An element with 20 protons and 20 electrons gains one electron. the electron goes into what type of orbital?

  3d

 20 electrons means that the one 4s orbital is filled up. Then the electrons will fill up the five 3d orbitals and finally the three 4p orbitals to complete energy level 4

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

Carbon based or cells?