Question

Ionisation energy of hydrogen is
A. Equal to that of chlorine
B. Lesser than that of chlorine
C. Slightly higher than that of chlorine
D. Much higher than that of chlorine

Answer 1

Hint: The minimum energy required to remove an electron from a gaseous atom is called ionisation energy. It is an endothermic process as this energy is absorbed by an atom to remove an electron. The concept of ionisation energy is used to determine strength and reactivity of chemical bonds. Units in which ionisation energy is measured in KJ/mol or electron volts.

Complete Step by Step Answer:
As we know that ionisation energy is the minimum energy required to remove an electron from its outermost shell, thus if the ionisation energy is very high it is difficult to remove the electron.

Electrons revolving in an orbit closer to the nucleus are strongly attracted toward the nucleus and thus experiences a very high effective nuclear charge that makes the removal of electrons difficult. Electrons revolving in an orbit farther away from the nucleus experiences less effective nuclear charge and are easier to remove from the shell. In case paired electrons, less ionisation energy is required as they revolve in the same orbit they experience electron-electron repulsion as well.

Hydrogen with atomic number 1 has only electrons in its only shell thus, very difficult to remove. Whereas chlorine with atomic number 17 also has 1 electron in its outermost shell but it is easier to remove it as the orbital is farther away from the nucleus and experiences a very less effective nuclear charge.

We can calculate ionisation energy using the following formula;
$E_{n}\Rightarrow-2.18×\left(\frac{Z^2}{n^2}\right)$; where, Z denotes the atomic number of an atom and n denotes the nth orbit. Using this formula, the ionisation energy of hydrogen is 1312 KJ/mol whereas that of chlorine is 1215 KJ/mol.
Hence, the correct option is C: Slightly higher than that of chlorine.

Note: First ionisation energy is the energy to remove the first electron from the first shell and in the same way the second ionisation energy is the energy required to remove the second electron. As the distance between orbital and nucleus is decreasing, its ionisation energy increases making the removal of electrons difficult. This third IE is higher than the second IE, and the second IE is higher than the first
IE.