Molecules & Bonding

Atoms rarely go it alone. They join together into molecules — and the way they join, the chemical bond, decides almost everything about the substance that results. Water, carbon dioxide, the proteins in your body: all of them are atoms held together by bonds.

Why atoms bond at all

Remember the outer shell from the atoms lesson? Atoms are “happiest” — most stable — when that outer shell is full. Most atoms don’t start out full, so they bond to reach a full shell, either by sharing electrons or by transferring them. That drive toward a full outer shell is the engine behind all of chemistry.

Covalent bonds: sharing

In a covalent bond, two atoms share a pair of electrons, and both get to count the shared pair toward a full shell. Non-metals bonding with non-metals almost always share this way.

• In hydrogen gas (H₂), two hydrogens share one pair.
• In oxygen gas (O₂), two oxygens share two pairs — a double bond.
• In methane (CH₄), one carbon shares with four hydrogens.

A molecule is the name for a group of atoms held together by covalent bonds.

Ionic bonds: transferring

In an ionic bond, one atom hands electrons over to another. A metal like sodium gives up its lone outer electron (becoming a +1 ion); a non-metal like chlorine takes it (becoming a −1 ion). The opposite charges then attract — and that attraction is the bond. Table salt, sodium chloride, is held together this way.

Why water bends

Water is H₂O — one oxygen sharing with two hydrogens. You might expect the three atoms to sit in a straight line, but they don’t: water is bent, at about 104.5°. Why?

Oxygen has two shared pairs (the bonds to hydrogen) and two unshared lone pairs of electrons. All four pairs repel each other and spread out as far as possible. The lone pairs push the two hydrogens down into a V shape.

That bend has enormous consequences. It makes water polar — slightly negative near the oxygen, slightly positive near the hydrogens — which is why water dissolves so many things and why it’s the medium for life.

Rotate some molecules

Drag to rotate, scroll to zoom, and switch between ball-and-stick and space-filling views. Compare bent water with straight-line CO₂, and see methane’s tetrahedral spread.

Vocabulary

Check yourself

We’ve built atoms, arranged them, and bonded them. Last for Phase 1: what happens to whole crowds of these particles when you heat them up — the states of matter.