1) For example, sodium oxide (Na₂O).
One one molecule of sodium oxide has one sodium atome (Na) and two oxygen atoms (O).
The subscript after element shows the number of elements in a molecule.
2) Yes, it possible for two different compounds to be made from the exact same two elements.
This are isomers, same molecular formula, but different arrangements of atoms.
For example, n-butane (C₄H₁₀) and 2-methylpropane (C₄H₁₀).
3) There are large number of compounds in this world, around 40 millions.
