A, B, C are three elements, B is an inert gas element other than helium; with this information, complete the table given below:
| Element | Atomic No. | No. of electrons in the valence shell | Group to which element belongs |
| A | Z - 1 | ||
| B | Z | ||
| C | Z+1 |
Also, explain the following:
(i) Electron gain enthalpy of element A has large negative value than that of C.
(ii) Ionisation enthalpy of element C is less than that of A.
(iii) Electron gain enthalpy of B is positive.
Let B be an inert gas element neon with atomic number Z=10. Therefore, atomic number of A=9 (halogen) and of C=11 (alkali metal).
The electronic configuration of elements are:
Groups:
Element A belongs to Group 17 (Halogens).
Element B belongs to Group 18 (Noble gases).
Element C belongs to Group 11 (Alkali metals).
(i) Electron gain enthalpy of element A has large negative value than that of C. The element A (fluorine) requires only one electron to have a noble gas configuration. Thus, electron gain enthalpy of A is much more than that of C (sodium).
(ii) Ionisation enthalpy of C is less than that of A. The atomic size of A is smaller (atomic radius = 71 pm) than that of C (atomic radius = 154 pm).Thus, it is easier to remove an electron from C as compared to A. Hence ionisation enthalpy of C is less than that of B.
(iii) Electron gain enthalpy of B is positive. B being a noble gas element has a symmetrical electronic configuration ( 1s-2s22p6). Added electron has to enter the next higher principal quantum level leading to the positive value of electron gain enthalpy.
