Hybridization & Polarity

Where a molecule's shape comes from — and how that shape decides whether the whole molecule is polar, even when its bonds are.

High schoolIntro Gen ChemUni Year 1
⏱️ About 20 min

Carbon dioxide has two strongly polar bonds — yet the molecule as a whole is nonpolar. Water has bonds that are barely different, yet the molecule is one of the most polar substances you'll meet. The twist? It isn't the bonds that decide — it's the shape.

💡
The big idea: Hybridization mixes an atom's orbitals to match its VSEPR shape (sp, sp², sp³). A single BOND is polar when the two atoms differ in electronegativity — but the whole MOLECULE is polar only if those bond dipoles don't cancel. Symmetry can make a molecule of polar bonds add up to zero.
🎯 By the end, you'll be able to
  • Assign sp, sp² or sp³ hybridization from the number of electron domains
  • Decide whether a bond is polar from the electronegativity difference
  • Add bond dipoles as vectors to find a molecule's net dipole
  • Explain why symmetric molecules (CO₂, CCl₄) are nonpolar despite polar bonds
📎 Helpful to know first

Hybridization: orbitals that fit the shape

A carbon atom's pure orbitals (one s, three p) don't point in the right directions to make four identical bonds at 109.5°. So the atom hybridizes: it blends those orbitals into a new set of equivalent ones that aim exactly where the bonds need to go. Hybridization is just the orbital story behind the shape VSEPR already told you.

The count is beautifully simple — the number of electron domains is the hybridization:

🔑 Domains → hybridization
2 domains → sp, linear (180°). 3 domains → sp², trigonal planar (120°). 4 domains → sp³, tetrahedral (109.5°). Count the domains from the Lewis structure and you have both the shape and the hybridization.
\[ \text{domains} = 2 \Rightarrow sp \qquad 3 \Rightarrow sp^2 \qquad 4 \Rightarrow sp^3 \]
The number of electron domains around an atom gives its hybridization directly.

Bond polarity: an unequal tug-of-war

A covalent bond shares a pair of electrons — but not always equally. The more electronegative atom pulls the shared pair toward itself, leaving it slightly negative (δ−) and its partner slightly positive (δ+). That separation of charge makes the bond polar, a little arrow (dipole) pointing toward the electron-hungry atom.

Bigger electronegativity difference → more polar bond. Equal atoms (as in O₂ or N₂) share perfectly evenly, so the bond is nonpolar.

✨ Electronegativity vs electron affinity
Don't confuse the two. Electronegativity is how strongly an atom pulls on electrons it is already sharing in a bond (a comparative scale, no units). Electron affinity is the measured energy change when a free atom gains an electron. Polarity is about electronegativity.

Molecular polarity: do the arrows cancel?

Here's the key move: a molecule's overall polarity is the vector sum of its bond dipoles. Draw each bond's dipole arrow, then add them up like forces.

If the molecule is symmetric, the arrows point in opposing directions and cancel to zero — the molecule is nonpolar even though every bond is polar. If the shape is lopsided (usually because of lone pairs), the arrows don't cancel and the molecule has a net dipole — it is polar.

⚠️ Polar bonds ≠ polar molecule
CO₂ (O=C=O) is linear: its two C=O dipoles point in exactly opposite directions and cancel → nonpolar. CCl₄ is tetrahedral and symmetric → nonpolar. But H₂O is bent, so its two O–H dipoles add to a strong downward resultant → polar. Shape is the deciding factor.
📝 Worked example: Carbon dioxide (CO₂) has two polar C=O bonds. Is the whole molecule polar or nonpolar?
  1. Each C=O bond is polar: oxygen is more electronegative than carbon, so each dipole points from C toward O.
  2. Find the shape: carbon has 2 electron domains (two double bonds, no lone pairs) → linear, O=C=O at 180°.
  3. Add the dipoles as vectors: the two arrows are equal in size and point in exactly opposite directions.
  4. Opposite, equal arrows cancel, so the net dipole is zero.
✓ Nonpolar — the two polar bonds cancel because the molecule is linear and symmetric. (Contrast bent H₂O, whose dipoles don't cancel.)
✏️ Practice: An atom at the centre of a molecule has 3 electron domains and no lone pairs (like the boron in BF₃). How many domains is that, i.e. what number tells you its hybridization is sp²?
domains
Solution
  1. Hybridization comes straight from the domain count.
  2. 3 electron domains means the orbitals blend into three sp² hybrids pointing 120° apart (trigonal planar).
  3. So the answer is 3 domains → sp². (BF₃ is also nonpolar: three identical B–F dipoles at 120° cancel.)

Check your understanding

1. A molecule has polar bonds but a symmetric shape (like CO₂). The molecule is…
Polarity is the vector sum of the bond dipoles. In a symmetric molecule they point in opposing directions and cancel, so the molecule is nonpolar despite its polar bonds.
2. An atom surrounded by four electron domains is hybridized as…
Four domains blend into four sp³ hybrid orbitals pointing toward the corners of a tetrahedron (109.5°).
3. Which best explains why water is polar but carbon dioxide is not?
Both have polar bonds. CO₂ is linear so its dipoles cancel (nonpolar); water is bent so its dipoles add to a net dipole (polar). Shape decides.
🔎 Related lessons
Previous: VSEPR & Molecular Geometry Next: Intermolecular Forces Lewis Structures
✅ Key takeaways
  • Hybridization matches orbitals to shape: 2 domains → sp, 3 → sp², 4 → sp³.
  • A bond is polar when the two atoms differ in electronegativity (δ+ / δ−).
  • A molecule's polarity is the vector sum of its bond dipoles.
  • Symmetric molecules (CO₂, CCl₄, BF₃) are nonpolar even though their bonds are polar — the dipoles cancel.
  • Electronegativity (pull within a bond) is not the same as electron affinity (energy of gaining an electron).
➡️ Now you can label a molecule polar or nonpolar. That single label controls how strongly one molecule tugs on the next — the intermolecular forces that set melting and boiling points, up next.
Want to test yourself on this? Try the Chemistry practice test →