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A molecular formula gives atom totals; a structural formula shows connectivity; a displayed formula shows every atom and bond. Skeletal notation omits carbon symbols at vertices and line ends, and omits hydrogens attached to carbon. Add enough hydrogens to give each carbon four bonds. A homologous series shares a functional group and general pattern, with successive members differing by CH2. Structural isomers share a molecular formula but differ in connectivity, including chain, positional and functional-group isomerism.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Interpret displayed, structural and skeletal formulae
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is shared by structural isomers?
Molecular formula. Their atoms are connected differently.
After watching 2: What is formed by homolytic bond fission?
Radicals. Each fragment receives one bonding electron.
What you'll be able to do
A molecular formula gives atom totals; a structural formula shows connectivity; a displayed formula shows every atom and bond. Skeletal notation omits carbon symbols at vertices and line ends, and omits hydrogens attached to carbon. Add enough hydrogens to give each carbon four bonds. A homologous series shares a functional group and general pattern, with successive members differing by CH2. Structural isomers share a molecular formula but differ in connectivity, including chain, positional and functional-group isomerism.
Choose the longest suitable carbon chain containing the principal functional group, number it to give that group an appropriate low locant, and identify substituents and unsaturation. Distinguish an alcohol suffix from a halogen prefix. A branched substituent does not necessarily increase the length of the chosen parent chain. When drawing an isomer, check the total atom count and avoid counting the same connectivity twice after rotating or flipping the drawing. A name is a compact description of structure, not a label to memorise in isolation.
Homolytic bond fission splits a pair, forming radicals; heterolytic fission gives both electrons to one atom or group. A nucleophile donates an electron pair, while an electrophile accepts a pair. A full curly arrow starts at a bond or lone pair and ends where that electron pair forms a bond or becomes a lone pair. A radical mechanism uses single-headed arrows for single electrons. Include intermediate charges and regenerate any catalyst; an arrow from a positive charge alone does not identify the electrons being moved.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Where must the curly arrow start when OH⁻ attacks an electron-deficient carbon?
At the nucleophile’s lone pair, not its negative-charge symbol alone.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.