isotopes of the same element can differ based on the:

number of protons that is present

number of electrons that is present

number of neutrons that is present

overall charge of the atom

Number of neutrons that are present.

Explanation: If there is a change in the protons, then the element itself changes (so does the charge of the atom). If there is a change in the electrons, there is a change in the charge of the atom. A change in the overall charge just makes it either a cation or an anion. That leaves us with just neutrons.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water