Hybridization

Carbon has a problem. Its ground-state electron configuration is 1s2 2s2 2p2, which gives it two unpaired electrons in two p orbitals. If carbon only used those unhybridized orbitals, it could only form two bonds - but we know carbon almost always forms four bonds. Something has to change.

The solution is hybridization. The s and p orbitals on carbon mix together to create a new set of equivalent hybrid orbitals, each with the same shape and energy. These hybrids point in specific directions, giving carbon its characteristic geometries: tetrahedral, trigonal planar, or linear.

Toggle sp³ / sp² / sp below to see the hybrid lobes rearrange and the unhybridized p orbitals appear. Bond-angle arcs snap to 109.5°, 120°, and 180° so you can correlate hybridization with geometry directly.

sp3 Hybridization

When one s orbital mixes with three p orbitals, the result is four sp3 hybrid orbitals. These four orbitals are identical in shape and energy, and they point toward the corners of a tetrahedron with 109.5-degree bond angles.

Every sp3 carbon has four regions of electron density (four sigma bonds, or some combination of bonds and lone pairs). No unhybridized p orbitals remain - all p orbitals participated in the mixing.

Common sp3 atoms in organic chemistry:

• Carbon in methane (CH4), ethane (C2H6), and any saturated carbon
• Nitrogen with three bonds and one lone pair (e.g., amines like NH3)
• Oxygen with two bonds and two lone pairs (e.g., water, alcohols)

sp2 Hybridization

When one s orbital mixes with two p orbitals, the result is three sp2 hybrid orbitals plus one unhybridized p orbital. The three sp2 hybrids lie in a plane at 120-degree angles (trigonal planar). The leftover p orbital sticks straight up and down, perpendicular to that plane.

That unhybridized p orbital is not just sitting there doing nothing. It is available for side-by-side overlap with a p orbital on a neighboring atom to form a pi bond. This is why sp2 carbons always participate in double bonds.

Common sp2 atoms:

• Carbon in alkenes (C=C), carbonyls (C=O), and aromatic rings
• The carbon in a carboxylic acid’s C=O
• Nitrogen in imines (C=N)

sp Hybridization

When one s orbital mixes with one p orbital, the result is two sp hybrid orbitals plus two unhybridized p orbitals. The two sp hybrids point in opposite directions at 180 degrees (linear). The two leftover p orbitals are perpendicular to each other and to the sp hybrids.

Those two unhybridized p orbitals can form two pi bonds - one in each perpendicular plane. This is why sp carbons participate in triple bonds (one sigma + two pi) or two cumulated double bonds (allenes).

Common sp atoms:

• Carbon in alkynes (C is identical to C)
• Carbon in nitriles (C is identical to N)
• Carbon in CO2 (O=C=O)

How to Assign Hybridization - The Quick Method

Here is the fastest way to determine any atom’s hybridization in an organic molecule:

Count the number of sigma bonds + lone pairs on the atom. That total equals the number of hybrid orbitals, which tells you the hybridization.

| Sigma bonds + lone pairs | Hybridization | Geometry | Bond angle |
|--------------------------|---------------|----------|------------|
| 4 | sp3 | Tetrahedral | 109.5 degrees |
| 3 | sp2 | Trigonal planar | 120 degrees |
| 2 | sp | Linear | 180 degrees |

Important: count sigma bonds, not total bonds. A double bond counts as one sigma bond (plus one pi bond). A triple bond counts as one sigma bond (plus two pi bonds).

Worked Examples

Methane (CH4): Carbon has four single bonds to hydrogen. Four sigma bonds + zero lone pairs = 4 regions. Hybridization = sp3. Geometry = tetrahedral. Bond angle = 109.5 degrees.

Ethene (H2C=CH2): Each carbon has two C-H sigma bonds + one C=C bond (which is one sigma + one pi). Three sigma bonds + zero lone pairs = 3 regions. Hybridization = sp2. Geometry = trigonal planar. Bond angle = 120 degrees.

Ethyne (HC is identical to CH): Each carbon has one C-H sigma bond + one C triple bond (which is one sigma + two pi). Two sigma bonds + zero lone pairs = 2 regions. Hybridization = sp. Geometry = linear. Bond angle = 180 degrees.

Ammonia (NH3): Nitrogen has three N-H sigma bonds + one lone pair. Three sigma bonds + one lone pair = 4 regions. Hybridization = sp3. Note: the electron geometry is tetrahedral, but the molecular geometry (shape you would see) is trigonal pyramidal because one “arm” of the tetrahedron is an invisible lone pair.

Water (H2O): Oxygen has two O-H sigma bonds + two lone pairs. Two sigma bonds + two lone pairs = 4 regions. Hybridization = sp3. Electron geometry = tetrahedral. Molecular geometry = bent.

The Connection Between Hybridization and Bonding

Hybridization determines three things simultaneously:

1. Geometry - The arrangement of atoms around the central atom
2. Bond angles - How far apart the bonded atoms are
3. Number of pi bonds - How many unhybridized p orbitals remain for pi bonding

This third point is the most important for organic chemistry. Every unhybridized p orbital is available to form a pi bond. If you know the hybridization, you know how many pi bonds that atom can form:

• sp3: zero unhybridized p orbitals, zero pi bonds
• sp2: one unhybridized p orbital, one pi bond maximum
• sp: two unhybridized p orbitals, two pi bonds maximum

Hybridization and Electronegativity

As s character increases, electrons are held closer to the nucleus. An sp orbital has 50% s character. An sp2 orbital has 33% s character. An sp3 orbital has 25% s character.

This means sp-hybridized atoms are effectively more electronegative than sp3-hybridized atoms of the same element. The practical consequence: sp carbons hold onto electrons more tightly than sp3 carbons.

This explains why terminal alkynes (RC is identical to CH) are weakly acidic (pKa around 25), while alkanes (RCH3) are essentially nonacidic (pKa around 50). The sp carbon in the alkyne stabilizes the resulting carbanion much better because it holds the negative charge closer to the nucleus.

| Hybridization | s Character | Relative electronegativity | Example pKa |
|---------------|------------|---------------------------|-------------|
| sp | 50% | Highest | ~25 (terminal alkyne) |
| sp2 | 33% | Middle | ~44 (alkene) |
| sp3 | 25% | Lowest | ~50 (alkane) |