What do the 100 or so atoms of the periodic table, in different combinations, make up

100 or so atoms of the periodic table, in different combinations, make up Compounds.

Explanation:

Atoms from different element in the periodic table combined together with the help of  bond and make a new product that is called compound.

For example

Hydrogen and oxygen are the elements of the periodic table. When one atom of oxygen and two atoms of hydrogen are combined together, a compound will be formed named as water (H20).

In another example, two atoms of oxygen and one atom of carbon combines toghther through chemical reaction and make compund named as Carbon dioxide (CO2).