If a substance has a large mass and a small volume how dense is it

Very dense.

Explanation:

Conceptually, you are taking a large amount of atoms and putting them into a small container.  This means the spaces between atoms has to be smaller and the substance is more dense. (see image)

Mathematically, the equation for density is mass divided by volume ( ). If you divide a big number by a small number, you still have  a pretty big number (ex 4/2=2 versus 6/1=6)

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates