Exothermic reactions Reactions which are accompanied by evolution of heat are called exothermic reactions. The quantity of heat produced is shown either along with the products with a ' + ' sign or in terms if $\Delta H$ with a ' - ' sign. e.g.,

$$\begin{aligned} \mathrm{C}(\mathrm{s})+\mathrm{O}_2(g) & \longrightarrow \mathrm{CO}_2(g)+393.5 \mathrm{~kJ} \\ \mathrm{H}_2(g)+\frac{1}{2} \mathrm{O}_2(g) & \longrightarrow \mathrm{H}_2 \mathrm{O}(l) ; \Delta H=-285.8 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}$$

Endothermic reactions Reactions which proceed with absorption of heat are called endothermic reactions. The quantity of heat absorbed is shown either along with the products with a ' - ' sign or in terms of $\Delta H$ with a ' + ' sign e.g.,

$$\begin{gathered} \mathrm{C}(\mathrm{s})+\mathrm{H}_2 \mathrm{O}(\mathrm{g}) \longrightarrow \mathrm{CO}(\mathrm{g})+\mathrm{H}_2(g)-131.4 \mathrm{~kJ} \\ \mathrm{~N}_2(g)+3 \mathrm{H}_2(g) \longrightarrow 2 \mathrm{NH}_3(g) ; \Delta H=+92.4 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{gathered}$$

The placing of elements are as

(i) Ionisation enthalpy of nitrogen $\left({ }_7 \mathrm{~N}=1 s^2, 2 s^2, 2 p^3\right)$ is greater than oxygen $\left({ }_8 \mathrm{O}=1 s^2\right.$, $2 s^2, 2 p^4$ ) due to extra stable exactly half-filled $2 p$-orbitals. Similarly, ionisation enthalpy of phosphorus $\left({ }_{15} \mathrm{P}=1 s^2, 2 s^2, 2 p^6, 3 s^2, 3 p^3\right)$ is greater than sulphur $\left({ }_{16} \mathrm{~S}=1 s^2, 2 s^2, 2 p^6, 3 s^2, 3 p^4\right)$.

On moving down the group, ionisation enthalpy decreases with increasing atomic size. So, the order is

$$\mathrm{S}<\mathrm{P}<\mathrm{O}<\mathrm{N} \rightarrow \text { First ionisation enthalpy increases. }$$

(ii) Non-metallic character across a period (left to right) increases but on moving down the group it decreases. So, the order is

$\mathrm{P}<\mathrm{S}<\mathrm{N}<\mathrm{O} \rightarrow$ Non-metallic character increases.

There is deviation of ionisation enthalpy of some elements from the general trend as shown in figure. The first ionisation enthalpy of B is lower than that of Be and in case of nitrogen, the first ionisation enthalpy is higher than that of $O$. (Also, refer to Q. 27)

(a) Across the period, the nuclear charge increases and the atomic radius decreases. As a result, the tendency of the atom of an element to attract the shared pair of electrons towards itself increases and hence the electronegativity of the element increases. e.g., electronegativity of the elements of the 2nd period increases regularly from left to right as follows $\mathrm{Li}(1.0), \mathrm{Be}(1.5), \mathrm{B}(2.0), \mathrm{C}(2.5), \mathrm{N}(3.0), \mathrm{O}(3.5)$ and .

(b) The ionisation enthalpy decreases regularly as we move from top to bottom, as explained below

(i) On moving down a group from top to bottom, the atomic size increases gradually due to the addition of a new principal energy shell at each succeding element. As a result, the distance between the nucleus and the valence shell increases. In other words, the force of attraction of the nucleus for the valence electrons decreases and hence the ionisation enthalpy should decrease.

(ii) With the addition of new shells, the number of inner shell which shield the valence electrons from the nucleus increases. In other words, the shielding effect or the screening effect increases.

As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionisation enthalpy should decrease.

(iii) Further, in a group from top to bottom nuclear charge increases with increase in atomic number. As a result, the force of attraction of the nucleus for the valence electrons increases and hence the ionisation enthalpy should increase.

The combined effect of the increase in atomic size and screening effect more than compensate the effect of the increased nuclear charge. Consequently, the valence electrons become less and less firmly held by the nucleus and hence the ionisation enthalpy gradually decreases as we move down the group.

As we move from left to right in a period, the number of valence electrons increases by one at each succeeding element but the number of shells remains same. Due to this effective nuclear charge increases. More is the effective nuclear charge, more is the attraction between nuclei and electron.

Hence, the tendency of the element to lose electrons decreases, this results in decrease in metallic character. Furthermore, the tendency of an element to gain electrons increases with increase in effective nuclear charge, so non-metallic character increases on moving from left to right in a period.