[Explained] Why do Non Metals have High Ionization Energy?

Non Metals are those elements which are found on the right hand side of the periodic table. They have very high ionization energy values.

Non-metals have very high ionization values because they have a small size, experience high nuclear charge, poor shielding effect, lower penetration effect and they generally have a very stable electron configuration.

On the other hand metals have low ionization energy.

We shall discuss each of them in detail and understand why non-metals are affected by them. But first let us acquaint ourselves with the general features of non-metals.

Features of Non-Metals

They are electronegative in nature which means that they accept electrons to attain the noble gas electron configuration. This makes them reactive.

They have an amorphic structure and they are present in the solid state or the gaseous state at room temperature.

They have a low density and are non-lustrous in appearance. Most of the non-metals found in nature are soft with the exception of diamond, the hardest substance known to man.

Non-metals are non-malleable, non ductile and non-sonorous.

They do not conduct heat and electricity well and have low melting and boiling points.

They have more valence electrons when compared to metals. Metals generally have 1 to 3 valence electrons whereas non-metals have 4-8 electrons in the valence shell.

They react with oxygen to form acidic oxides and they generally don't react with acids.

Examples of non-metals are halogens, phosphorus, sulfur, etc.

Now let us understand what ionization energy is.

Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom to form a gaseous ion.

It is expressed in the unit of kJ mol^-1

Non-Metals have high ionization energy due to the following factors which govern ionization energy:

• Size of the atom
• Nuclear Charge
• Screening Effect also known as Shielding effect
• Penetration Effect
• Electronic Configuration

We will discuss all of these factors with respect to  non-metals and the reason behind them having such low ionization energy.

Size of the Atom

Size of non-metals are very small compared to metals. In smaller atoms of non-metals, the force of attraction experienced by the electrons is very high, this is because the valence electrons of non-metals are very close to the nucleus.

The attractive force between the electrons and the nucleus is inversely proportional to the distance between them. Therefore closer the electron from the nucleus, more would be the force of attraction on it and more difficult would it be to remove it. 

Nuclear Charge

Owing to the small size of the atoms of non-metals the nuclear charge experienced by the valence electrons is very high.

And because of this when going from left to right across a period, i.e. going from metal elements to non metal elements in the periodic table, the number of shells remain the same but nuclear charge increases with each succeeding element. Thus, the electrons in the valence shell experience a greater pull towards the nucleus.

Screening effect/Shielding effect of the electrons

The electrons present between the nucleus and the valence electron shield the valence electron from the nucleus.

This in turn reduces the pull exerted by the nucleus on valence electrons.

The inner electrons of the non-metallic atoms provide very poor shielding effect to the valence electrons. This also contributes to the ionization energy being very high in non-metals.

Penetration Effect of Electrons

In a multielectron 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 the nucleus and experience a greater pull by it.

For the same value of n (principle quantum number) 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, which is in turn greater than that of a f electron. 

Electron Configuration of the element

Non-metals mostly have the most stable electron configuration of all the elements present in the periodic table.

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 np_x¹ np_y¹ np_z¹. 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.

Learn: Trends of Ionization Energy in the Periodic Table

Also Read: Why does Ionization Energy decrease down the group?