Which of the following electronegativity order is incorrect? (1) Al < Mg < B < N (2) Al < Si < C < N (3) Mg < Be < B < N (4) S < Cl < O < F

Key Concepts

1. Electronegativity (EN) is a measure of an atom’s tendency to attract the bonding pair of electrons.

2. General Trends

   • Across a period (left → right): EN increases because nuclear charge rises but shielding remains nearly the same.
   • Down a group (top → bottom): EN decreases because added shells push valence electrons farther away.

3. Typical Pauling values (needed here)

   \text{Element} & Li & Be & B & C & N & O & F & \dots\\ \hline \chi_{\text{Pauling}} & 1.0 & 1.5 & 2.0 & 2.5 & 3.0 & 3.5 & 4.0 & \dots\end{array}$$ $$\begin{array}{c|cccccccc} \text{Element} & Na & Mg & Al & Si & P & S & Cl & \dots\\ \hline \chi_{\text{Pauling}} & 0.9 & 1.2 & 1.5 & 1.8 & 2.1 & 2.5 & 3.0 & \dots\end{array}$$

4. Method to check each list

   • Write the approximate EN value under every symbol.
   • See if the numbers rise from left to right in the given order. If any step falls or stays the same, that list is wrong.

Thought process for each option

1. Option (1) $\text{Al} < \text{Mg} < \text{B} < \text{N}$
   • Values: $1.5 \ ?\ 1.2 \ ?\ 2.0 \ ?\ 3.0$
   • First step: $1.5 \not< 1.2$ — immediately violates the rule.
2. Option (2) $\text{Al} < \text{Si} < \text{C} < \text{N}$
   • $1.5 < 1.8 < 2.5 < 3.0$ ✔️ correct.
3. Option (3) $\text{Mg} < \text{Be} < \text{B} < \text{N}$
   • $1.2 < 1.5 < 2.0 < 3.0$ ✔️ correct.
4. Option (4) $\text{S} < \text{Cl} < \text{O} < \text{F}$
   • $2.5 < 3.0 < 3.5 < 4.0$ ✔️ correct.

Hence, Option (1) is the only incorrect order.

What is the question?

We have to spot which list shows the wrong order of how strongly atoms pull electrons (this strength is called electronegativity).

How to think about it (like a story)

1. Imagine atoms sitting in rows (periods) and columns (groups) on the periodic table.
2. If you move right in a row, atoms get hungrier for electrons.
3. If you move down in a column, atoms get less hungry.
4. We only need rough sizes:
   • Second period (row-2): $\text{Li (1.0)} < \text{Be (1.5)} < \text{B (2.0)} < \text{C (2.5)} < \text{N (3.0)} < \text{O (3.5)} < \text{F (4.0)}$
   • Third period (row-3): $\text{Na (0.9)} < \text{Mg (1.2)} < \text{Al (1.5)} < \text{Si (1.8)} < \text{P (2.1)} < \text{S (2.5)} < \text{Cl (3.0)}$
5. Compare each option with these trend numbers—whichever breaks the rule is the wrong one.

Step-by-Step Solution

1. Write down approximate Pauling electronegativity values

   \text{Element} & Mg & Al & Si & P & S & Cl & B & C & N & O & F & Be\\ \hline \chi & 1.2 & 1.5 & 1.8 & 2.1 & 2.5 & 3.0 & 2.0 & 2.5 & 3.0 & 3.5 & 4.0 & 1.5\end{array}$$

2. Check each proposed order

   (1) Al < Mg < B < N $1.5 \stackrel{?}{<} 1.2 \;\text{(False)}$ Immediately wrong.

   (2) Al < Si < C < N $1.5 < 1.8 < 2.5 < 3.0 \;\text{(True)}$

   (3) Mg < Be < B < N $1.2 < 1.5 < 2.0 < 3.0 \;\text{(True)}$

   (4) S < Cl < O < F $2.5 < 3.0 < 3.5 < 4.0 \;\text{(True)}$

3. Conclusion Option (1) is the only incorrect electronegativity order.

Final Answer: (1) Al < Mg < B < N