Lowest First Ionisation Enthalpy Element Concept

❓ Question

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

1. 35
2. 19
3. 32
4. 87

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🖼 Question Image

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✍️ Short Explanation

This problem is based on:

👉 Periodic trends
👉 First ionisation enthalpy
👉 Metallic character.

Main idea:

$$\boxed{ \text{Ionisation enthalpy decreases down a group} }$$

and

$$\boxed{ \text{Highly metallic elements have low IE} }$$

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🔷 Step 1 — Identify the Elements 💯

Atomic Number	Element
35	Br
19	K
32	Ge
87	Fr

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🔷 Step 2 — Recall Ionisation Enthalpy Trend

Ionisation enthalpy:

✔ Decreases down the group

✔ Increases across the period

Lowest IE is shown by:

$$\boxed{ \text{Alkali metals at bottom} }$$

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🔷 Step 3 — Compare Given Elements

Bromine (35)

Non-metal.

High ionisation enthalpy.

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Potassium (19)

Alkali metal.

Low ionisation enthalpy.

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Germanium (32)

Metalloid.

Moderate ionisation enthalpy.

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Francium (87)

Bottom-most alkali metal.

Maximum atomic size.

Weakest attraction on outer electron.

Hence:

$$\boxed{ \text{Lowest first ionisation enthalpy} }$$

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🔷 Step 4 — Final Identification

$$\boxed{ 87 }$$

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🔷 Step 5 — JEE Trap Alert 🚨

❌ Atomic size trend bhool jaana

❌ Metallic character ignore kar dena

❌ Potassium choose kar lena instead of Francium

Remember:

$$\boxed{ \text{Down group} \Rightarrow \text{IE decreases} }$$

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✅ Final Answer

$$\boxed{ 87 }$$

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📚 Related Topics

• MOI of Combined Bodies Using Parallel Axis
• Van’t Hoff Factor Shortcut for Percentage Dissociation
• Quadratic Equation Roots Reciprocal Relation Trick