From the previous module you know that atoms are most stable when their outer shell is full, and that they can get there by losing, gaining, or sharing electrons. Each of those three strategies corresponds to a distinct kind of chemical bond. This module walks through all three.
Ionic bonds — the transfer
Established When one atom strongly wants to lose electrons and another strongly wants to gain them, they just transfer. The classic case: sodium (1 valence electron) meets chlorine (7 valence electrons).
Sodium dumps its lone electron onto chlorine. Sodium becomes Na⁺ — a positive ion, because it lost a negative charge. Chlorine becomes Cl⁻ — a negative ion, because it gained one. The positive and negative ions then attract each other electrostatically, and that attraction is the "bond." The result is sodium chloride — table salt.
Ionic bonds are strong, especially in solid form. They form crystals (lots of ions in a 3D lattice), dissolve in water (water molecules separate the ions), and conduct electricity when dissolved because the free-moving charged ions can carry current.
There's no such thing as a discrete "NaCl molecule" in salt. It's a lattice of alternating Na⁺ and Cl⁻ ions extending in all directions. The formula NaCl tells you the ratio, not a molecule.
Covalent bonds — the share
Established When two atoms both want electrons (neither wants to dump them), they share. They overlap their orbitals so that the same pair of electrons "belongs" to both atoms, completing both their octets simultaneously.
Two hydrogen atoms, for example. Each has 1 electron and each needs 2 for a full shell 1. They share their two electrons between them, and each sees two electrons, so both are happy. The result is H₂.
Methane, CH₄, is a richer example. Carbon has 4 valence electrons and needs 4 more. Each hydrogen has 1 and needs 1 more. So carbon shares one electron with each of 4 hydrogens. Carbon now sees 8 electrons (its own 4 plus 4 shared), and each hydrogen sees 2. Everyone's happy.
Variations: a single bond is one shared pair (H-H, C-H). A double bond is two shared pairs (O=O, C=C). A triple bond is three shared pairs (N≡N). More shared pairs make a stronger and shorter bond.
Polar covalent bonds — the unfair share
When two different atoms share electrons, they often don't share equally. The atom that pulls harder on electrons — the more electronegative one — gets a slightly bigger share. The result is a bond that's covalent but lopsided, with partial charges at each end.
The most important example is water, H₂O. Oxygen is much more electronegative than hydrogen. The two O-H bonds share electrons, but oxygen pulls them closer to itself. The oxygen end has a slight negative charge (written δ⁻); the hydrogen ends have slight positive charges (δ⁺).
This is why water is polar — it has a positive end and a negative end despite being electrically neutral overall. Almost every weird and wonderful property of water (surface tension, high boiling point, ice floating, ability to dissolve salts) comes from this polarity. The next module (M-Chem-04) develops the consequences.
Metallic bonds (briefly)
Established In metals, atoms pool their valence electrons into a "sea" that flows freely through a lattice of positive ions. This is why metals conduct electricity (free electrons), are malleable (the lattice can slide without breaking bonds), and are shiny (free electrons absorb and re-emit light easily).
You won't need this much for biology or the protomolecule, but it's the third major bond type, and you should know it exists.
Magnesium has 2 valence electrons. Oxygen has 6. What kind of bond forms between them and what is the resulting compound?
Show answer
Magnesium dumps 2 electrons; oxygen takes them. You get Mg²⁺ and O²⁻, an ionic compound: MgO, magnesium oxide. It's the white stuff that forms when magnesium burns — and it burns very brightly, which is why old camera flashes used magnesium ribbon.