How Do Atoms Combine with Each Other?

How Do Atoms Combine with Each Other?
A couple of millennia ago, the Greeks believed that water was one of the
four elements of nature, along with fire, air, and earth, and that all things
were made from combinations of these elements. Today, we of course
know that there are more than a hundred elements. And, in fact, water is
not a single element but a combination of atoms of two elements, namely
hydrogen (H) and oxygen (O). When two or more atoms of the same or
different elements combine together, molecules are formed. Therefore,
water is a molecule. The chemical formula for a water molecule (H2O) is
probably the most widely quoted of all chemical formulas. A chemical
formula is merely a molecule's atomic recipe. Thus, for each molecule of
water, two hydrogen atoms (subscript 2 behind H) are bound to one
oxygen atom (no subscript, so 1 is implied).
  From our previous description of the size of atoms you can imagine
then that an ordinary glass of water must contain millions of water mol-
ecules. In fact, we can use water to tidy up our understanding of elements,
atoms, and molecules. If we have an 8 ounce (oz) glass of pure water, we
can say that the container is accommodating millions of molecules of
water, and thus millions of atoms; however, only two elements are pres-
ent, oxygen and hydrogen.