How do the number of electrons in the second energy shell of an atom change going across period 2 in the periodic table? a. the number of electrons increases by 1, from left to right. b. atoms in period 2 do not have a second energy shell, instead the first energy shell increases in electrons from left to right. c. the number of electrons in the second energy shell is full, the number of electrons in the third energy shell increase by 1. d. the number of electrons increases by 1, from right to left.