The atomic number of the element from the following with lowest 1 st ionisation enthalpy is : A 32 B 35 C 87 You Marked D 19

Key Concepts You Need

1. Ionisation Enthalpy ($\Delta_iH_1$)
   The energy required to remove the outermost (first) electron from a gaseous atom.

2. Periodic Trend
   • Across a period (left → right): Effective nuclear charge increases, ionisation enthalpy increases.
   • Down a group (top → bottom): Atomic size increases, shielding effect increases, ionisation enthalpy decreases.

3. Groups Involved
   • Atomic number 19: Potassium ($K$) → Group 1, Period 4
   • Atomic number 32: Germanium ($Ge$) → Group 14, Period 4
   • Atomic number 35: Bromine ($Br$) → Group 17, Period 4
   • Atomic number 87: Francium ($Fr$) → Group 1, Period 7

Logical Path to the Answer

1. Locate each element on the periodic table.
2. Identify their groups. Group 1 elements always have the lowest first ionisation enthalpy in their respective periods.
3. Between the two Group 1 elements (19 and 87), Francium (87) is lower in the group than Potassium (19).
4. Because ionisation enthalpy decreases down a group, Francium must have the lowest $\Delta_iH_1$.

Imagine Pulling a Balloon Away from a Magnet

1. Atoms are like little magnets that hold on to their balloons (electrons).
2. Ionisation enthalpy is how hard you have to pull to snatch the first balloon away.
3. Big atoms (lower in a group) hold their balloons loosely—like a weak magnet—so you can pull them off easily.
4. Small atoms (higher in a group) grip balloons tighter—like a strong magnet—so they’re harder to pull.
5. From the list, the biggest and ‘loosest’ magnet is the element with atomic number 87 (Francium), so it is the easiest to take an electron from.

Step-by-Step Calculation-Free Solution

1. Write each atomic number with its element:
   19 → $K$, 32 → $Ge$, 35 → $Br$, 87 → $Fr$

2. Group Identification:
   • $K$ & $Fr$ → Group 1 (alkali metals)
   • $Ge$ → Group 14
   • $Br$ → Group 17

3. Use the Trend:
   Alkali metals have the lowest first ionisation enthalpies in any period.

4. Compare Down Group 1:
   Ionisation enthalpy decreases down the group:

   $\Delta_iH_1(\text{Li}) > \Delta_iH_1(\text{Na}) > \Delta_iH_1(\text{K}) > \Delta_iH_1(\text{Rb}) > \Delta_iH_1(\text{Cs}) > \Delta_iH_1(\text{Fr})$

5. Conclusion:
   The smallest value is for Francium, atomic number 87.

Final Answer: Option C (87)