Ionisation enthalpy of elements is defined as the amount of energy required by an isolated gaseous atom to lose an electron in its ground state. Losing of electrons results in cation formation.
The first ionization energy of element A is defined as the energy required by an atom to form A+ ions. The unit of ionisation energy is given as KJ mol-1.
A (g) → A+ (g) + e–
In the same way second ionisation energy is described as the energy needed to remove the second electron from its valence shell. It can be explained by the equation given below:
A+ (g) → A2+ (g) + e–
Whenever an electron is removed from an atom, a specific amount of energy is required to remove it, hence the ionisation enthalpies of chemical elements are always positive. The second most outer electron will be more attracted by the nucleus than the first outer electron. Therefore the second ionization energy will be greater than the first ionisation energy. In the same way, third ionisation enthalpy will be greater than the second one.
The ionisation energy depends on two factors:
- The force of attraction between electrons and the nucleus.
- The force of repulsion between electrons.
The effective nuclear charge felt by the outermost electrons will be less than the actual nuclear charge. This is because the inner electrons will shield the outermost electrons by hindering the path of nuclear charge. This effect is known as the shielding effect. For example: In Na, the 3s1 electrons will be shielded by its core electrons (1s2, 2s2 and 2p6). Generally, shielding effect is more prominent when the inner orbitals are completely filled.
General periodic trends:
- In a group while moving from top to bottom it decreases.
- It increases from left to right across a period.
Trends in ionisation enthalpy in a group: The first ionisation enthalpy of elements decreases as we move down in a group. While moving down in a group, atomic number increases and the number of shells also increases. Outermost electrons are far away from the nucleus and thus can be removed easily. The second factor that decreases the ionisation energy is the shielding effect due to an increasing number of shells as we move down a group.
Trends in ionisation enthalpy across a period: As we move from left to right across a period, ionisation energy of elements increases. This is due to the decrease in size of atoms across a period. The valence electrons get closer to the nucleus of an atom as we move from left to right due to increased nuclear charge. The force of attraction between the nucleus and the electrons increases and hence more energy is required to remove an electron from the valence shell.
This was just a brief layout about the ionisation energy and its trend in the periodic table. To know more about properties of elements (such as ionization enthalpy) and their trends in the periodic table, please download BYJU’S – the learning app.