Final answer:
A group on the periodic table determines the number of valence electrons, while the period determines the number of energy shells an atom has. Chlorine, located in group 17 and period 3, has three energy shells and seven valence electrons.
Step-by-step explanation:
The group on a periodic table determines a. The number of valence electrons an element possesses, which are the electrons in its outermost shell that contribute to chemical reactions. Conversely, the period on a periodic table determines b. The number of energy shells an atom has. This is because as we move from left to right across a period, we add one proton and one electron, but the electrons are added to the same valence shell.
Chlorine, which is found in group 17 and period 3 of the periodic table, has three energy shells (answer b. 3) since it is located in the third row. For the number of valence electrons, chlorine has seven (answer c. 7) because it is in group 17, and the group number typically signifies the number of valence electrons for non-transition metals.