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Chemistry Question 56 – JEE-MAIN 2025

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

Ionization enthalpy is the energy required to remove the most loosely held electron from an isolated gaseous atom.

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Ninja Strategy•Largest Alkali Metal

The element with the lowest ionization enthalpy will be the largest alkali metal among the choices, as ionization enthalpy decreases down a group and is lowest for Group 1 elements.

Step 1: Identify Elements and their Positions✦ Active

Let's identify the elements corresponding to the given atomic numbers and their positions in the periodic table:

Atomic number 19: Potassium (K), Group 1, Period 4 Atomic number 87: Francium (Fr), Group 1, Period 7 Atomic number 35: Bromine (Br), Group 17, Period 4 Atomic number 32: Germanium (Ge), Group 14, Period 4
Step 2: Recall Periodic Trends for First Ionization Enthalpy○ Expand

The first ionization enthalpy (IE1) generally decreases down a group due to increasing atomic size and shielding effect, which reduces the effective nuclear charge on the valence electrons. It generally increases across a period from left to right due to increasing effective nuclear charge and decreasing atomic size.

💡 Teacher's Secret Hint

Remember that alkali metals (Group 1) have the lowest ionization enthalpies in their respective periods.

Step 3: Compare Elements and Determine Lowest IE1○ Expand

Comparing elements in Period 4 (K, Ge, Br): Potassium (K) is in Group 1, Germanium (Ge) in Group 14, and Bromine (Br) in Group 17. Since ionization enthalpy increases across a period, K will have the lowest IE1 among these three.

Now, compare Potassium (K, atomic number 19) and Francium (Fr, atomic number 87). Both are alkali metals (Group 1). Francium (87) is located below Potassium (19) in Group 1. As ionization enthalpy decreases down a group, Francium (87) will have a lower IE1 than Potassium (19).

Therefore, Francium (atomic number 87) has the lowest 1st ionization enthalpy among the given options.

💡 Teacher's Secret Hint

The element with the largest atomic radius and lowest effective nuclear charge on its valence electrons will have the lowest ionization enthalpy.

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