The atomic number of the element from the following with lowest 1st ionisation enthalpy is :
- 32
- 35
- 19
- 87
Answer
(4) 87
Solution
Position of each element in the periodic table:
Z = 32 (Ge): period 4, group 14
Z = 35 (Br): period 4, group 17
Z = 19 (K): period 4, group 1
Z = 87 (Fr): period 7, group 1
Ionisation enthalpy increases across a period, the alkali metals K and Fr have the lowest values in their periods.
Down a group the atomic size increases and the outer electron is farther from the nucleus (more shielding), ionisation enthalpy decreases.
Fr (period 7) has a lower 1st ionisation enthalpy than K (period 4).
The element with the lowest 1st ionisation enthalpy has Z = 87.
Answer: (4)