Question

Why aren't H2O and CO2 in the periodic table?

Answer 1

It’s not in the periodic table because
They are not composed from a single atom.

Answer 2

The periodic table contains an organized table of elements. H2O and CO2 are compounds, but they are made up of elements. For example, the compound H2O is made up of the elements hydrogen and oxygen (which would be found on the periodic table).