3d. The covalent bonding in the hydrogen molecule H₂

Covalent bonding diagram for HYDROGEN covalent molecule, molecular formula H₂

Two hydrogen atoms (1) form the molecule of the element hydrogen H₂

Hydrogen, a H atom, is one short of a full shell like helium, so two hydrogen atoms share each others electron to have a full outer shell.

 and combine to form where both atoms have a pseudo helium structure of 2 outer electrons around each atom's nucleus. Any covalent bond is formed from the mutual attraction of two positive hydrogen nuclei and negative electrons between them (i.e. effectively 'electron sharing'), in this case a single H-H bond.

Incidentally, the nuclei would be a very tiny dot in the middle of where the H symbols are drawn! H valency is 1.

The Lewis diagram of hydrogen is the most simplified 'dot and cross' style of electronic diagram for the covalently bonded diatomic molecule hydrogen molecule.

The hydrogen molecule is held together by the strong hydrogen–hydrogen single covalent bond H–H (displayed formula, a simple representation of the single bond but no electronic structure detail). The Venn style diagram on the left is the best style for hydrogen, clearly showing the sharing of the pair of electrons for the single covalent bond in the hydrogen molecule (in a sort of Venn diagram style).

Remember, electronically, hydrogen is simply 1 and becomes like helium 2, so the hydrogen atoms effectively have a full outer shell in forming the covalent bonds when the atoms share their outer electrons. This last diagram of the hydrogen molecule shows the sharing of electrons bond in an overlap 'Venn diagram' style.

The - line in H-H represents the single covalent bond, in this simplest possible displayed formula

Comments

Hydrogen is a colourless gas at room temperature.

Melting point of hydrogen -259 ^oC

Boiling point of hydrogen -253 ^oC

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

Helium is the only element with a lower melting point and boiling point than hydrogen.

If hydrogen is mixed with air, the strong H-H bond stops it reacting with oxygen - too high an activation energy.

However, a flame provides enough energy to initiate the reaction - hence the 'squeaky pop' test for hydrogen!

Extra notes on  Hydrogen for e.g. GCSE/IGCSE level Chemistry Revision Notes

Properties of Hydrogen

• Symbol: H
• Atomic number: 1
• Molecular form: H₂ (diatomic gas)
• Physical properties:
  • Colourless, odourless gas
  • Very light (least dense gas)
  • Insoluble in water
• Chemical properties:
  • Burns with a pale blue flame → forms water (H₂ + ½O₂ → H₂O)
  • Explosive when mixed with air/oxygen
  • Reducing agent (can remove oxygen from metal oxides)

Laboratory Preparation

• Method: Reaction of a metal with dilute acid
  • Example: Zn + 2HCl → ZnCl₂ + H₂↑
• Collection: Over water or by upward delivery (because hydrogen is lighter than air)
• Test for hydrogen: Burns with a ‘pop’ sound when a lit splint is applied (due to explosive reaction with oxygen).
• Methods of preparing and collecting gases revision notes
• Tests for gases, water and non-metallic elements
• QUIZ and notes on Chemical tests for identification section 23

Industrial Manufacture

• Steam reforming of methane (main method):
  • CH₄ + H₂O → CO + 3H₂
  • Followed by water-gas shift reaction: CO + H₂O → CO₂ + H₂
• Electrolysis of water:
  • 2H₂O → 2H₂ + O₂
• Other processes:
  • Cracking of hydrocarbons in petroleum refining
  • By-product in chlor-alkali industry (electrolysis of brine)

Uses of Hydrogen

• Industrial:
  • Manufacture of ammonia (Haber process)
  • The Haber Synthesis of ammonia
  • Hydrogenation of oils (making margarine)
  • Production of methanol
• Energy:
  • Fuel cells (clean energy source, only water as product)
  • Rocket fuel (liquid hydrogen + liquid oxygen)
• Laboratory:
  • Reducing agent in metallurgy
  • Carrier gas in gas chromatography

Typical Exam Board Requirements

Student Exam Tips

• Always write H₂, not H when referring to hydrogen gas.
• Pop test: Mention “squeaky pop” and explain it shows hydrogen reacting explosively with oxygen.
• Industrial manufacture: Be precise—steam reforming is the main method, not electrolysis (electrolysis is too expensive for large-scale use).
• Uses: Link to syllabus keywords (Haber process, hydrogenation, fuel cells).
• Equations: Balance them correctly (common exam mark lost here).
• Collection method: State upward delivery or over water clearly.

Common Misconceptions

•  Thinking hydrogen is collected by downward delivery (it’s lighter than air, so upward delivery is correct).
•  Believing hydrogen is soluble in water (it is almost insoluble).
•  Confusing hydrogen with helium (both are light gases, but helium is inert and non-flammable).
•  Assuming electrolysis is the main industrial method (actually steam reforming of methane).
•  Forgetting hydrogen is diatomic (H₂, not H).