Why Does Ionization Energy Increases Across A Period: Detailed Explanations

This article answers the question why does ionization energy increase across a period? We will study basics of periodic table first.

Then carry our discussion to trends followed by ionisation energy of different elements in the periodic table. It is important to know the meaning of ionisation energy first, so we shall discuss about ionisation energy and then continue with our discussion.

What is ionisation energy?

If we want to remove an electron from an atom, it is logical to remove the most loosely packed electron of that atom.

Ionisation energy is the name given to this energy that is required to remove the loosely packed electron. Without this energy, we won’t be able to remove the electron from the influence of nuclear force of attraction.

Image credits: Double sharp, First Ionization Energy blocks, CC BY-SA 4.0

What is periodic table?

Periodic table is a table which represents different chemical elements found on Earth. These elements are given a specific atomic number and are then arranged in ascending order of atomic number.

Periodic table has many divisions in it. These divisions are called as blocks. With all the rows being called as period and all the columns being called as groups. We shall study about the trends of ionisation energy across both periods and groups.

What is atomic number?

Atomic number is considered as the fingerprint of the chemical element. It is simply the total number of protons present inside the atom.

Number of protons for every chemical element is unique hence it is considered as the fingerprint of that particular chemical element. The arrangement of chemical elements in the periodic table is done in increasing values of atomic numbers.

What is a period?

A period is simply a row in periodic table. Horizontal arrangement of chemical elements in periodic table is termed as period.

The atomic number increases by 1 as we move ahead in a period. The last element of any period is a noble gas. Noble gases have no free electron revolving around the nucleus. Noble gases are considered as the most stable elements in the periodic table.

What is a group?

As discussed above, rows of periodic table are called as period. Similarly, the columns are called as groups.

Here as we move down the group, the atomic number increases but not by 1. These groups divide metals from non metals and noble gases and alkali metals. Even the groups follow a trend for different properties. We shall discuss about them in later sections of this article.

Periodic trends

Different properties follow different trends as we move from left to right in a period. There are certain exceptions as well which don’t fit inside the trend.

We shall discuss about different trends in a period in the section given below-

• Atomic radius– The atomic radius or the size of an atom generally decreases as we move across a period from left to right. This is due to the fact that the magnitude of nuclear charge is same but number of electrons keep increasing in the shell.
• Ionsiation energy- Ionisation energy depends on the atomic radius. As the radius decreases acrosss a period, the ionisation energy keeps on increasing as we move across a period. It is maximum for nobel gases.
• Electorn affinity– This property is exactly opposite to ionisation energy. Energy is released when an electron is stuffed into an atom meaning it is added to an atom. Electron affinity will increase while moving towards right in the periodic table.
• Electronegativity – This property increases its value when we move towards right in the period. Metallicity- The metals are situated at the left hand side of the periodic table and non metals are situated at the right hand side of the periodic table. We can conclude that the metallicity value decreases when we move towards right in a period.

Group trends

A column of periodic table is called as group. The properties exhibited by these elements follow different trends along the group. We shall discuss about these trends in section given below-

• Atomic radius- As we move down the group, an extra shell is added in the elements. We can say that due to addition of an extra shell, the atomic radius increases as we go down along the group.
• Ionisation energy – The minimum value of energy required to pull out an electron from the influence of nucleus is called as ionisation energy. As the atomic radius increases as we move down the group, the influence of nucleus decreases on the electron and hence it becomes easier to remove electron. So we can conclude that the value of ionisation energy decreases as we go down along the group.
• Electron affinity– Its meaning is right opposite to that of ionisation energy. Atom will release energy if an electron is plucked from it or stuffed into it. Similar to the trend of ionisation energy, electron affinity decreases while moving towards bottom in a group.
• Electronegativity– While going towards the bottom of group, electronegativity keeps decreasing.
• Metallicity- Metallicity can becompared to the tendency of an atom to lose electron. Metallicity increases while going towards the bottom of group in periodic table.

Why does ionization energy increase across a period?

Coming to the most important question in this article that is why does ionization energy increase across a period? The answer is already discussed n above sections, but we shall discuss it again.

Atomic radius is a deciding factor behind the energy required to remove the loosely packed electron. This is because smaller the radius, closer the electron will be to the nucleus. Hence greater will be the attractive force of nucleus towards electron. Hence ionisation energies of Hydrogen is low and keeps on increasing as we move towards right in the period. Only Oxygen has an exception because it forms electron pairs, due to repulsive forces, the electron is easily removed.

Why is second ionisation energy greater than first ionisation energy?

The name itself suggests that first ionisation energy is related to the first electron. It is the energy required to pluck out the first electron from the atom.

Similarly the second ionisation energy is used to remove second electron from the already electron deficit atom. The influence of nucleus on electron increases as and when we dig deeper into the atom. Hence it becomes difficult to remove that electron from the influence of nucleus thus justifying the fact that second ionisation energy is more than first ionisation energy.