1. Electron Domains:

The oxygen atom in H₂O is sp³ hybridized.

It has 4 electron domains (2 bonding pairs with hydrogen 2 lone pairs).

2. Ideal Tetrahedral Angle:

A perfect tetrahedral shape has a bond angle of 109.5° (like in CH₄).

But in H₂O, there are 2 lone pairs, which exert more repulsion than bonding pairs.

3. Lone Pair Repulsion:

Lone pairs are more spread out and repel bonding pairs more strongly.

This pushes the H-O-H bonds closer together, reducing the angle from 109.5° to 104.5°.

Let’s analyze each option:

A. NH₃ (Ammonia)

Has a lone pair on nitrogen

Donates that electron pair

➡️ It is a Lewis base, not an acid

B. H₂O (Water)

Has two lone pairs on oxygen

Can donate electron pairs

➡️ So, it’s a Lewis base.

C. BF₃ (Boron trifluoride) ✅

Boron has only 6 electrons in its outer shell in BF₃

Electron-deficient, wants to accept electron pairs

➡️ This makes it a Lewis acid.

D. CH₄ (Methane)

Carbon is fully satisfied with 8 electrons

No empty orbitals to accept electron pairs

➡️ Not a Lewis acid.

🧠 Summary Table:

Compound Lewis Acid/Base Reason

NH₃ Base Has lone pair on N

H₂O Base Has lone pairs on O

BF₃ Acid ✅ Electron-deficient boron

CH₄ Neutral No ability to accept electrons

Explanation: What is Resonance?

Resonance occurs when:

A molecule or ion has two or more valid Lewis structures (called resonance structures).

The actual structure is a hybrid of all these forms.

This usually happens when pi bonds (double/triple bonds) and lone pairs are delocalized over multiple atoms.

🧪 Let’s analyze each option:

A. CH₄ (Methane)

Single bonds only (sigma bonds).

No delocalized electrons.

➡️ ❌ No resonance

B. C₂H₆ (Ethane)

All single bonds.

No π-bonds or lone pairs involved in delocalization.

➡️ ❌ No resonance

C. CO₃²⁻ (Carbonate ion)

Has three oxygen atoms bonded to carbon.

Only one double bond at a time can be drawn with carbon, but the double bond shifts between the three O atoms.

This gives three resonance structures: