The Periodic Table is the universal catalog of everything you can drop on your foot. Everything tangible is made of elements. Your foot is made mostly of oxygen, with quite a bit of carbon joining it, giving structure to the organic molecules that define you as an example of carbon-based life. Oxygen is a clear, colorless gas, yet it makes up 3/5 of the weight of your body. How can that be?
Elements have two faces:
• their pure state, and
• the range of chemical compounds they form when they combine with other elements.
Oxygen in pure form is a gas, but when it reacts with silicon they become together the strong silicate minerals that compose the majority of the earth's crust. When oxygen combines withhydrogen and carbon, the result can be anything from water to carbon monoxide to sugar. Oxygen atoms are still present in these compounds, no matter how the substance may appear. And the oxygen atoms can always be extracted back out and returned to pure gaseous form.
But (shot of nuclear disintegration) each oxygen atom can never itself be broken down into something simpler. This property of indivisibilityis what makes an element an element.