3g. The covalent bonding in the water H₂O molecule and the hydrogen sulfide molecule H₂S

 Covalent bonding diagram for WATER covalent molecule, molecular formula H₂O

Two atoms of hydrogen (1) combine with one atom of oxygen (2.6) to form the molecule of the compound water H₂O

Hydrogen is one electron short of a full shell, oxygen is two electrons short of a full outer shell of 8, so two hydrogen atoms share their electrons with the six outer electrons of oxygen, so all three atoms now have a full outer shell.

 and and combine to form so that the hydrogen atoms are electronically like helium and the oxygen atom becomes like neon (2.8, but only the outer shell of oxygen's electrons are shown).

Electronically, hydrogen (1) becomes like helium (2) and oxygen (2.6) becomes like neon (2.8), so the hydrogen and oxygen atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

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

The water molecule is held together by the strong H–O hydrogen–oxygen single covalent bonds by sharing electrons.

Note that the inner shell of oxygen's electrons are shown in this diagram, but NOT in the bonding diagram above. Only the outer shell of oxygen's electrons are involved in the covalent bonding here.

Although these electron arrangement diagrams show how the covalent bond is formed and the electronic structure of the molecule, they do not give any idea on the relative size of the atoms in the molecule or the 3D spatial arrangement of the atoms in the molecules, nor indeed how the molecules stack together in a crystal lattice.

The molecule can be shown as (displayed formula of water) with two hydrogen – oxygen single covalent bonds.

The two pairs of double dots represent pairs of electrons not involved in the covalent bonding in water. Its actually referred to as a bent or V shape (A level comment).

Advanced note: water has a V or bent shape, the H–O–H bond angle is 105^o.

The 'pros and cons' of molecular diagrams and understand the limitations of diagrams:

The dot and cross diagram (left) is good for showing the electronic detail of the bonding but does not show the relative size of the atoms or the 3D shape. It can also show whether a bond is single (ox) or double (oxox).

This displayed formula (right) may give you, but not usually for larger molecules, some idea of the shape, but not the full electron detail, nor does it show the relative size of the atoms. Importantly, what it does show is precisely how all the atoms are connected e.g. which atoms connect to each other (H-O-H NOT H-H-O) and whether the bonds are single (as in water H-O-H) or double (as in carbon dioxide O=C=O).

The ball and stick model (left) gives you an idea of the shape of the molecule, but nothing about the relative size of the atoms and nothing about the electronic structure of the molecule (which you get from the dot and cross diagrams).

A space filling model (right) gives you the best 2D representation of the real 3D shape of the molecule and the relative size of the atoms and the space they fill. However, it does not say anything about the electronic structure and it doesn't indicate whether the covalent bonds are double, single or treble.

These comments apply to the rest of the molecular structures described below.

You should appreciate different styles of representing a molecule's structure all having something useful to say!

On the above right is the ball and stick diagram for the water molecule.

The diagrams on the left and right are the full 'dot and cross' electronic diagram for the covalent bonding in the water molecule. All the electrons are shown in this electronic dot & cross Lewis diagrams for covalent bonding in the water molecule.

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

The dot and cross diagram for hydrogen sulfide will be similar (just swap the O for an S).

The dot and cross diagram does not tell you anything about the size of the atoms in the molecule or the accurate 3D spatial arrangement of the atoms, that is, the shape of the molecule

Comments

Melting point of water 0 ^oC

Boiling point of water 100 ^oC

You would expect low values because of the weak intermolecular forces between small covalent molecules like water.

Water is a colourless liquid at room temperature

HYDROGEN SULFIDE

Hydrogen sulphide will be similar, since sulphur (electrons 2.8.6) is in the same Group 6 as oxygen.

Valency of oxygen is 2 and sulphur is also 2 here.

The dot and cross Lewis diagram for hydrogen sulfide is similar to water.

Sulfur is in group 6 with six outer shell electrons, two of which pair up with the electrons from the two hydrogen atoms.

It has a bent shape like water.

Comments

Melting point of hydrogen sulfide -86 ^oC

Boiling point of hydrogen sulfide -61 ^oC

You would expect low values because of the weak intermolecular forces between small covalent molecules like hydrogen sulfide.

Hydrogen sulfide is a poisonous egg smelling gas at room temperature.

 

A periodic table group 6 note

Oxygen and sulfur are in the same group 6 of the periodic table, so that you would expect the electronic diagrams of the covalently bonded molecules, based on the outer electrons only, to be identical and also the 'bent' shape of the molecule..

Extra information on Water (H₂O) e.g. for GCSE/IGCSE Level Chemistry Revision Notes

(and a mention of the similar molecule hydrogen sulfide.

Properties of Water

• Formula: H₂O
• Physical properties:
  • Colourless, odourless liquid at room temperature
  • Freezes at 0 °C, boils at 100 °C (at 1 atm)
  • High specific heat capacity (absorbs lots of heat before temperature rises)
  • Good solvent (dissolves many ionic and polar substances)
• Chemical properties:
  • Neutral pH (pure water ≈ 7)
  • Reacts with some metals → hydrogen gas (e.g., Ca + 2H₂O → Ca(OH)₂ + H₂)
  • Reacts with non-metal oxides → acidic solutions (e.g., CO₂ + H₂O → H₂CO₃)
  • Reacts with metal oxides → alkaline solutions (e.g., Na₂O + H₂O → 2NaOH)

Laboratory Preparation-Purification

• Distillation: Purifies water by boiling and condensing vapour
• Deionisation: Removes dissolved ions using ion-exchange resins
• Testing for purity:
  • Pure water boils at 100 °C and freezes at 0 °C
  • Conductivity test (pure water is a poor conductor)

Industrial Importance

• Solvent: Used in chemical reactions and processes
• Coolant: High heat capacity makes it useful in cooling systems
• Raw material: Electrolysis of water → hydrogen and oxygen
  • See notes on the Electrolysis of water
• Agriculture & domestic use: Irrigation, drinking water, sanitation

Uses of Water

• Drinking, washing, agriculture
• Food and drinks industries.
• Industrial cooling
• Solvent in reactions
• Electrolysis (production of H₂ and O₂)
• Medium for transport in biological systems

Hydrogen Sulphide (H₂S) – Brief Notes

• Formula: H₂S
• Properties:
  • Colourless gas
  • Smells of rotten eggs
  • Toxic
  • Slightly soluble in water, weak acid
• Uses: Limited industrial use (e.g., in chemical synthesis); mainly studied for toxicity and environmental impact.
• Exam relevance: Often contrasted with water (similar formula, very different properties).

Typical Exam Board Requirements for water

See also:

1. Water cycle, potable water, water treatment, pollution, ion tests

2. Colloids - sols, foams, soaps, detergents and emulsions

3. Hard and soft water - causes and treatment

4. Gas and salt solubility in water and solubility curves

5. Determination and calculation of water of crystallisation

Student Exam Tips

• Distinguish between pure water and potable water: Potable water is safe to drink but not chemically pure.
• Always write H₂O, not HO or OH₂.
• Electrolysis: Remember products (H₂ at cathode, O₂ at anode).
• Purity test: State boiling/freezing points clearly.
• Reactions: Link water with oxides → acidic/alkaline solutions.
• Exam wording: “Water is a universal solvent” → explain it dissolves many substances, not all.

Common Misconceptions

•  Thinking potable water = pure water (potable water contains dissolved minerals).
•  Assuming pure water conducts electricity well (it doesn’t; ions in solution do).
•  Forgetting electrolysis products (hydrogen at cathode, oxygen at anode).
•  Confusing water’s neutral pH with “always safe” — contaminated water can be harmful.
•  Believing hydrogen sulphide behaves like water (similar formula but very different properties).