Every living thing is made of atoms, and you already know from Electric Charge that atoms hold positive protons and negative electrons. Life uses only a few kinds of atoms, joined in a few kinds of ways.
Key idea
Living things are built mostly of carbon, hydrogen, oxygen and nitrogen, with phosphorus and sulfur (CHNOPS). Atoms inside a molecule share electrons in covalent bonds. Ionic bonds hold oppositely charged ions together, and weak hydrogen bonds attract molecules (or parts of one large molecule) to each other.
Oxygen, carbon, hydrogen and nitrogen make up about 96% of the mass of the human body (about 65%, 18%, 10% and 3%). Much of the oxygen and hydrogen is in water. Carbon is the backbone of life's large molecules because each carbon atom can form four covalent bonds, building chains, branches and rings.
| Bond | What holds it | Example |
|---|---|---|
| Covalent (strong) | Two atoms share electrons | O–H inside water; C–C in sugar |
| Ionic (strong when dry, weaker in water) | A positive and a negative ion attract | Na⁺ and Cl⁻ in salt |
| Hydrogen (weak) | A δ+ H on one molecule and a δ− O or N nearby attract | Between water molecules; between DNA strands |
Why it works
In a water molecule, oxygen pulls the shared electrons closer to itself than hydrogen does. That makes the bond polar covalent: the oxygen end is slightly negative (δ−) and the hydrogen ends slightly positive (δ+). Opposite partial charges on neighboring molecules attract. That attraction is the hydrogen bond. It is weak enough to break and re-form constantly at body temperature, which is exactly why it is useful: DNA's two strands can unzip, and proteins can fold and unfold.
Worked example: Which bond is it?
Name the bond: (a) between the oxygen and a hydrogen in one water molecule; (b) between the oxygen of one water molecule and a hydrogen of the next; (c) between Na⁺ and Cl⁻ in a salt crystal.
- (a) The atoms share electrons inside one molecule: covalent (polar covalent).
- (b) It is an attraction between two different molecules through a δ+ hydrogen and a δ− oxygen: a hydrogen bond.
- (c) Two ions with opposite charges: ionic.
Answer: (a) covalent, (b) hydrogen bond, (c) ionic
Worked example: Count the bonds
How many covalent bonds does each atom usually form: H, O, N, C?
- Each atom shares enough electrons to fill its outer shell.
- Hydrogen forms 1, oxygen 2, nitrogen 3 and carbon 4.
- Check with methane, CH₄: one carbon with four hydrogens, every H with one bond.
Answer: H 1, O 2, N 3, C 4
Common mistake
Thinking a hydrogen bond is a covalent bond to hydrogen. The O–H bond inside water is covalent. A hydrogen bond is the weak attraction between a hydrogen that already belongs to one molecule and an O or N of another molecule (or another part of the same big molecule). Quick check: does it join two separate molecules, or two parts far apart along a chain? Then it is a hydrogen bond.