Question

usually, elements within the same group have the same number of valence electrons. which of the following elements does not have the same number of valence electrons as the rest of its group?
a. carbon
b. hydrogen
c. helium
d. oxygen

Explanation:

Carbon (Group 14) has 4 valence electrons (matches group), hydrogen (Group 1) has 1 valence electron (matches group), oxygen (Group 16) has 6 valence electrons (matches group). Helium is in Group 18 (noble gases), but unlike other noble gases (which have 8 valence electrons), helium has only 2 valence electrons (its only shell is full with 2 electrons). Thus, helium does not match the valence electron count of its group.

Answer:

C. helium