Boron has lesser ionization enthalpy than Beryllium, because:

The energy of an electron in the ground state of the hydrogen atom is $–2.18\times {10}^{-18}J$.  The ionization enthalpy of atomic hydrogen in terms of J ${\mathrm{mol}}^{-1}$ is:

1. 2.81 × 10⁶ J mol${}^{-1}$

2. 1.31 × 10⁶ J mol${}^{-1}$

3. 2.31 × 10⁶ J mol${}^{-1}$

4. 1.81 × 10⁶ J mol${}^{-1}$

The explanation for the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium would be :

The first ionization enthalpy values (in kJ mol^–1) of group 13 elements are :

The explanation for the deviation from the general trend can be -

1. Ga has lower ionization enthalpy than Al.

2. Ga has higher ionization enthalpy than Al.

3. Al has higher ionization enthalpy than Ga. 

4. Ga has a lesser valence electron than Al.

The values of the first ionization enthalpies for two isotopes would be:

The reactivity of alkali metals increases, whereas halogen decreases down the group, because:

1st (∆H₁) and 2nd  (∆H₂) Ionization Enthalpies (in kJ mol^–1) and the (∆_egH) Electron Gain Enthalpy (in kJ mol^–1) of a few elements are given below:

The most reactive metal is:

The incorrect statement about ionization enthalpy is: