A recipe in a few letters

About this video

A recipe card for pancakes says what goes in and how much of each: 2 eggs, 1 cup of flour, 1 cup of milk. Chemists have a recipe card for substances too, and you have almost certainly seen one: H₂O, the formula for water.

A chemical formula is built from two things. The chemical symbols say which elements: H for hydrogen, O for oxygen, C for carbon. The small numbers, written a little lower, say how many atoms there are of the element just before them. No number means one. So H₂O is two hydrogen atoms and one oxygen atom: exactly one water molecule. The 2 belongs to the hydrogen, not to the oxygen.

Read a few more, one symbol at a time. O₂ is two oxygen atoms, the oxygen molecule in the air. CO₂ is one carbon atom and two oxygen atoms: carbon dioxide, the gas you breathe out. CH₄ is one carbon atom and four hydrogen atoms: methane, the natural gas that burns in many cooking stoves, five atoms in all. Some formulas are long: table sugar is C₁₂H₂₂O₁₁, twelve carbon, twenty-two hydrogen and eleven oxygen atoms, 45 atoms in a single sugar molecule.

One atom makes a big difference. CO₂, carbon dioxide, is the harmless gas in your breath and in the bubbles of fizzy drinks. CO, carbon monoxide, has one oxygen atom fewer, and it is a poisonous gas that can come from a stove or heater that burns badly; that is why many homes have an alarm for it. Which atoms join, and how many of each, decides what a substance is like, and a formula shows that recipe in a few letters.

Transcript

1. 0:00A recipe says what goes in, and how much. Chemists have recipes too.
2. 0:07H two O. The small two counts the hydrogen just before it. No number means one oxygen.
3. 0:17O two is oxygen in the air. C O two, carbon dioxide. C H four, methane, five atoms.
4. 0:28Sugar's formula is long. Count them all, and one sugar molecule has 45 atoms.
5. 0:36Carbon dioxide is in your breath. Lose one oxygen atom, and it is a poison.
6. 0:46A formula is a recipe. Which atoms, and how many.