Electronic Energy Levels of Atoms - Introduction

Types of bonds formed by different atoms and their characteristic properties depend on ionization energy. The term ionization highlights the formation of ions. So let us first understand briefly what ions are. When a neutral atom loses or gains an electron, a positive or negative charged specie is formed which is termed as ion. When an atom loses an electron, a positive charged ion is formed which is known as cation. When it gains an electron, a negative charged specie is formed known as anion.

An example of the formation of a cation is given below. Sodium loses one electron to form a cation. Fluorine can gain one electron to complete its’ valence shell. This results in forming a F-.

There is a force of attraction between electron and nucleus. Therefore, in order to remove the electron from the influence of nucleus, energy must be provided. What is ionization energy? It is the amount of energy required to remove an electron from the influence of the nucleus of an isolated atom in gaseous state. Ionization energies are expressed in electron volt per atom or in kj/mol of atoms.

Ionization energy of carbon is shown here.Let us try to remove an electron from the C and see how its’ ionization energy varies.The removal of one electron from a neutral C atom takes 1086.2 kj/mol.

The removal of another electron from C⁺ takes 2352 kj/mol. This is almost double the energy needed to remove an electron from a neutral carbon atom.

So what could be the reason behind the rise in these ionization energies although we are only removing 1 electron each time? To answer this question let’s first assign the removal of 1st electron a term known as 1st ionization energy. Let’s assign the removal of 2nd electron the term 2nd ionization energy and removal of 3rd electron 3rd ionization energy.

We can say that the 3rd ionization energy is greater than the second. The second ionization is greater than the first. As the figure illustrates there are 6 electrons and 6 protons in a neutral carbon atom. Let’s see what happens when we remove electrons from it.

When we remove 1 electron from this neutral atom, there are 6 protons and 5 electrons remaining. After removal of 1 more electron from the C⁺ we now have 6 protons and 4 electrons remaining.

What happens when we remove another electron from C⁺² ion? It will give us a C⁺³ with 6 protons and 3 electrons. We can see that the number of protons are constant at a value of 6. However, the number of electrons are decreasing.

1st , 2nd and 3rd ionization energies of oxygen and phosphorous are shown here. How can we explain this behavior? We know that a greater number of positively charged protons will attract a less number of negatively charged electrons more strongly. Hence it would be much more difficult to remove an electron highly attracted to the nucleus. This means more energy will be required to do so.

Another factor that causes the rise in successive ionization energies is that there exists forces of repulsion between electrons. After removal of an electron, this force of repulsion is reduced. As a result, due to less repulsion between electrons, they are more strongly attracted to the nucleus.Distance between valence electrons of C⁺ is decreased. Therefore the repulsion between electrons is also decreased. This means due to lack of repulsion, more energy will be required to remove an electron from a C⁺ as compared to neutral Carbon atom.