The amount of electrons and their position in the atom is the main thing. The atomic number of an element is it's amount of protons. That makes the periods (rows). Groups are put in place using where...
You can tell if a substance is an element or not by it's atoms. If it's a pure element, it will only have one type of atom (e.g. Copper atoms). If it's not an element it will have two or more types...