What is common to all elements in the same group in the periodic table?

A. They have the same number of neutrons.

B. They are all nonmetals.

C. They have the same number of outer electrons.

D. Their properties are very different.

C is the correct answer. Elements in the periodic table are grouped based on having similar properties. For example, the noble gases are all non-reactive and non-metallic. The electronic structure of an atom is the way the electrons are arranged within it, and this affects where they are located in the periodic table. The number of electrons in an element is the same as its group number in the periodic table (with the exception of Group 0).