The first ionization enthalpy values (in kJ/mol) of group 13 elements are:B= 801Al= 577Ga= 579In=558Tl=589How would you explain this deviation from the general trend?

The first ionization enthalpy is a measure of the energy required to remove one electron from a neutral atom in the gas phase. In general, the ionization enthalpy tends to increase across a period from left to right in the periodic table due to two main factors: increasing effective nuclear charge and decreasing atomic radius.

However, the group 13 elements (boron, aluminum, gallium, indium, and thallium) show a deviation from this general trend. This deviation can be explained by considering the electronic configurations and the stability of different electronic subshells.

When we examine the electronic configurations of these elements, we observe that they all have a completely filled or half-filled p orbital in their valence shell. For example, boron has a configuration of 1s²2s²2p¹, aluminum has 1s²2s²2p⁶3s²3p¹, gallium has 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p¹, indium has 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p¹, and thallium has 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p¹.

The half-filled or completely filled p orbitals in these elements provide extra stability, known as the "extra stability of half-filled and completely filled subshells." The presence of this extra stability makes it easier to remove an electron from the p orbital, resulting in a lower ionization enthalpy compared to what would be expected based solely on the increasing effective nuclear charge and decreasing atomic radius trends.

This deviation from the general trend is most prominent in boron, where the first ionization enthalpy is significantly higher compared to the other group 13 elements. This is because boron does not have a completely filled or half-filled p orbital, making the removal of an electron from its 2p orbital less favorable.

In summary, the deviation from the general trend in the first ionization enthalpy of group 13 elements can be explained by the extra stability associated with half-filled and completely filled subshells in the p orbital. This extra stability lowers the ionization enthalpy compared to the trend expected solely based on increasing effective nuclear charge and decreasing atomic radius.