Saturday Breakfast, Atom by Atom
1Odile Brennan, a seventh grader, cooks Saturday breakfast with her grandfather, Mr. Ruben Castellano, a retired cook who keeps every jar and box in the pantry labeled. This week her science class is studying models of atoms, so she made a chart of substances in the kitchen and sketched an atom model of each one. Her younger brother, Anselmo, asked questions the whole time, and she wrote them down to bring to class.
2Many substances are made of molecules. A molecule is a group of atoms joined together, and every molecule of a pure substance has the same kinds and numbers of atoms, joined in the same way. A molecule's formula tells how many of each kind of atom it has. Water, H₂O, has 2 hydrogen atoms and 1 oxygen atom. Some molecules are small, like water or carbon dioxide, CO₂, the gas in soda bubbles. Others are large: one of the main molecules in olive oil, triolein, has the formula C₅₇H₁₀₄O₆.
3Other substances are extended structures. Their atoms are joined in a pattern that repeats in every direction, so there are no separate molecules, and a crystal is one big connected piece. For these substances, the formula gives the ratio of atoms. In table salt, NaCl, there is 1 sodium atom for every chlorine atom. In baking soda, NaHCO₃, there are 1 sodium, 1 hydrogen and 1 carbon atom for every 3 oxygen atoms. Metals, such as the aluminum in foil and the copper on the bottom of a pot, are extended structures made of a single kind of atom.
4In a mixture, each substance keeps its own particles. When sugar is stirred into tea, the sugar molecules spread out among the water molecules but stay whole. In ice, H₂O molecules are held near each other in a repeating pattern, but they are not joined to each other the way the atoms inside a molecule are joined.