explained using the electron configuration of Beryllium.


Trend of Ionization Energies in Second Row of Elements in the Periodic Table
Group 1 2 13 14 15 16 17 18
Ionization Energy
in kJ/mol

2. Nuclear charge 

In this section we shall see, how nuclear charge affects ionization energy?

How does nuclear charge affect ionization energy?

With the increase in the magnitude of nuclear charge, the valence shell electrons experience greater pull by the nucleus. Therefore, it becomes more and more difficult to remove the valence shell electron. Hence, ionization energy increases with increase in nuclear charge.

3. Screening or Shielding Effect 

In this section we shall see, how nuclear charge affects ionization energy?

How does screening effect or shielding effect affect ionization energy?

In a multi-electron atom, the electrons present between the nucleus and the valence shell electron ( the inner electrons) shield the valence electron present from the nucleus. This decreases the effect of the nucleus on the valence electron, due to the presence of inner electrons, the nucleus exerts less force of attraction on the valence electron.

This effect is called screening effect or shielding effect and decreases the nuclear charge felt by the valence electron. The actual charge felt by the valence electron is known as effective nuclear charge (Z*) and is given by

Z* = Z - S

where Z is the actual nuclear charge and S is a constant known as screening or shielding constant. The screening effect depends upon the number of inner electrons.

Greater the number of inner electrons, greater would be the screening effect and consequently less would be the effective nuclear charge experienced by the valence electron. Hence, if other factors remain the same, the ionization energy decreases with an increase in the number of inner electrons.

4. Penetration Effect of the Nucleus 

In this section we shall see, how penetration effect affects ionization energy?
How does penetration effect affect Ionization energy?

In a multi electron atom, the electron clouds of various electrons do not maintain distinct boundaries. Instead, the electron cloud of one electron  penetrates into the electron cloud of some other inner electron.

This effect is known as the penetration effect. Due to this effect, electrons shift towards nucleus and experience a greater pull by it.

For a given value of n (principal quantum number) the probability of finding an electron near the nucleus decreases as the value of l ( azimuthal quantum number) increases. Since the value of l decides the nature of sub shell, the probability of finding electron near the nucleus follows the following order

s > p > d > f

This means that the s electron can penetrate better than a p electron which in turn penetrates better than a d electron. f electrons possess the least tendency to penetrate.

When the penetration power of an electron is more, it will be closer to the nucleus and more energy will be required to remove it. Thus for the same value of n, the ionization energy of s electron is greater than that of a  p electron which in turn is greater than that of a d electron, and so on. Hence for the same shell, the ionization energy follows the order


5. Electron Configuration 

In this section we shall see, how electron configuration affects ionization energy?

How does electron configuration affect ionization energy?

The electronic configuration of an atom may also affect the value of its ionization energy.

Half filled and completely filled shells are found to possess extra stability. The atoms having completely filled  shells are said to possess stable electronic configuration.

The atoms having filled shells also show an extra stability. Such atoms possess a tendency to lose the valence electron and consequently have higher values of ionization energies.

1. Helium possesses a stable electronic configuration 1s2. In it the K shell is completely filled . This is why the ionization energy of helium is much greater than that of hydrogen. Other gases also possess completely filled shells with stable configuration ns2 np6. Therefore the noble gases have very high values of ionization energy.

2. Elements like Be, Mg etc. possess electronic configuration of the type of ns2 in which orbitals are completely filled. Therefore these elements also have higher ionization energies. 

3. Elements like N, P, etc. possess configuration of the type ns2 npx1 npy1 npz1. In these electronic configurations the p orbitals belonging to the valence shell are exactly half filled. Therefore, these elements show higher stability and have relatively higher ionization energies.

Also Read: Trend of Ionization Energy in the Periodic Table

Frequently asked questions on Ionization energy:

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