NH4- is formed from NH3 and H- by

Analyze the question: The question (written as NH4⁻ and H⁻, but it should be NH₄⁺ formed from NH₃ and H⁺) asks what type of bond forms when ammonia combines with a hydrogen ion.

Note: the ion is actually NH₄⁺ (ammonium) and the species added is H⁺ (a proton). The "⁻" signs in the question are most likely typing errors.

Concept:

• In ammonia, nitrogen has 5 valence electrons. It uses 3 to bond with three hydrogen atoms, leaving one lone pair.
• The H⁺ ion has no electrons, but has an empty 1s orbital.
• When both electrons in the bond come from the same atom (here nitrogen), the bond is a dative (coordinate covalent) bond.

Step-by-step solution:

1. NH₃ has a lone pair on N: H₃N:
2. H⁺ has an empty orbital.
3. Nitrogen donates its lone pair into the empty orbital of H⁺.
4. A bond forms: NH₃ + H⁺ → [NH₄]⁺
5. Once formed, all four N-H bonds in NH₄⁺ are identical, but the way the fourth bond was created is by dative bonding.

Checking the options:

• A. Covalent bonding: a dative bond is technically a type of covalent bond, but it is not the specific answer being tested here.
• B. Dative bonding: correct, since both shared electrons come from nitrogen.
• C. Hydrogen bonding: this is an intermolecular attraction, not the bond formed here.
• D. Ionic bonding: this involves electron transfer, which does not happen here.

**Answer: B. Dative bonding.

Exam tip: Whenever you see NH₄⁺, H₃O⁺ (hydroxonium), or NH₃→BF₃, think "lone pair + empty orbital = dative bond." Also note that the question likely has a typo (H⁺, not H⁻), so read the context rather than panicking.