Question

The atomic number of the element from the following with lowest $ 1^{\text{st}} $ ionization enthalpy is:

• A. 87
• B. 19
• C. 32
• D. 35

Hint:

- Ionization energy \(\propto \frac{1}{\text{atomic size}}\) - Alkali metals have lowest ionization energies in their periods - Within Group 1, ionization energy decreases down the group - Francium has the lowest known ionization energy (380 kJ/mol)

Answer 1

The Correct Option is A

The question is about determining which element has the lowest first ionization enthalpy from the given atomic numbers. Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion. Typically, elements with a lower ionization enthalpy can lose electrons more easily.

Step-by-step Explanation:

1. The atomic numbers given are: 87, 19, 32, 35.
2. To find the corresponding elements, we refer to the periodic table:
   • Atomic number 87: Francium (Fr)
   • Atomic number 19: Potassium (K)
   • Atomic number 32: Germanium (Ge)
   • Atomic number 35: Bromine (Br)
3. Ionization enthalpy tends to decrease as we move down a group in the periodic table because the outermost electrons are further away from the nucleus and are more shielded by inner electrons.
4. Among these elements:
   • Francium (Fr) is an alkali metal and belongs to Group 1, which is known for having low ionization energies.
   • Potassium (K) is also in Group 1 but is higher up in the column compared to Francium.
   • Germanium (Ge) and Bromine (Br) are further right in the periodic table and have higher ionization enthalpy compared to Group 1 elements.
5. Thus, Francium, being lower in the periodic table specifically in Group 1, has the lowest ionization enthalpy among the options provided.

Conclusion: The element with the lowest first ionization enthalpy among the options is Francium (Atomic number 87).

Answer 2

Concept:
The first ionization enthalpy decreases:
- Down a group (atomic size increases)
- From right to left across a period (effective nuclear charge decreases)

Analysis of Options:
Option 1 (87): Francium (Fr) - Group 1, Period 7 element - Largest atomic size in periodic table - Lowest effective nuclear charge on valence electron - Lowest ionization energy among given options.
Option 2 (19): Potassium (K) - Group 1, Period 4 - Higher ionization energy than Fr (smaller size).
Option 3 (32): Germanium (Ge) - Group 14, Period 4 - Much higher ionization energy than alkali metals.
Option 4 (35): Bromine (Br) - Group 17, Period 4 - Highest ionization energy among options (high effective nuclear charge).

Periodic Trend:
Ionization Energy Order: Fr (87) < K (19) < Ge (32) < Br (35)