Which element is less reactive, an element who's atoms have seven valence electrons or an element whose atoms have eight valence electrons? explain.

An element whose atoms have eight valence electrons each is much less reactive, because such an element already has the energy-minimizing electron configuration, a closed octet, that other elements achieve by reaction with one another. Elements with seven electrons in each valence shell have a strong drive to abstract another one from a less electronegative atom such as a metal.

The answer is frictional force

Multiply the 3 dimensions, assuming it is a rectangular cuboid, to get the volume in cm³ divide mass by volume to get density in g/cm³