Answer: Combustion reactions are characterized by the reaction of a substance with O2 to form one or more oxygen, often water. Combustion reactions emit heat, and they are important because most of society's energy is derived from them. Ex.: CH4 (g) + 2O2 (g) -- CO2 (g) + 2H2O (g)
Electrons fill the electron orbitals (s, p, d, or f) starting from the lowest energy level going to the highest energy level.
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