sp2 hybridization is the mixing of one s orbital and two p orbitals on an atom to form three equivalent hybrid orbitals arranged 120° apart in a trigonal planar shape, leaving one unhybridized p orbital available to form a pi bond.
sp2 hybridization happens when an atom mixes one s orbital with two p orbitals to make three identical hybrid orbitals. Each one is 33% s character and 67% p character, and they spread out as far from each other as possible, landing 120° apart in a flat, trigonal planar arrangement. The quick mental shortcut is to count regions of electron density around the atom. Three regions (any combo of bonds and lone pairs, where a double or triple bond counts as ONE region) means sp2.
Here's the part that connects everything in Topic 2.7. Since only two of the three p orbitals got mixed in, one p orbital is left over, unhybridized and sticking out perpendicular to the plane. That leftover p orbital is what overlaps side-by-side with a neighboring atom's p orbital to form a pi bond. So when you see a carbon with a double bond, like in ethene or formaldehyde, you're looking at an sp2 atom. The hybrid orbitals handle the three sigma bonds in the plane, and the unhybridized p orbital handles the pi bond.
sp2 hybridization lives in Topic 2.7 (VSEPR and Bond Hybridization) in Unit 2: Compound Structure and Properties, supporting learning objective 2.7.A. The CED asks you to connect Lewis diagrams and VSEPR theory to predict molecular geometry, bond angles, and bond order, and hybridization is the orbital-level explanation behind those predictions. VSEPR tells you electron pairs repel each other (EK 2.7.A.1) and spread out, and sp2 hybridization is the orbital picture of three electron regions doing exactly that at 120°. It's also your bridge to explaining double bonds: sp2 atoms are the ones with a leftover p orbital for pi bonding, which feeds directly into bond order, bond energy, and resonance arguments later in Unit 2.
Keep studying AP Chemistry Unit 2
Pi Bond (Unit 2)
sp2 hybridization and pi bonds come as a package deal. The unhybridized p orbital left over after sp2 mixing is exactly what overlaps sideways to make a pi bond, so an atom with a double bond is almost always sp2.
Molecular Geometry (Unit 2)
Three sp2 orbitals pointing 120° apart give trigonal planar geometry when all three regions are bonds. Add a lone pair and the molecule bends, but the electron geometry stays trigonal planar. Hybridization and VSEPR are two languages describing the same shape.
Resonance Structures (Unit 2)
Resonance requires delocalized pi electrons, and delocalized pi electrons require lined-up unhybridized p orbitals. That's why every atom in a resonance system like the carbonate ion or benzene is sp2. The flat geometry isn't a coincidence, it's the whole mechanism.
sp3 Hybridization (Unit 2)
Same idea, one more p orbital in the mix. Four electron regions give sp3 (109.5°, tetrahedral) with no leftover p orbital, which is why sp3 atoms only form sigma bonds while sp2 atoms can form one pi bond.
This is multiple-choice territory. Typical stems describe a central atom (for example, 'sp2 hybridized with three bonding pairs and no lone pairs') and ask you to name the molecular geometry, or they flip it and describe geometry or bond angles and ask you to name the hybridization. You may also get a lineup of molecules and have to spot which one has an sp2 central atom. The skill being tested is fast translation between four representations: Lewis diagram, number of electron regions, hybridization label, and geometry with bond angles. No released FRQ asks for the word 'sp2' by itself, but FRQs on bonding regularly ask you to explain geometry, bond angles, or sigma/pi bonding, and hybridization is the vocabulary that makes those explanations work. Memorize the chain: 3 regions → sp2 → trigonal planar → 120° → one unhybridized p orbital → can form one pi bond.
Count the electron regions, not the bonds. sp2 means three regions of electron density (trigonal planar, 120°), while sp3 means four regions (tetrahedral, 109.5°). The trap is double bonds. A double bond is two bonds but only ONE electron region, so the carbon in ethene (H₂C=CH₂) is sp2, not sp3. The other big difference is the leftover orbital. sp2 keeps one unhybridized p orbital for pi bonding, while sp3 uses all three p orbitals, so sp3 atoms form only sigma bonds.
sp2 hybridization mixes one s orbital and two p orbitals into three equivalent hybrid orbitals arranged 120° apart in a trigonal planar plane.
An atom is sp2 hybridized when it has exactly three regions of electron density, and a double bond counts as just one region.
sp2 hybridization leaves one unhybridized p orbital perpendicular to the plane, and that orbital is what forms a pi bond.
If all three regions are bonding pairs, the molecular geometry is trigonal planar with 120° bond angles, like in BF₃ or formaldehyde.
Every atom in a resonance system, like carbonate or benzene, is sp2, because delocalized pi electrons need aligned unhybridized p orbitals.
On the exam, translate quickly in both directions: 120° angles or trigonal planar shape means sp2, and sp2 means three electron regions.
It's the mixing of one s orbital and two p orbitals into three equivalent hybrid orbitals, each 33% s and 67% p character, arranged 120° apart in a trigonal planar shape. It shows up in Topic 2.7 alongside VSEPR theory.
No. A double bond (or triple bond) counts as one region of electron density. That's why the carbons in ethene have three regions each and are sp2, not sp3, even though they form four total bonds.
sp2 comes from three electron regions and gives trigonal planar geometry with 120° angles, while sp3 comes from four regions and gives tetrahedral geometry with 109.5° angles. Also, sp2 atoms keep one unhybridized p orbital for pi bonding; sp3 atoms have none.
Only two of the three p orbitals get mixed into the hybrids, so one p orbital is left unhybridized and perpendicular to the molecular plane. That leftover p orbital overlaps side-by-side with a neighboring p orbital to form the pi bond of a double bond.
Yes. It's part of Topic 2.7 (VSEPR and Bond Hybridization) under learning objective 2.7.A. It's tested mostly in multiple choice, where you match hybridization to geometry, bond angles, or specific molecules like BF₃.