Why is density equal to mass/volume?

Will you chech and finish these for me, because i am stumped with them.

Which of the following are not typically included in the periodic table? a. atomic mass b. element symbol c. isotopes d. number of electrons

Select all the correct answers. which changes will increase the rate of reaction during combustion? decreasing the area of contact between the reactants adding more oxygen to the reaction removing heat from the reaction changing the reactants from solid form to powdered form lowering the exposure of the reactants to air