Molecules and their sizes
Every substance has its own properties: sugar is sweet, salt is salty. However finely we grind sugar, down to a certain size its taste stays. The smallest particle that keeps the properties of a substance is called a molecule. A molecule consists of one or several atoms: a water molecule H₂O has 2 hydrogen atoms and 1 oxygen atom; an oxygen molecule O₂ has 2 oxygen atoms; carbon dioxide CO₂ has 1 carbon atom and 2 oxygen atoms. All molecules of one substance are identical: a molecule of spring water is no different from one of water evaporated from seawater. Molecules cannot be seen with the naked eye or even with the best optical microscope; to estimate their size a drop of oil is placed on a water surface, where it spreads into a layer one molecule thick. If the drop’s volume V and the area S of the spot are known, the thickness is d = V : S — a molecule’s diameter is about 0.0000002 mm. Special electron microscopes can image some large molecules and atoms.
“Molecule builders”: from plasticine balls (hydrogen — small white, oxygen — large red, carbon — black) and sticks build models of H₂O, O₂ and CO₂. Count the atoms in each molecule. Safety: do not put plasticine or sticks in your mouth.