What element has 7 valence electrons and 4 electron shells

Idon’t know how to solve this question

Write the empirical chemical formula of calcium with a mass percent of 38.8, phosphorus with a mass percent of 20.0, and oxygen with a mass percent of 41.3.

Choose an equation that represents an enzyme-catalyzed reaction. (a) enzyme + substrate → enzyme-substrate complex (b) enzyme + substrate ←−→ enzyme + products (c) enzyme + substrate ←−→ enzyme-substrate complex → enzyme + products (d) enzyme + substrate ←−→ enzyme-substrate complex → enzyme-substrate complex + products