The picture shows a close-up of one carbon atom. A hydrogen atom has one proton as the nucleus and one electron in the region outside the nucleus. The electron and proton are attracted to each other.

The formula H₂O shows that water is made from two atoms of hydrogen (H) bonded to one atom of oxygen (O). Katie: Time for some science. And today it's a chemistry classic, the periodic table.

At the heart of our biological composition are four elements: oxygen (O), carbon (C), hydrogen (H ... These are almost one-quarter of the periodic table and are mainly found in the first four ...