Dative Covalent Bonds

Introduction

Most earlier dot-and-cross examples place one electron from each atom into a shared pair. A coordinate, or dative covalent, bond forms when one participant supplies both electrons of that pair. Ammonium formation is a clear example because ammonia already has a lone pair and an incoming proton has no electron to contribute.

Core explanation

Ammonia, NH₃, has three N–H bonding pairs and one nitrogen lone pair. A proton, H⁺, is a hydrogen nucleus without an electron in this simple description. When ammonia accepts that proton, nitrogen's lone pair becomes the shared pair in a new N–H bond.

The product is ammonium, NH₄⁺. Its four N–H bonds contain eight valence electrons. Counting directly gives five from nitrogen plus four from neutral hydrogen atoms minus one for the positive charge, also eight. Alternatively, the reactants supply eight from NH₃ and zero from H⁺. Both routes give the same inventory.

An arrow from nitrogen towards the incoming hydrogen can mark the donor-to-acceptor direction during bond formation. In a dot-and-cross formation drawing, the new pair uses two nitrogen-origin symbols, while the original N–H pairs can show one symbol from each original atom.

Once the ion forms, its four N–H bonds are equivalent in the ideal isolated tetrahedral ammonium ion. One is not permanently a weaker or chemically different “special” bond because its electrons were labelled as coming from the same donor. The arrow records a formation description, not an enduring tag on one pair of indistinguishable electrons.

Draw brackets around the entire NH₄ group with + outside. Do not put an independent + charge on each hydrogen. The ion's overall charge is a property of its complete proton and electron inventory. In ammonium chloride, covalent bonds hold the ammonium ion together and ionic attractions connect NH₄⁺ with Cl⁻ in the solid.

Step-by-step reasoning

1. Locate an available donor lone pair on ammonia's nitrogen. 2. Recognise that H⁺ contributes no electron to the new bond. 3. Use both lone-pair electrons to make the fourth N–H shared pair. 4. Check eight valence electrons, four hydrogen duets and a nitrogen octet, then enclose the whole ammonium ion in brackets with charge +1.

Visual explanation

Draw NH₃ with its nitrogen lone pair pointing towards H⁺. Use a donor arrow N→H in the formation sketch. Beside it draw the final bracketed NH₄⁺ ion with four N–H connections and no nitrogen lone pair.

Real-world analogy

Two people can share a meal even if one person supplied both portions. The source of the portions describes how the sharing began, not a different kind of sharing afterwards. Coordinate bonding similarly records the origin of the shared pair in the formation model.

Real-world example

Ammonium salts occur in many nitrogen-containing materials, including some fertilisers. Their formulas combine NH₄⁺ with counterions in charge-balanced ratios. Recognising covalent bonding inside ammonium prevents the mistaken claim that every connection in an ionic salt must itself be an ionic bond.

Why?

Why does ammonium have charge +1 if it has four ordinary-looking N–H bonds? The incoming proton adds positive charge without adding an electron. The total remains +1 even though the shared pairs complete local duet and octet counts.

Common misconception

“A coordinate bond contains only one electron because only one atom contributes.” The donor contributes a pair, so the new bond contains two electrons in the introductory model. One donor does not mean one electron.

Worked example

Audit a drawing of NH₃ + H⁺ that adds a new hydrogen cross to the fourth bond while retaining both nitrogen lone-pair dots. It invents an electron on H⁺ and gives three electrons to the new region. Remove the invented cross and use the two nitrogen dots as the shared pair. The final ion has four pairs and charge +1.

Quick check

1. Which atom supplies both electrons of the new bond when ammonia accepts a proton? Answer: Nitrogen supplies its existing lone pair.

Exam focus

Point the formation arrow from donor to acceptor. Show a two-electron pair, and put the final positive charge outside brackets enclosing the entire ammonium group.

Advanced insight

The same donor–acceptor language extends to ligands bonding to metal centres in coordination chemistry. Detailed bonding can also involve electron donation in more than one direction, so the simple arrow is a starting model rather than a complete orbital description.

Summary

A dative bond uses a shared pair supplied by one donor. Ammonia donates its lone pair to H⁺, forming NH₄⁺ with four equivalent N–H bonds in the ideal ion. Correct accounting adds a proton but no extra electron, preserving the overall positive charge.

Practice questions

1. How many lone pairs remain on nitrogen in the standard NH₄⁺ structure? Answer: None; the former lone pair is now the fourth bonding pair. 2. How many valence electrons are represented in NH₄⁺? Answer: Eight, calculated as 5 + 4 − 1. 3. Does the donor arrow prove that one ammonium bond remains permanently weaker than the other three? Answer: No. It describes electron-pair origin during formation; the ideal ion has equivalent N–H bonds.