What is ionization energy and why second ionization energy is greater than first ionization energy?
What is ionization energy ?
The ionization energy of an element is defined as the amount
of energy required to remove an outer
most valence electron from an isolated gaseous atom of that element resulting
in the formation of a positive ion that is a cation. Ionization energy is denoted by ‘I’ .
What is first ionization energy ?
The amount of energy required to remove
the first outer most valence electron from an isolated gaseous atom resulting
in the formation of one unit
positive charge , is known as
first ionization energy.
Similarly , the energy required to remove second or third electron ,is known as second or third ionization energy respectively.
Unit of ionization energy .
The unit of ionization energy is KJ / mol or Kcal / mol or electron volt per atom .
Ionization energies are generally measured by spectroscopic
techniques.
Why second ionization energy is greater than first ionization energy ?
If one electron has been taken out of an atom, it becomes
increasingly difficult to remove the second and subsequent electrons from the
resulting positively charged ion on account of electrostatic attraction.
This is due to the fact that after the removal of an electron , the number of electrons decreases while the nuclear charge remains the same.
Consequently , the remaining electrons are held more tightly by the nucleus and it becomes difficult to remove the second electron.
Therefore, the second ionization energy is greater than the first ionization energy .
This is due to the fact that after the removal of an electron , the number of electrons decreases while the nuclear charge remains the same.
Consequently , the remaining electrons are held more tightly by the nucleus and it becomes difficult to remove the second electron.
Therefore, the second ionization energy is greater than the first ionization energy .
Similarly, the third ionization energy is greater than the second ionization energy and so on .
Thus, I1< I2< I3
For example, the first ionization energy of chlorine is 1251 kJ/mol where as second ionization energy of chlorine is 2300 kJ/mol.
Variation of ionization energy in the periodic table .
In general, the ionization energy decreases as we move from top to bottom along a group and ionization energy increases when we move from left to right along a period.
The variation of ionization energy with atomic number in the second period are shown below graphically.
Practice problem:
How ionization energy depends on atomic or ionic size ?
Why second ionization is higher than first ionization energy ?
What is shielding or screening effect ?
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