Q.Which of the following species has tetrahedral geometry?

The Problem: One Picture Isn't Enough

Imagine you're trying to draw a photograph of a friend who is laughing. A single still frame captures one expression, but it misses the movement, the energy, the in-between of the laugh. A single Lewis structure does the same thing to certain molecules — it freezes them into one arrangement of electrons, but the real molecule is more like a short video clip, with electrons moving smoothly between positions.

Take ozone, $O_3$. If you try to draw a Lewis structure, you get a dilemma. You can put the double bond on the left:

Or on the right:

Both satisfy the octet rule. Both have the same atoms. But which one is correct? Neither, alone. The real ozone molecule has two identical $O-O$ bonds — each is halfway between a single and a double bond. No single Lewis picture can show that.

The Solution: Resonance Structures

Resonance structures are a set of two or more Lewis structures that collectively describe the actual electronic structure of a molecule where a single Lewis structure is inadequate. They are connected by a double-headed arrow ($\leftrightarrow$) to show they are not different molecules, but different ways of drawing the same molecule.

The Rules (Precise Statement)

1. Same atomic positions. Only electrons (pi bonds and lone pairs) move; atoms never move.
2. Same total number of electrons. You are redistributing, not adding or removing.
3. Valid Lewis structures. Each resonance form must obey the octet rule (for second-period elements) and have correct formal charges.
4. Curved arrows show electron movement. An arrow from a lone pair or a pi bond points to where those electrons go next.

How to Draw Them: The Curved Arrow Method

Take the nitrate ion, $NO_3^-$. Start with one valid Lewis structure:

Now, push electrons:

• Take the lone pair on the top oxygen (the one with the negative charge) and push it down to form a double bond with nitrogen.
• Simultaneously, push the existing double bond on the right up to become a lone pair on that oxygen.

You get a second structure:

Repeat the process from this new structure, and you get a third. All three are resonance structures of $NO_3^-$.

The Hybrid: What the Molecule Actually Looks Like

The resonance hybrid is not an average of the bond lengths — it is the actual molecule. In $NO_3^-$, all three $N-O$ bonds are identical, with a bond order of $1\frac{1}{3}$ (one and one-third). The negative charge is spread equally over all three oxygens, not stuck on one.

You represent the hybrid by drawing dashed lines for partial bonds and placing the charge in a circle (or using fractional charges) to show delocalization.

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