Geometrical isomerism: cis–trans, E–Z notation is a core Organic Chemistry concept in Chemistry. This guide explains what it is, walks through a fully worked example, and lists the key equations you need — with a short quiz to test yourself.

Key equations and worked example

Water (H₂O) is bent: the two O–H bonds make an angle of 104.5°, not 180°, because oxygen's two lone pairs squeeze the bonding pairs together (VSEPR theory). Methane (CH₄) is tetrahedral with 109.5° between every pair of C–H bonds — the angle that keeps four electron pairs as far apart as possible. Switch molecules in the viewer above and compare their shapes.

• <code>VSEPR: 2 pairs → linear (180°)</code>
• <code>3 pairs → trigonal planar (120°)</code>
• <code>4 pairs → tetrahedral (109.5°)</code>
• <code>Lone pairs compress bond angles: NH₃ 107°, H₂O 104.5°</code>

Geometrical isomerism: cis–trans, E–Z notation in detail

Geometrical isomerism: cis–trans, E–Z notation is one of the central ideas in Organic Chemistry, and it appears in Chemistry curricula under Isomerism and Stereochemistry. It is worth learning deeply because it connects to so many other topics in this section.

Molecular shape is decided by electron-pair repulsion: bonding pairs and lone pairs arrange themselves to stay as far apart as possible. Two pairs give a linear shape (CO₂, 180°), three give trigonal planar (120°), four give tetrahedral (109.5°), and lone pairs compress these ideal angles — which is why ammonia (NH₃) is pyramidal at 107° and water is bent at 104.5°. Shape decides polarity, and polarity decides properties like boiling point and solubility.

For exams, the pattern is predictable: first a definition or statement of the result, then a direct numerical application of one of the equations above, then a "why" question — why the formula takes that form, or what changes when a variable is doubled or halved. The worked example and quiz below cover exactly that progression.

Quick self-check:

• Q: Why is water bent instead of linear?<br />A: Oxygen has two lone pairs which repel the two O–H bonding pairs, squeezing the H–O–H angle down to 104.5°.
• Q: What is the bond angle in methane, and why?<br />A: 109.5° — four bonding pairs arrange tetrahedrally to maximise their separation (VSEPR).
• Q: How does molecular shape affect properties?<br />A: Shape decides whether bond dipoles cancel (non-polar, e.g. CO₂) or add up (polar, e.g. H₂O) — polarity then governs boiling point, solubility and reactivity.
• Q: What does VSEPR stand for and what does it predict?<br />A: Valence Shell Electron Pair Repulsion — it predicts molecular geometry from the number of electron pairs around the central atom.