Question

Why does ammonia, NH3, behave as a base when it reacts with an acid?z

Answer 1

The nitrogen atom in ammonia comes with a lone pair of electrons.

• The ammonia molecule can accept a proton from an acid with that lone pair. As a result, ammonia is a Bronsted-Lowry base.
• Ammonia can also "donate" that lone pair to an acid. As a result, ammonia is a Lewis base.

Explanation

• A substance is a base in the Bronsted-Lowry Acid-base Theory if it can accept one or more protons.
• A substance is a base in the Lewis Acid-base Theory if it can donate a lone pair of electrons.

Consider the feature of an ammonia molecule:

• Three H atoms bonded to a central N atom.
• One lone pair of electrons on the central N atom.

The central N atom carries a partial positive charge. That N atom would attract protons, which are positive. The molecule can bond to a proton to form an ammonium NH₄⁺ ion. As a result, NH₃ is a Bronsted-Lowry Base.

The central N atom might also share that lone pair of electrons with an electrophile, which is a substance that would readily bond to lone pairs of electrons (typically one that is short of electrons.) H⁺ and BF₃ are examples of electrophiles. Ammonia would react with either readily and share its lone pair of electron. It would thus act as a Lewis base.