Defined as the amount of energy necessary to remove the outer most electron from an isolated neutral atom in the gaseous stat to infinity. This is also known as 1^st ionization energy. Abbreviation is E _i , it has units of kJ/mol.

As the effective nuclear charge increases along a row from left to right, electrons are held more tightly from left to right. Therefore, the energy required to remove an electron, i . e . ionization energy will increase from left to right in a row.

Similarly, in a group on moving from top to bottom effective nuclear charge decreases, hence, electron are held more loosely on moving from top to bottom, consequently, ionization energy decreases.

The 1^st ionization energy is always lower compared to 2 ^nd , 3 ^rd , so on, ionization energy. This is because after the removal of 1 st electron, the system becomes mono positive. Due to this reason electrons are more tightly hold by the nucleus. Therefore, more energy is required to remove electron further.

The 1^st ionization potential of N or P is more compared to O or S.

In N the outer electronic configuration is 2 s ² 2 p ³ . The three electrons in 2p orbital are all in same spin and provide a stable configuration. In O the outer electronic configuration is 2 s ² 2 s ⁴ . In this case one of the three p orbitals occupies two electrons that are anti parallel to each other. Because of having two electrons in a same sub shell a repulsion between electron occurs which lower the electron releasing or ionization energy.

(E) Electron affinity:

Electron affinity reflects the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a gaseous atom. This is also known as 1^st electron affinity.

Abbreviation is E _a , it has units of kJ/mol.