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Atoms, Elements, Compounds
 t
the time the Curies
discovered polonium and radium, identifying new chemical elements was
one of the highest goals a scientist could hope to reach. A chemical
element is a substance that
contains only one kind of atom.
If you keep dividing up such a substance, you finally get to the tiny
atoms. Nobody had been able to divide an atom further, into smaller pieces.
 Everything
in the world around us is made up of the atoms of the chemical elements,
combined with one another in countless ways in compounds.
For example, water is not an element but a compound of two true elements,
hydrogen and oxygen. When chemists describe water as H2O
they mean that the smallest particle of water is made of two atoms of
hydrogen and one atom of oxygen. For scientists of Marie Curie’s
time, it was a great mystery why atoms of different elements had different
chemical properties—for example, why it was in the nature of oxygen
atoms to combine in this way with hydrogen to make a wet liquid. Experiments
using radioactivity helped bring the answer after many years.
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