Atoms, Bonds, and Drawings

Why this rung exists

Every reaction in organic chemistry is electrons moving to a better home. That sentence is useless until three words underneath it mean something physical: electron, bond, and whatever that zigzag line on the page is supposed to be.

So we start here. Nothing on this page assumes you have met chemistry before. Everything on every later page assumes you have met this.

The Electron Ledger

An atom has a heavy centre — the nucleus — and light electrons around it. The nucleus is fixed. It does not change in any reaction in this book. The electrons are the negotiable part, and they are the entire subject.

Electrons sit in layers, and only the outermost layer trades. Those are the valence electrons. Think of each atom as running a small account: it has a certain number of valence electrons, and it wants its outer layer full. For most atoms we care about, full means eight — the octet.

The octet is a bookkeeping rule, not a law of physics. It is astonishingly good at predicting what happens, which is why we use it, and it breaks in places we will name when we get there.

• Spare pair — electrons with no bond to do
• Bond — electrons already committed

Notice the spare pairs on nitrogen and oxygen. They are not spectators. Those uncommitted electrons are the most reactive things in most molecules, because they are the ones free to go somewhere else.

The Shared Pair

An atom short of electrons has two options. It can take them outright, which gives charged particles and is mostly not our subject. Or it can share, and this is what carbon does.

A covalent bond is two electrons sitting between two nuclei, counted by both. Each atom gets to include that pair in its own ledger, so both books balance from one pair of electrons. That double-counting is the trick that makes molecules possible.

• Electrons — the part that moves
• Nuclei — fixed throughout

Two consequences worth carrying upward. First, a bond is a store of energy: the shared position is lower in energy than the separate one, so breaking a bond costs and making one pays. Second, a bond is a place where electrons are — which means a bond can itself be the thing that moves in a reaction, not just the thing that breaks. Almost nobody expects that on first meeting, and it is central later.

Skeleton Shorthand

Drawing every atom of a real molecule is unbearable. A modest fatty acid has fifty-plus atoms, most of them hydrogens doing nothing interesting. So chemists draw almost none of them.

The skeletal convention has four rules, and once you have them you can read any structure in any chemistry book.

• Written explicitly — not carbon
• Implied carbon — never drawn in practice
• Bonds

The hydrogens are recoverable, which is why leaving them out is safe rather than sloppy. Carbon takes four bonds. Count the lines at any corner, subtract from four, and that is how many hydrogens are there. A corner with two lines has two hydrogens. You never have to be told.

Grok check

Prediction, not recall. If the rung is solid you can answer these without looking anything up.

1. A skeletal drawing shows a corner with three lines meeting at it. How many hydrogens are on that carbon, and how did you know without being told?
2. Nitrogen forms three bonds and keeps one spare pair. Predict how many bonds fluorine forms, given that it has seven valence electrons.
3. Breaking a bond costs energy and making one pays. What does that tell you about a reaction that breaks one bond and makes two?
4. Why is a spare pair on oxygen more likely to start a reaction than the electrons in an O–H bond?

Question 4 is the one that matters. It is the first hint of the idea the whole book runs on, and it gets its proper name two rungs up on the curly-arrow alphabet. Next rung: orbitals, shape, and polarity.