An element’s ability to deliver its outermost electrons to form positive ions is manifested in the amount of energy supplied to its atoms sufficiently to remove the electrons from it. This energy is known as the ionization energy. Simply, the Ionisation Energy is the energy supplied to an atom or molecule to knock out its most loosely bound valence shell electron for form a posetive ion. Its unit is either electron-volt eV or kJ / mol and is measured in an electrical discharge tube where a fast-moving electron collides with a gas element to eject one of its electrons. The lesser Ionisation Energy, the better ability to form cations.
This can be explained with Bohr atomic model, in that it considers a hydrogen-like atom in which, due to the columbic force of attraction, an electron revolves around a positively charged nucleus and the electron can have only fixed or quantized energy levels. A Bohr model electron’s energy is quantizied and given as below:
Where, Z is the atomic number and n is the quantum number where n is an integer. Ionization energy for a hydrogen atom is 13.6eV.
The Ionisation Energy (eV) is the energy needed to take the electron to infinity from n= 1 (ground or most stable state). The Ionisation Energy can therefore be written at 0 (eV) reference at infinity.
The concept of Ionisation Energy supports the evidence Bohr atomic model that the electron can rotate around the nucleus in a fixed or discrete energy level or shell represented by the quantum number ‘n’. ‘ As the first electron leaves the vicinity of the positive nucleus, there is a need for greater energy to remove the next loosely bound electron as the electrostatic force of attraction increases, i.e. the second Ionisation Energy is greater than the first.
For ex. the first ionization energy of Sodium (Na) is:
And the second Ionisation Energy is:
Hence, IE2 > IE1 (eV). This is true if there are K number of ionisations, then IE1 < IE2 < IE3……….< IEk
Metals have low energy of ionization. Low ionization Energy improved element conductivity. For example, the Silver conductivity (Ag, atomic number Z = 47) is 6.30 × 107 s/m and its Ionisation Energy is 7.575 eV and for Copper (Cu, Z = 29) is 5.76 × 107 s/m and its Ionisation Energy is 7.726eV. The low ionization energy in conductors causes the electrons to move through the positively charged lattice, forming a cloud of electrons.
Factors Affecting Ionisation Energy
The general trend in the periodic table is that the Ionization Energy grows from left to right and decreases from top to bottom. Thus the factors affecting the ionization energy can be summarized below:
Atom size: The energy of ionization decreases with the size of the atom as the atomic radius increases the columbic force of attraction between the nucleus and the outermost electron decreases and vice versa.
Shielding effect: The presence of electrons shielding of inner shell or weakening the attraction of columbic force between the nucleus and the valence shell electrons. Therefore, the energy of ionization decreases. More shielding means the number of internal electrons. But Ionisation Energy of gold greater than silver , even if the size of gold is is more than silver. This is due to the weak shielding provided by the inner d and f orbitals in case of gold.
Nuclear charge: The higher the nuclear charge, the more difficult it will be to ionize the atom due to the increased force of attraction between nucleus and electrons.
Electronic configuration: The more stable the atom’s electronic configuration, the more difficult it is for an electron to be withdrawn, hence more ionization energy.
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