AX is a covalent diatomic molecule, where A and X are second row elements of periodic table. Based o

Q: Why can't the answer be 13?

While 13 electrons (like in the CN radical) do give a bond order of 2.5, CN is highly reactive and doesn't exist as a stable neutral gas under normal conditions. NO (15 electrons) is a classic, stable neutral molecule.

Q: Does this shortcut work for all molecules?

This specific 14-electron rule shortcut works beautifully for diatomic molecules of the second period (from $B_2$ to $Ne_2$). It is a massive time-saver for JEE!

Q: How do we calculate bond order formally?

The formal formula is $B.O. = \frac{N_b - N_a}{2}$, where $N_b$ is the number of bonding electrons and $N_a$ is the number of antibonding electrons, derived from the full Molecular Orbital diagram.

Q: Is NO paramagnetic or diamagnetic?

NO has 15 electrons, which is an odd number. Therefore, it must have at least one unpaired electron, making it paramagnetic.

Q: What happens to the bond order if NO loses an electron to become $NO^+$?

If NO (15 electrons) loses an electron, it becomes $NO^+$ with 14 electrons. According to our rule, 14 electrons correspond to a bond order of 3. So, the bond becomes stronger!

Question

AX is a covalent diatomic molecule, where A and X are second row elements of periodic table. Based on molecular orbital theory, the bond order of AX is 2.5. The total number of electrons in AX is ……… . (Round off to the nearest integer).

Sigma Percentile · Accepted Answer

### The Power of the 14-Electron Rule

When dealing with diatomic molecules of the second period, drawing the full Molecular Orbital (MO) diagram can be incredibly time-consuming.

Fortunately, there is a brilliant shortcut that every JEE aspirant must know: **The 14-Electron Rule**.

This rule states that a diatomic molecule with exactly $14$ electrons (like $N_2$) has a maximum bond order of $3$.

For every electron you add or remove from this magic number of $14$, the bond order decreases by exactly $0.5$.

### Decoding the Bond Order

In our problem, we are given a bond order of $2.5$.

Using our shortcut, we know that a bond order of $2.5$ is just one step away from the maximum bond order of $3$.

This means the total number of electrons must be either $14 - 1 = 13$, or $14 + 1 = 15$.

So, our mystery molecule $AX$ has either $13$ or $15$ electrons. But which one is it?

### The Chemical Reality Check

Let's analyze the $13$-electron possibility first.

If the molecule has $13$ electrons and is made of second-row elements, the most likely combination is Carbon ($6$ electrons) and Nitrogen ($7$ electrons), forming $CN$.

However, $CN$ is a highly reactive free radical. It does not exist as a stable, neutral diatomic gas under normal conditions.

Now, let's look at the $15$-electron possibility.

This corresponds to Nitrogen ($7$ electrons) and Oxygen ($8$ electrons), forming $NO$ (Nitric Oxide).

Nitric Oxide is a very well-known, stable, and neutral diatomic molecule.

Therefore, the molecule $AX$ must be $NO$, and the total number of electrons is **$15$**.

Enter Numerical Value:

Visualized Solution

\text{The Setup}

\text{The Trick: Bond Order vs Total Electrons}

\text{Finding the Electron Count}

\text{Analyzing the 13-Electron Case}

\text{Analyzing the 15-Electron Case}

\text{Final Conclusion}

The Sigma Insight: Molecular Orbital Theory

The Power of the 14-Electron Rule

Decoding the Bond Order

The Chemical Reality Check

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