Chemistry 2e · Chemical Bonding and Molecular Geometry
Covalent Bonding
On this page 9 sections
In 30 seconds
When two atoms have similar electronegativities, neither can pull an electron completely away from the other — so instead of transferring electrons, they share them. That sharing is a covalent bond A bond formed by sharing a pair of electrons between two atoms Full entry →. Each shared pair counts toward both atoms' octets, which is why two chlorine atoms, each needing one electron, form a single bond in Cl₂, and two nitrogen atoms, each needing three, form a triple bond in N₂. Covalent bonding is the glue of the molecular world: water, oxygen, proteins, DNA, plastics, and fuels are all held together by shared electron pairs. This topic covers what covalent bonds are, how bond order Number of shared electron pairs between two atoms (1, 2, or 3) Full entry → changes strength and length, how electronegativity An atom's ability to attract shared electrons makes bonds polar, and coordinate covalent bonds.
Why this matters
Most of the chemistry of living things is covalent chemistry. Water's ability to dissolve, transport, and stabilize biomolecules; the strength of the O₂ bond that respiration must break; the folding of proteins; the pairing of DNA strands — all trace back to covalent bonds and their polarity. In health care, drug solubility and membrane permeability depend on whether molecules are polar or nonpolar, and blood gas transport (O₂ and CO₂) involves covalent molecules moving through the body. On exams, covalent bonding is the foundation for Lewis structures, molecular shape, polarity, and nearly every later chapter.
The college version
Core Concepts
Sharing electrons completes octets
In H₂, each hydrogen brings one electron; the shared pair gives each a full duet, matching helium. In Cl₂, each chlorine shares one of its seven valence electrons and counts eight: six lone-pair electrons plus the two shared. A single line between atoms represents one shared pair — one bond.
Bond order: single, double, and triple
Atoms can share more than one pair. Bond order is the number of shared pairs between two atoms: O₂ has a double bond (O=O), N₂ a triple bond (N≡N). More shared pairs mean more electron density between the nuclei, making the bond shorter and stronger: N–N averages about 163 kJ/mol, N=N about 418 kJ/mol, and N≡N about 945 kJ/mol. The Lewis model has limits — it cannot explain why O₂ is paramagnetic — but it remains the standard first tool for counting bonds.
Electronegativity and bond polarity
No two atoms share equally unless they are identical. Electronegativity measures how strongly an atom attracts shared electrons, and the difference ΔEN between bonded atoms decides bond character:
- ΔEN < 0.4: nonpolar covalent bond A bond with equal (or nearly equal) sharing Full entry → (equal sharing, e.g., H–H).
- 0.4 ≤ ΔEN ≤ 1.7: polar covalent bond A bond with unequal sharing, producing δ+ and δ- ends Full entry → (unequal sharing, e.g., H–Cl).
- ΔEN > 1.7: largely ionic (transfer, e.g., Na–Cl).
In a polar bond, the more electronegative atom carries a partial negative charge, δ-, and its partner carries δ+. These partial charges create a bond dipole Separation of charge within a bond or molecule Full entry → — a separation of charge within the bond — which drives solubility, intermolecular forces, and molecular polarity later in the chapter.
Coordinate covalent bonds
In most covalent bonds, each atom contributes one electron to the shared pair. In a coordinate covalent bond A bond in which one atom supplies both shared electrons Full entry →, one atom supplies both. Ammonia has a lone pair A pair of valence electrons not involved in bonding Full entry → on nitrogen; when it reacts with H⁺, nitrogen donates that pair into a new N–H bond, forming NH₄⁺. Once formed, a coordinate bond is indistinguishable from any other covalent bond. The same pattern produces H₃O⁺ when water accepts an H⁺.
Properties of molecular compounds
Covalent compounds form discrete molecules held together by relatively weak intermolecular forces, so they tend to have low melting and boiling points, are often gases or liquids at room temperature, and do not conduct electricity — a sharp contrast with the ionic lattices of Topic 1. Water is a famous exception: its polar O–H bonds allow hydrogen bonding between molecules, raising its boiling point far above what its molar mass suggests.
How It Works / Step-by-Step Process
- Identify the atoms (typically nonmetals) and count valence electrons.
- Decide how many electrons each atom must share to reach an octet (duet for H).
- Draw the structure: shared pairs as lines, lone pairs as dots.
- Classify each bond by ΔEN: nonpolar, polar covalent, or largely ionic.
- Predict physical properties from the molecular model: discrete molecules, weak forces, low melting points.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| "Sharing" meaning equal sharing | Polar covalent sharing | Unequal sharing happens whenever ΔEN > 0; polar bonds have δ+/δ- ends |
| A double bond being exactly twice as strong | A double bond being stronger than a single bond | C=C (614 kJ/mol) beats C–C (348 kJ/mol) but not by double |
| "Covalent compounds conduct when dissolved" | Ionic compounds conduct when dissolved | Sugar dissolves but does not conduct; salts do — they produce ions |
| The Lewis structure being a complete description | A useful model with limits | The Lewis model does not predict O₂'s paramagnetism |
| All bonds being either ionic or covalent | A bonding continuum | Most real bonds are mixed; ΔEN is a guideline |
| A coordinate bond being special forever | An ordinary bond after formation | Once formed, it is identical to any other covalent bond |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Two friends each want a full set of marbles, but neither can take the other's without a fight — so they agree to share, and both feel complete. A covalent bond is exactly that: two atoms share a pair of electrons, sometimes two pairs or even three, so each counts a full outer shell.
Worked example
Example 1: Classifying bonds with electronegativity differences
Use Pauling electronegativities (H = 2.20, Cl = 3.16, Na = 0.93) to classify three bonds.
For H–H:
ΔEN = 2.20 - 2.20 = 0.00
so the bond is nonpolar covalent — identical atoms share equally.
For H–Cl:
ΔEN = 3.16 - 2.20 = 0.96
so the bond is polar covalent, with δ- on chlorine and δ+ on hydrogen.
For Na–Cl:
ΔEN = 3.16 - 0.93 = 2.23
so the bond is largely ionic — chlorine essentially takes the electron. Together the three examples span the continuum from pure sharing to near-complete transfer.
Example 2: Bond order, strength, and reactivity in nitrogen
Compare the nitrogen–nitrogen bonds in three molecules: hydrazine, N₂H₄, has an N–N single bond (about 163 kJ/mol); diazene, N₂H₂, an N=N double bond (about 418 kJ/mol); and nitrogen gas, N₂, an N≡N triple bond (about 945 kJ/mol). Each additional shared pair roughly doubles the bond energy and shortens the bond.
The enormous triple-bond energy of N₂ explains a real-world puzzle: nitrogen gas is everywhere in the air, yet it is unreactive at ordinary temperatures because breaking that bond costs so much energy. Nitrogen fixation — converting N₂ into usable fertilizer compounds — therefore demands high temperatures, high pressures, or specialized catalysts, and the human body cannot use atmospheric nitrogen directly.
Key takeaways
- Covalent bonds share electron pairs between nonmetals; each shared pair counts toward both atoms' octets (duet for H).
- Bond order 1, 2, or 3: more shared pairs mean shorter, stronger bonds (N≡N 945 kJ/mol vs N–N 163 kJ/mol).
- ΔEN: <0.4 nonpolar, 0.4–1.7 polar covalent, >1.7 largely ionic — a guideline, not a law.
- Polar bonds carry partial charges: δ- on the more electronegative atom, δ+ on the other.
- Coordinate covalent bonds: one atom donates both electrons (NH₃ + H⁺ → NH₄⁺); the product bond is ordinary afterward.
- Molecular compounds generally have low melting points and do not conduct; hydrogen bonding explains water's high boiling point.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why does nitrogen gas form a triple bond rather than a single bond?
Show answer
Each nitrogen atom has five valence electrons and needs three more to reach an octet, so the atoms share three pairs — a triple bond.
Which bond is more polar, H–F or H–Cl?
Show answer
H–F. Fluorine (3.98) and hydrogen (2.20) differ by ΔEN = 1.78, while H–Cl differs by 0.96; the larger difference means more unequal sharing.
What is unusual about the new N–H bond in NH₄⁺?
Show answer
Nitrogen donates its lone pair to the H⁺ ion — both electrons of the new bond come from nitrogen. It is a coordinate covalent bond.
How many shared pairs hold two oxygen atoms together in O₂?
Show answer
Two shared pairs — a double bond — plus two lone pairs on each oxygen.
Water is covalent yet boils at 100 °C. Does this contradict the low-boiling-point trend for molecular compounds?
Show answer
No. The trend holds for the bonding model; water's unusually high boiling point is explained by strong hydrogen bonding between its polar molecules, an intermolecular effect.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- covalent bond
- A bond formed by sharing a pair of electrons between two atoms
- bond order
- Number of shared electron pairs between two atoms (1, 2, or 3)
- electronegativity
- An atom's ability to attract shared electrons
- polar covalent bond
- A bond with unequal sharing, producing δ+ and δ- ends
- nonpolar covalent bond
- A bond with equal (or nearly equal) sharing
- coordinate covalent bond
- A bond in which one atom supplies both shared electrons
- dipole
- Separation of charge within a bond or molecule
- lone pair
- A pair of valence electrons not involved in bonding
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.