3h. Describing and explaining the covalent bonding in the ammonia molecule NH₃ with extra notes on properties and uses

Covalent bonding diagrams for AMMONIA covalent molecule, molecular formula NH₃

Three atoms of hydrogen (1) combine with one atom of nitrogen (2.5) to form the molecule of the compound ammonia NH₃

Each hydrogen atom is one electron short of a helium structure (full shell) and nitrogen is three electrons short of a full outer shell (of 8), so three hydrogen atoms share their electrons with the five outer electrons of nitrogen, so all four atoms effectively have full outer shells.

three of  and one combine to form so that the hydrogen atoms are electronically like helium and the nitrogen atom becomes like neon (only the outer shell of nitrogen's electrons are shown).

Electronically, hydrogen (1) becomes like helium (2) and nitrogen (2.5) becomes like neon (2.8), so the hydrogen and nitrogen atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

(Lewis diagram of ammonia) simplified 'dot and cross' electronic diagram for the covalently bonded ammonia molecule

The triatomic ammonia molecule is held together by the strong N–H nitrogen–hydrogen single covalent bonds by sharing a pair of electrons.

The Venn diagram above on the far right is the best style for ammonia, clearly showing the sharing of the pairs of electrons for the single covalent bonds in the ammonia molecule (in a sort of Venn diagram style).

Note that the inner shell of nitrogen's electrons are not shown (as in the diagrams on the right), only the outer shell of nitrogen's electrons are involved in the covalent bonding here.

 

The molecule can be shown as  (displayed formula) with three nitrogen – hydrogen single covalent bonds (AS note: called a trigonal pyramid shape, the H–N–H bond angle is 107^o.  This displayed formula does indicate the shape of the ammonia molecule as well as how the three N-H single covalent bonds are arranged, but no relative size of atoms or electronic detail of covalent bond formation by electron sharing. It does indicate the pyramidal shape of the molecule (A level comment).

The double dots represent a pair of electrons not involved in the covalent bonding in ammonia. PH₃ will be similar since phosphorus (2.8.5) is in the same Group 5 as nitrogen. Valency of nitrogen or phosphorus is 3 here.

Above on the right two of the full 'dot and cross' electronic Lewis diagram for the covalent bonding in the ammonia molecule.

Covalent bond molecule diagrams come in a variety of forms e.g. for ammonia, as well as those above ...

dot and cross electronic diagram, 3D ball and stick model, 3D space-filling model and 2D displayed formula and all represent ammonia!

At advanced level chemistry this is described as a trigonal pyramid shape.

See advanced level chemistry notes on the shapes of molecules

 

Its worth making the following comments on the different representations of simple covalent molecules of >2 atoms.

So, reminders:

(i) Dot and cross diagrams are good for showing the electronic detail of the structure, and whether the bonds are single (ox) or double (ox) etc. However, it gives no idea on the shape of the ammonia molecule i.e. the 3D spatial arrangement of the bonds and atoms (its effectively a 2D diagram, but the molecule might not be flat!) and no information about the relative size of the atoms.

(ii) Displayed formulae clearly shows how the atoms are bonded together, e.g. the arrangement of the three N-H single bonds, but only gives a 2D view of the molecule.

(iii) It needs a 3D ball and stick model diagram to give an idea of the spatial arrangement of the atoms in ammonia, but not the relative size of the atoms. A space filling model would give the shape of the molecule and the relative size of the atoms. However, neither of these two models show any electronic details of how the covalent bond is formed.

Comments on ammonia's bonding and properties

Melting point of ammonia -78 ^oC

Boiling point of ammonia -33 ^oC

You would expect these low state change values for ammonia because of the weak intermolecular forces between small covalent molecules.

However, the melting point and boiling point are higher than expected for such a small molecule because the strongest intermolecular bonding force of hydrogen bonding acts to attract ammonia molecules.

Ammonia is a colourless pungent smelling gas at room temperature.

Ammonia forms an alkaline solution when dissolved in water where some of the ammonia reacts with water to give ammonium ions and hydroxide ions.

NH₃(aq)  +  H₂O(l)    NH₄⁺(aq)  +  OH^-(aq)

Extra structured and exam-board-aligned notes on ammonia (NH₃)

Tailored for GCSE/IGCSE Chemistry students across WJEC, CCEA, CIE, AQA, Edexcel, OCR Gateway, and OCR 21st Century specifications.

Bonding and Structure of Ammonia (NH₃)

Type of Bonding in ammonia

• Covalent bonding between nitrogen and hydrogen atoms.
• Nitrogen shares three electrons with three hydrogen atoms.
• Each covalent bond involves a shared pair of electrons.

Electron Configurations for ammonia

• Nitrogen: 5 outer electrons → needs 3 more for a full shell.
• Hydrogen: 1 outer electron → needs 1 more.
• Dot-and-cross diagrams show three bonding pairs and one lone pair on nitrogen.

Molecular Geometry of ammonia (advanced note)

• Trigonal pyramidal shape.
• Bond angle ≈ 107° due to lone pair repulsion.
• Not a perfect tetrahedron (109.5°) because lone pairs repel more strongly than bonding pairs.

Intermolecular Forces in ammonia

• Weak van der Waals forces between molecules, hence the low melting and boiling points.
• Hydrogen bonding possible due to lone pair on nitrogen and hydrogen atoms.

Physical Properties of ammonia

Uses of Ammonia NH₃

Industrial Uses of ammonia

Laboratory and industrial uses

• Test for halide ions (via silver nitrate and ammonia). (advanced level)
• Preparation of ammonium salts - used in fertilisers by neutralising nitric acid and sulfuric acid.

• For more details on ammonium salts see:

• Acid reactions with metals/oxides/hydroxides/carbonates/ammonia, neutralisation to give salts

• More on reactions of bases-alkalis like ammonia and sodium hydroxide (overlaps with Part 4.)

• For more details on industrial manufacture and uses see:

• The Haber Synthesis of ammonia - nitrogen fixation

• The manufacture and uses of ammonia-nitric acid-fertilisers (and preparation of ammonium salts)

• Use of NPK fertilisers and environmental problems

Common Misconceptions about ammonia

Exam Tips about questions involving ammonia

• Use dot-and-cross diagrams to show bonding clearly (Venn diagram (xo) style best).
• Mention lone pair when explaining shape and bond angles (advanced level).
• Link structure to properties (e.g. low boiling point due to weak forces).
• Know the Haber Process conditions: e.g. 450°C, 200 atm, iron catalyst.
• Be precise: distinguish between ammonia (NH₃) as a gas or in solution and ammonium ion (NH₄⁺) in ammonium salts - crystals or in aqueous solution.