The Lone Pair Gift

Why it matters

Ammonia has a lone pair just sitting there, unused. Along comes a bare H⁺ ion with no electrons at all — and ammonia donates BOTH electrons of its lone pair to bond with it, forming the ammonium ion (NH₄⁺). One partner pays the entire bill, yet the new bond turns out identical to the other three.

Predict first

When ammonia (NH₃) bonds with a hydrogen ion (H⁺) to form NH₄⁺, where do the two electrons in the new bond come from?

Nitrogen donates its whole lone pair to the empty H⁺, forming a coordinate (dative) bond shown with an arrow (→). Once formed, all four N–H bonds in NH₄⁺ are identical — you cannot tell which one was the coordinate bond. The very same idea builds the hydronium ion (H₃O⁺) and the NH₃→BF₃ adduct.

• One from nitrogen and one from the H⁺ ion, like a normal covalent bond
• Both come from the nitrogen lone pair; the H⁺ brings none — correct
• Both come from the hydrogen ion
• They are pulled from the surrounding water

What you do

Nitrogen has bonded to three hydrogens and still holds one lone pair. A bare H⁺ ion arrives with no electrons of its own — donate the nitrogen lone pair as a coordinate bond (→) so the H⁺ also reaches a full shell. One atom supplies both shared electrons.

Check yourself

Which symbol represents a coordinate bond?

The arrow points from the donor (the atom that supplies the lone pair) to the acceptor.

In the ammonium ion (NH₄⁺), how do the four N–H bonds compare?

Once formed, a coordinate bond is identical to a normal covalent bond — all four N–H bonds in NH₄⁺ are equivalent, with no way to tell which began as the dative bond.

A lone pair can also be donated to an atom with an incomplete octet. Which is such an acceptor?

Boron in BF₃ has only 6 valence electrons and an empty orbital, so it readily accepts a donated lone pair — for example from ammonia, forming H₃N→BF₃.

• A single line (—)
• An arrow (→) pointing from donor to acceptor — correct
• A double line (=)
• A dotted line (···)
• The coordinate bond is weaker than the other three
• All four are identical and indistinguishable — correct
• The coordinate bond is ionic, the others covalent
• Three are single bonds and one is a double bond
• Boron in BF₃ (only 6 valence electrons) — correct
• Neon, which already has a full octet
• Sodium, which tends to lose electrons
• Argon, an inert noble gas