Raven writes two formulas on a card and props it on the Bench: H2O and H2O2.
"Same letters," Rocket says. "One extra 2. Big deal."
Raven taps the card. "What do you notice? Which atom got the extra one?"
"Oxygen," Rocket says. "Two hydrogen and two oxygen." He adds a big white bean to his water model.
Nova hovers above the two models. "One of these you drink every day," she says. "The other you never would. Same atoms, different substance."
Rocket looks from one to the other. "So the recipe matters. Change one number and you get something else."
"That is why chemists write formulas so carefully," Raven says. "Every letter and every little number means something."
A chemical formula shows the elements in a compound and the relative proportions of those elements.
A molecular formula shows the kinds and numbers of atoms present in one molecule. Each element gets its symbol: H for hydrogen, O for oxygen, C for carbon.
The number of atoms of each kind is written as a small number after the symbol. If there is only one atom of a kind, no number is written.
So H2O means two hydrogen atoms and one oxygen atom. CO2 means one carbon atom and two oxygen atoms.
Water is composed of hydrogen and oxygen in a two to one ratio. Every water molecule follows that exact recipe.
| Formula | Name | Atoms in one molecule | Total atoms |
|---|---|---|---|
| H2O | water | 2 hydrogen, 1 oxygen | 3 |
| H2O2 | hydrogen peroxide | 2 hydrogen, 2 oxygen | 4 |
| CO2 | carbon dioxide | 1 carbon, 2 oxygen | 3 |
| O2 | oxygen gas | 2 oxygen | 2 |
| O3 | ozone | 3 oxygen | 3 |
Water is H2O. Hydrogen peroxide is H2O2. Both are made of only hydrogen and oxygen atoms, yet they are different substances.
Oxygen gas is O2 and ozone is O3. Both are made of only oxygen atoms. Two atoms make one substance, and three make another.
A compound is a substance that contains two or more elements chemically combined in a fixed proportion.
In methane there are always four times as many hydrogen particles as carbon particles. Change the proportion and it is no longer methane.
That is the difference from last week's mixtures. A mixture can have any proportion. A compound has one fixed recipe.
Not every solid is made of separate molecules. Some solids are extended structures with repeating units, such as crystals.
Table salt is a compound of equal parts of the elements sodium and chlorine. Its formula is written NaCl, one sodium for every chlorine.
In a salt crystal, sodium and chlorine atoms repeat in a pattern that goes on and on. There is no single salt molecule to point to.
Diamond is another example of an extended structure, a pattern that repeats through the whole crystal.
| Statement | True or false? |
|---|---|
| H2O and H2O2 are the same substance. | ? |
| If no number is written after a symbol, there is one atom of that element. | ? |
| Table salt is made of separate NaCl molecules. | ? |
| A compound has a fixed proportion of its elements. | ? |
| Ozone has three oxygen atoms. | ? |
Sharp reading. Tomorrow is Bench Lab: you build a whole set of molecules and a salt crystal from kitchen objects.