The atomic number of the element from the following with lowest 1ˢᵗ ionisation enthalpy is :

Solution

Chemistry ma’am · 4 steps

First place each atomic number in the periodic table; the answer is the lowest alkali metal on the list.

1. Locate each element. Find the period and group for each atomic number. Ionisation enthalpy falls as you go down a group and rises as you go across a period, so position decides the order.

   Atomic number	Element	Position	First IE (approx.)
   32	Ge	period 4, group 14	762 kJ mol⁻¹
   35	Br	period 4, group 17	1140 kJ mol⁻¹
   19	K	period 4, group 1	419 kJ mol⁻¹
   87	Fr	period 7, group 1	about 393 kJ mol⁻¹

2. Across period 4. K, Ge and Br are all in period 4. Nuclear charge grows from left to right while the shell stays the same, so K has the lowest value of the three.
3. Down group 1. Fr sits in the same group as K, but three periods lower. Its outermost 7s electron is much farther from the nucleus and better shielded, so it needs less energy to remove.
4. So. Francium, atomic number 87, has the lowest first ionisation enthalpy of these four. The answer is (d).

The idea to keep. To compare ionisation enthalpies, place each element in the table first. Group 1 is lowest in any period, and within a group the element lower down is lower still.

Answer from the NTA final answer key. Solution by PrepWiser.