Source: Lecture notes / slide deck
Tags: orbital hybridisation, sp3, sp2, sp, hybrid orbitals, s orbital, p orbital, electron configuration, carbon, mixing orbitals
Difficulty: Intermediate. Prerequisites: electron configurations (1s, 2s, 2p notation), understanding of atomic orbitals (s and p shapes), and VSEPR theory.
VSEPR tells you what shape a molecule will adopt, but hybridisation explains why it adopts that shape at the orbital level. Before carbon can form four equivalent bonds in methane, its orbitals need to mix (hybridise) into a new set of equivalent orbitals. This concept underpins almost everything in organic chemistry, because carbon's hybridisation state determines bond angles, bond lengths, and the types of bonds (sigma or pi) a molecule can form. If VSEPR is the "what," hybridisation is the "how."
Hybridisation is the mixing of atomic orbitals (s and p) on an atom to create new, equivalent hybrid orbitals suited for bonding. sp3 hybridisation mixes one s and three p orbitals to give four equivalent orbitals (tetrahedral, 109.5 degrees). sp2 mixes one s and two p orbitals for three equivalent orbitals (trigonal planar, 120 degrees), leaving one unhybridised p orbital for pi bonding. sp mixes one s and one p for two equivalent orbitals (linear, 180 degrees), leaving two unhybridised p orbitals.
Hybridisation (orbital hybridisation)
The process of mixing atomic orbitals on the same atom to produce new, equivalent hybrid orbitals. Think of it as blending different-shaped containers into identical new containers that are better suited for forming bonds.
sp3 hybridisation
The mixing of one s orbital and three p orbitals to produce four equivalent sp3 hybrid orbitals. Each sp3 orbital is 25% s character and 75% p character. Results in a tetrahedral arrangement with 109.5 degree bond angles.
sp2 hybridisation
The mixing of one s orbital and two p orbitals to produce three equivalent sp2 hybrid orbitals. Each sp2 orbital is 33% s character and 67% p character. Results in a trigonal planar arrangement with 120 degree bond angles. One unhybridised p orbital remains, available for pi bonding.
sp hybridisation
The mixing of one s orbital and one p orbital to produce two equivalent sp hybrid orbitals. Each sp orbital is 50% s character and 50% p character. Results in a linear arrangement with 180 degree bond angles. Two unhybridised p orbitals remain.
Unhybridised p orbital
A p orbital that was not included in the hybridisation process and remains in its original form. These orbitals are available for forming pi bonds by overlapping side-on with another unhybridised p orbital.
Electron configuration (valence shell)
The arrangement of electrons in the outermost (valence) energy level of an atom. For carbon: 2s2 2p2. Hybridisation rearranges these electrons into the hybrid orbitals before bonding occurs.
Carbon's ground-state electron configuration is 2s2 2p2 (two electrons in the 2s orbital, two in 2p orbitals, one p orbital empty)
In this ground state, carbon would only be able to form two bonds (using the two half-filled p orbitals)
To form four equivalent bonds (as in methane), the s and p orbitals must mix before bonding occurs
Hybridisation always happens before bonding, not during or after
All available orbitals are mixed, even if one of them (like 2pz) was originally empty
One 2s orbital + three 2p orbitals mix to form four equivalent sp3 orbitals
Each sp3 orbital has parallel electron spins (one electron per orbital, following Hund's rule)
Each sp3 orbital is 25% s character and 75% p character
Shape: longer, lobe-shaped orbitals arranged tetrahedrally
The four sp3 orbitals form four equivalent sigma bonds with four hydrogen 1s orbitals
Carbon has a stronger pull on bonding electrons than hydrogen because carbon's valence shell is at a higher energy level
One 2s orbital + two 2p orbitals mix to form three equivalent sp2 orbitals
The 2pz orbital remains unhybridised and empty (available for pi bonding)
The 2py orbital also stays in its original state (it was not mixed)
Each sp2 orbital is 33% s character and 67% p character
Shape: shorter and squatter than sp3 orbitals, arranged in a trigonal planar geometry (120 degree angles)
sp2 orbitals can form both single bonds and contribute to double bonds
In ethylene, each carbon uses its three sp2 orbitals to form sigma bonds (two C-H and one C-C), while the unhybridised p orbital forms the pi bond between the two carbons
Nitrogen has 5 valence electrons (electron configuration: 1s2 2s2 2p3)
N2 has a triple bond: one sigma bond and two pi bonds
The two pi bonds require two unhybridised p orbitals per atom
Therefore, only the 2s and one 2p orbital hybridise, forming two sp orbitals per nitrogen
Each sp orbital is 50% s character and 50% p character
Shape: oblong, mushroom-like, more elongated than sp2 or sp3
One sp orbital forms the sigma bond between the two nitrogen atoms; the other holds the lone pair
The two remaining unhybridised p orbitals on each nitrogen form the two pi bonds
sp3: longest, most lobe-shaped
sp2: shorter and squatter than sp3
sp: oblong, mushroom-like shape, most elongated along the bonding axis
As s character increases (sp3 to sp2 to sp), the orbital becomes shorter and fatter
Hybridisation | Orbitals Mixed | Hybrid Orbitals Produced | Unhybridised p Orbitals | s Character | p Character | Geometry | Bond Angle | Example |
|---|---|---|---|---|---|---|---|---|
sp3 | 1s + 3p | 4 | 0 | 25% | 75% | Tetrahedral | 109.5° | CH4 |
sp2 | 1s + 2p | 3 | 1 | 33% | 67% | Trigonal planar | 120° | C2H4 |
sp | 1s + 1p | 2 | 2 | 50% | 50% | Linear | 180° | N2 |
The quick rule: the number after "sp" tells you how many p orbitals were mixed in. sp3 = 3 p orbitals mixed, sp2 = 2, sp = 1.
Hybridisation determines bond lengths and strengths, which directly affects a molecule's reactivity. The sp2 hybridised carbons in ethylene make it reactive at the double bond site, which is the basis for polymerisation (how polyethylene plastic is made). Understanding hybridisation also explains why diamond (all sp3 carbon) and graphite (all sp2 carbon) have such different physical properties despite both being pure carbon.
Students often think hybridisation happens during bonding. It does not. Hybridisation occurs before bonding, as a preparatory rearrangement of orbitals on the atom.
Students sometimes believe that an empty p orbital is not mixed during hybridisation. All orbitals of the correct type are mixed, regardless of whether they contain electrons. In methane, the empty 2pz is mixed along with the filled 2s, 2px, and 2py.
Students frequently confuse the number of hybrid orbitals with the number of bonds. The number of hybrid orbitals equals the number of orbitals mixed, but the bonds formed also include pi bonds from unhybridised p orbitals.
Students may assume that sp3, sp2, and sp orbitals all look the same. They do not. The shape changes as the ratio of s to p character changes.
Be able to determine hybridisation from a Lewis structure: count the number of electron groups (bonding groups + lone pairs) around the central atom. 4 = sp3, 3 = sp2, 2 = sp.
Know the s and p character percentages for each hybridisation type.
Understand which orbitals remain unhybridised and what they do (form pi bonds).
Be prepared to draw energy-level diagrams showing the mixing process.
Know how hybridisation relates to bond angles and molecular geometry.
Fill in the blank: sp2 hybridisation produces ______ equivalent hybrid orbitals. (Three)
True or false: Hybridisation occurs after bonds are formed. (False, it occurs before bonding.)
Fill in the blank: An sp orbital is ______% s character and ______% p character. (50%, 50%)
True or false: In sp2 hybridisation, all three p orbitals are mixed with the s orbital. (False, only two p orbitals are mixed; one remains unhybridised.)
Fill in the blank: The number of unhybridised p orbitals in sp hybridisation is ______. (Two)
Q: What is the hybridisation of carbon in methane, and how do you determine it?
A: sp3. Carbon has four electron groups (four C-H bonds, no lone pairs), so it mixes one s orbital and three p orbitals to form four equivalent sp3 hybrid orbitals.
Q: Why does carbon need to hybridise before forming bonds in methane?
A: In its ground state (2s2 2p2), carbon has only two unpaired electrons and could only form two bonds. By hybridising into four sp3 orbitals (each with one electron), carbon can form four equivalent bonds.
Q: What is the hybridisation of each carbon in ethylene (C2H4)?
A: sp2. Each carbon has three electron groups (two C-H bonds and one C-C double bond counted as one group). One s and two p orbitals mix to form three sp2 orbitals; the remaining unhybridised p orbital forms the pi bond.
Q: In N2, why is the hybridisation sp rather than sp2 or sp3?
A: Nitrogen in N2 forms a triple bond (one sigma and two pi bonds) and has one lone pair. The sigma bond and lone pair require two hybrid orbitals, so only one s and one p orbital are mixed (sp). The two remaining unhybridised p orbitals form the two pi bonds.
Q: How does the s character of a hybrid orbital affect its shape?
A: More s character makes the orbital shorter and more spherical. sp3 (25% s) is the longest and most lobe-shaped, sp2 (33% s) is shorter and squatter, and sp (50% s) is the most oblong and mushroom-like.
Hybridisation connects directly to sigma and pi bonding, because the type of hybridisation determines how many of each bond type an atom can form. It also connects to VSEPR theory, since the number of hybrid orbitals matches the number of electron groups VSEPR counts. Later topics like stereochemistry, conjugation, and aromaticity all depend on understanding sp2 and sp hybridisation.
Orbital hybridisation, hybrid orbitals, sp3, sp2, sp, s orbital, p orbital, orbital mixing, electron configuration, valence shell, carbon hybridisation, tetrahedral, trigonal planar, linear geometry, Hund's rule, parallel spins, unhybridised p orbital, bond angles, 109.5, 120, 180 degrees, methane CH4, ethylene C2H4, dinitrogen N2, s character, p character, organic chemistry orbitals