What do elements of the same period have in common? number of valence electrons
O number of shells
O number of protons
O atomic mass

its actually the number of shells

Explanation:

valence electrons are the same down any group. atomic orbitals, or shells, are the same across periods

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.