Why is calcium harder than potassium?

The answer for #3 is c but i don't know why

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

Select all that apply. a beta particle: is electromagnetic energy is an electron has zero charge is emitted from the nucleus has a +2 charge has a -1 charge