Difficulty: Foundational | Prerequisites: Atomic orbitals, electron configuration, Lewis structures (general chemistry).
These four topics form the structural toolkit of organic chemistry. Hybridisation tells you the shape around each atom. VSEPR refines that shape by accounting for lone pairs. Resonance explains why some molecules are more stable than you would predict from a single Lewis structure. Functional groups are the vocabulary for the rest of the course: every reaction type you will study is defined by which functional group is involved. If you are behind, this is the place to catch up before reaction mechanisms begin.
Hybridisation (sp, sp2, sp3) tells you the geometry around each atom based on its electron domain count. VSEPR extends this by distinguishing between bonding pairs and lone pairs to predict the actual molecular shape. Resonance shows how electrons can be delocalised across a molecule, and molecules with more resonance structures tend to be more stable. Functional groups are the reactive units of organic molecules, and you need to recognise them on sight because the entire rest of the course is organised around them.
Hybridisation (orbital hybridisation)
The mixing of atomic orbitals on an atom to form new hybrid orbitals that better explain the observed geometry. Think of it as the atom reshuffling its orbitals to make room for the bonds it needs to form.
Electron domain (electron group)
Any region of electron density around a central atom: a lone pair, a single bond, a double bond, or a triple bond each count as one electron domain. In simple terms, count each "blob" of electrons, regardless of how many bonds it contains.
sp hybridisation
Two electron domains, producing a linear geometry (180°). Found in alkynes and molecules with two groups around a central atom.
sp2 hybridisation
Three electron domains, producing a trigonal planar geometry (120°). Found in alkenes, carbonyls, and aromatic rings.
sp3 hybridisation
Four electron domains, producing a tetrahedral geometry (109.5°). Found in alkanes, amines, and most atoms with no double or triple bonds.
VSEPR (Valence Shell Electron Pair Repulsion)
A model that predicts molecular geometry by assuming electron groups repel each other and arrange themselves as far apart as possible. In simple terms, electrons push each other away, and the shape of the molecule is a consequence of that repulsion.
Molecular geometry vs electron-pair geometry
Electron-pair geometry describes the arrangement of all electron domains (including lone pairs). Molecular geometry describes only the positions of the atoms. The two are different whenever lone pairs are present.
Resonance
A way of representing the delocalisation of electrons within a molecule using two or more Lewis structures (resonance structures). The real molecule is a weighted average of all contributing structures. Think of it as the molecule being a blend of several drawings, not any single one.
Functional group
A specific arrangement of atoms within a molecule that determines its chemical reactivity. In simple terms, the functional group is the "business end" of the molecule, the part that reacts.
The hybridisation of an atom is determined by how many electron domains surround it. Each of the following counts as exactly one electron domain: a lone pair, a single bond, a double bond, a triple bond.
Electron domains | Hybridisation | Geometry | Bond angle |
|---|---|---|---|
2 | sp | Linear | 180° |
3 | sp² | Trigonal planar | 120° |
4 | sp³ | Tetrahedral | 109.5° |
5 | sp³d | Trigonal bipyramidal | 90°/120° |
6 | sp³d² | Octahedral | 90° |
In organic chemistry, sp, sp², and sp³ are by far the most common. sp³d and sp³d² appear mainly in inorganic or expanded-octet contexts.
A quick worked example: in an amino acid, the carbonyl carbon (C=O) has three electron domains (double bond to O, single bond to N or OH, single bond to the alpha carbon), so it is sp². The alpha carbon, with four single bonds, is sp³.
VSEPR takes the electron-domain count one step further by distinguishing bonding pairs from lone pairs. The electron-pair geometry is set by the total domain count, but the molecular geometry is determined by the positions of atoms only.
Domains | Bonds | Lone pairs | Electron-pair geometry | Molecular geometry | Angle |
|---|---|---|---|---|---|
2 | 2 | 0 | Linear | Linear | 180° |
2 | 1 | 1 | Linear | Linear | n/a |
3 | 3 | 0 | Trigonal planar | Trigonal planar | 120° |
3 | 2 | 1 | Trigonal planar | Bent | ~118° |
3 | 1 | 2 | Trigonal planar | Linear | n/a |
4 | 4 | 0 | Tetrahedral | Tetrahedral | 109.5° |
4 | 3 | 1 | Tetrahedral | Trigonal pyramidal | ~107° |
4 | 2 | 2 | Tetrahedral | Bent | ~104.5° |
4 | 1 | 3 | Tetrahedral | Linear | n/a |
The key insight: lone pairs take up more space than bonding pairs, so they compress bond angles slightly below the ideal values.
Resonance structures are different Lewis structures for the same molecule that differ only in the placement of electrons (not atoms).
The real molecule is a hybrid (blend) of all valid resonance structures.
Molecules with more resonance possibilities tend to be more stable, because the electron density is spread out (delocalised) over a larger area.
When drawing a resonance structure, move lone pairs or pi electrons, never sigma bonds. Use curved arrows to show electron movement from electron-rich to electron-poor sites.
Charge separation in a resonance structure (one atom gaining a formal positive charge, another gaining a formal negative charge) is acceptable if it follows the rules of electron movement.
These are the reactive units you need to recognise on sight:
Ether: An oxygen atom bonded to two carbon groups (C-O-C). No carbonyl, no hydroxyl.
Ketone: A carbonyl group (C=O) bonded to two carbon substituents. The carbon on each side is a carbon, not a hydrogen.
Aldehyde: A carbonyl group (C=O) with at least one hydrogen substituent directly on the carbonyl carbon.
Carboxylic acid: A carbonyl group (C=O) with a hydroxyl group (-OH) on the same carbon. Written as -COOH.
Ester: A carbonyl group (C=O) bonded to an oxygen, which is in turn bonded to a carbon substituent (C-O-C=O pattern). Think of it as a carboxylic acid where the H of the -OH has been replaced by a carbon group.
Amine: A nitrogen atom bonded to carbon groups and/or hydrogens (NR₃ pattern, where R can be H or carbon chains). The nitrogen carries a lone pair.
Amide: A carbonyl group (C=O) bonded to a nitrogen (NR₂ group). Think of it as a carboxylic acid derivative where the -OH has been replaced by -NR₂.
Functional group recognition is not just an exam skill. Medicinal chemists scan molecular structures for functional groups to predict reactivity, solubility, and how a drug will be metabolised. Esters, for instance, are commonly used as prodrugs because the body's esterases cleave them to release the active compound.
Students often think a double bond counts as two electron domains. It does not. A double bond is one electron domain, a triple bond is one electron domain. Count the connections, not the electrons.
Students confuse electron-pair geometry with molecular geometry. A molecule with four electron domains but one lone pair (e.g. ammonia) has a tetrahedral electron-pair geometry but a trigonal pyramidal molecular geometry. Exams will test whether you know the difference.
Students sometimes think resonance structures are different molecules that the compound flickers between. They are not. The molecule does not switch back and forth. The real structure is a single, blended hybrid of all the resonance contributors.
Students mix up ketones and aldehydes. The distinguishing feature is whether the carbonyl carbon is bonded to at least one hydrogen (aldehyde) or only to carbons (ketone). Check what is directly attached to the C=O carbon.
Students sometimes forget that an ester is not the same as a carboxylic acid. In an ester, the hydrogen of the -OH group has been replaced by a carbon group. If there is still an -OH directly attached to the carbonyl carbon, it is a carboxylic acid.
⚠️ "Determine the hybridisation of atom X" is a standard exam question. Count electron domains, state the hybridisation. Fast, mechanical, worth easy marks.
⚠️ VSEPR geometry questions will ask you to name the molecular geometry (not just the electron-pair geometry). Lone pairs change the answer.
⚠️ Drawing valid resonance structures is tested frequently. You must move electrons (lone pairs or pi bonds), not atoms, and you must use curved arrows.
⚠️ Functional group identification is foundational: expect a structure and the instruction "identify all functional groups present." If you cannot do this quickly, every later topic will be harder.
⚠️ Distinguishing between closely related functional groups (ketone vs aldehyde, carboxylic acid vs ester, amine vs amide) is a favourite exam question.
Fill in the blank: A carbon with three electron domains is ______ hybridised. (sp²)
True or false: A double bond counts as two electron domains. (False. It counts as one.)
Fill in the blank: A molecule with four electron domains and one lone pair has a ______ molecular geometry. (trigonal pyramidal)
True or false: Resonance structures differ in the positions of atoms. (False. They differ only in the placement of electrons.)
Fill in the blank: A carbonyl group bonded to a nitrogen is called an ______. (amide)
Q: A nitrogen atom in an amine has three bonds to other atoms and one lone pair. What is its hybridisation, and what is the molecular geometry around it?
A: sp³ hybridisation (four electron domains: three bonds plus one lone pair). The molecular geometry is trigonal pyramidal.
Q: A carbon atom is double-bonded to an oxygen and single-bonded to an -OH group and a -CH₃ group. What functional group is this? What is the hybridisation of the carbonyl carbon?
A: This is a carboxylic acid. The carbonyl carbon has three electron domains (one double bond, two single bonds), so it is sp².
Q: Draw a valid resonance structure for a molecule where a nitrogen lone pair is adjacent to a carbonyl group (as in an amide). What happens to the formal charges?
A: The nitrogen lone pair donates into the C=O pi system: the nitrogen gains a formal positive charge (it has shared its lone pair into a bond), and the oxygen gains a formal negative charge (it now holds the electrons that were in the C=O pi bond as a lone pair). The C-N bond becomes a double bond, and the C=O double bond becomes a single bond.
Q: What is the difference between molecular geometry and electron-pair geometry for water (H₂O)?
A: Water has four electron domains (two bonding pairs, two lone pairs), so the electron-pair geometry is tetrahedral. The molecular geometry, which counts only atom positions, is bent (~104.5°).
Q: You see a molecule with a C=O group bonded to an -OCH₃ on one side and a -CH₃ on the other. Is this an ester, a carboxylic acid, or a ketone?
A: This is an ester. The carbonyl carbon is bonded to an oxygen that leads to a carbon group (-OCH₃), which is the defining pattern of an ester.
Hybridisation connects directly to stereochemistry: only sp³ carbons can be stereocenters, because sp² and sp carbons are planar. VSEPR geometry explains why molecules adopt the shapes that dictate their reactivity in later reaction mechanism topics.
Resonance is central to understanding acidity and basicity (a conjugate base stabilised by resonance is a weaker base, meaning its parent acid is stronger), aromatic chemistry, and the stability of reaction intermediates like carbocations and enolates.
Functional groups are the organising principle for the rest of the course. Every major reaction chapter (alcohols, ethers, carbonyls, amines) is built around a specific functional group.
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