Answer:
1.
Groups - Verticle columns in the periodic table, signifies the number of valence electrons in the element.
vs.
Periods - Horizontal rows in the periodic table, signifies the number of shells in the element.
2. Because the group number corresponds to the number of valence electrons in an atom an element in group 5 (like Phosphorus) has 5 valence electrons.
Added information: a Valence electron is an electron in the outermost shell of an atom.