In the period table, atoms are arranged in rows called periods and columns called groups. The arrangement of atoms in groups is based on the number of their valence electrons while the arrangement of atoms across the periods is based on their number of electrons shells. Atoms are arranged across the periods in increasing order of their atomic number i.e., according to their number of electrons. Potassium has 19 electrons while copper has 29 electrons. Both has four electrons shells according to their electronic configuration.
