Based on their relative positions in the periodic table, which two elements would have the same number of valence electrons?

Silicon and Carbon

Explanation:

Electrons present in the outermost shell are called Valence electrons.  

Carbon and silicon, both belong to group 14 of the periodic table and have the same number of electrons (4) in the outer-most shell. The periodic table has columns which are called groups and elements in the same group share many similar properties. Though the size of Carbon is smaller than Silicon, they have the same number of valence electrons.

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%