Answer:
Orbitals
Shells
Energy Levels 
Step-by-step explanation:
Elements in the same period will have the same energy levels which is the same as orbitals and shells. 
- The period of an element indicates the energy level in an atom. 
- Elements in period 1 will have one energy level or shell or orbitals. 
- Down a group this is not the case. 
- The energy level increases from top to bottom. 
- As we go down the group, we are transiting from one period to another.