How are the isotope nitrogen-14 and nitrogren-15 different?

The number after the dash is the mass number, which is the number of neutrons plus the number of protons. Since the number of protons can’t change (it would have become a new element if so), this means the number of neutrons must have changed.

Nitrogen has 7 protons, which means nitrogen-14 must have 7 neutrons and nitrogen-15 must have 8 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