Ionisation Energy
- What is Ionisation energy ?
- What is Ionisation Energy formula ?
- What is Ionisation energy of helium ?
- What is Ionisation Energy of hydrogen?
- What is Ionisation energy trend in periodic table ?
- what are the Factors influencing ionisation energy?
lonisation Energy (IE) of an element is defined as the amount of energy required to remove an electron from an isolated gaseous atom of that element resulting in
the formation of a positive ion.
Characteristics : (A) The energy required to remove the outermost electron from an atom is called first ionisation energy (IE), After removal of one electron, the atom changes into monovalent positive ion.
M(g) + IE, ——————–> M(g) + e-
(B) The minimum amount of energy required to remove an electron from monovalent positive ion of the element is known as second ionisation energy (IE),
M+(g) + IE2—————-> M2+(g) + e-
(C) The first, second etc. ionisation energies are collectively known as successive ionisation energies.
M2+(g) + IE, M3+(g) + e-
In general (IE)1 < (IE)2 < (IE)3 so on, because, as the number of electrons decreases, the attraction between the nucleus and the remaining electrons increases
considerably and hence subsequent ionization energies increase.
(D) Units : lonisation energy is expressed either in terms of electron volts per atom (eV/atom) or Kilojoules per mole of atoms (KJ mol – ‘) or K cal mol-1
1 eVlatom=96.49 KJ/mol = 23.06 Kcal/mol = 1.602 x 10-19 J/atom
Factors influencing ionisation energy:
(A) Size of the atom : lonisation energy decreases with increase in atomic size. As the distance between the outermost electrons and the nucleus increases, the force of attraction between the valence shell electrons and the nucleus decreases. As a result, outermost electrons are held less firmly and lesser amount of energy is required to knock them out. For example, ionisation energy decreases in a group from top to bottom with increase in atomic size.
(B) Nuclear charge : The ionisation energy increases with increase in the nuclear charge. This is due to the fact that with increase in the nuclear charge, the electrons of the outermost shell are more firmly held by the nucleus and thus greater amount of energy is required to pull out an electron from the atom. For example, ionization energy increases as we move from left to right along a period due to increase in nuclear charge.
(C) Shielding effect: The electrons in the inner shells act as a screen or shield between the nucleus and the electrons in the outermost shell. This is called shielding effect or screening effect. Larger the number effect and smaller the force of attraction and thus of electrons in the inner shells, greater is the screening lonisation energy decreases.
(D) Penetration effect of the electrons : The ionization electrons increases. It is a well known fact that the electrons of the s-orbital have the maximum probability
of being found near the nucleus and this probability goes on decreasing in case of p, d and f orbitals of the same energy level. Greater the penetration effect of electrons more firmly the electrons will be held by the nucleus and thus higher will be the ionization energy of the atom. For example, ionisation energy of aluminum is
comparatively less than magnesium as outermost electron is to be removed from p-orbital (having less penetration effect) in aluminum, whereas in magnesium it will be removed from s-orbital (having larger penetration effect) of the same energy level.
Note:
Within the same energy level, the penetration effect decreases in the order s > p >d>f