Question

Element First ionization energy (KJ/mol)

Lithium	520
Sodium	496
Rubidium	403
Cesium	376

According to the above table, which is most likely to be the first ionization energy for potassium?
A. 536 kJ/mol
B. 504 kJ/mol
C. 419 kJ/mol
D.391 kJ/mol
E.358 kJ/mol

Answer 1

In the periodic table, the ionization energy of the element decreases when the atomic size of the elements increases. This is due to the shielding effect. As the atomic size increases the force of attraction between the nucleus and the valence electrons present in the outermost orbital decreases, because of the presence of inner shells electrons.

Complete step by step answer:
The measure of the ability of electrons (attractive forces) to be held in its place or measure of the struggle for removing the electrons from the atom is termed as ionization energy.
Lithium is the first element present in group 1. Its first ionization energy is 520 kJ/mol.
Sodium is the second element present just below lithium. Its first ionization energy is 496 kJ/mol.
Potassium is the third element present below sodium.
Rubidium is the fourth element present below sodium. Its first ionization energy is 403 kJ/mol.
Cesium is the fifth element present below the rubidium. Its first ionization energy is 376 kJ/mol.
As potassium (K) is present between the elements sodium and rubidium, its first ionization energy will be between 496 kJ/mol and 403 kJ/mol.
Thus, the first ionization energy of potassium (K) is 419 kJ/mol.

Thus, the correct option is option (C).

Note:
As we move down in the group the atomic size increases due to the increases in the number of shells. When the ionization energy of an element is high, then the removal of the electron will become more difficult.