A lump of uranium that is dug out of the ground will be made from three different isotopes , that is three different types of uranium with different numbers of neutrons in their nuclei . Most will probably be uranium-238 , which has 238 protons and neutrons , a small amount will probably be uranium-235 with 235 protons and neutrons , and a very small amount will be uranium-234 , with 234 protons and neutrons . The uranium nucleus binds between 141 and 146 neutrons , establishing six isotopes , the most common of which are uranium-238 ( 146 neutrons ) and uranium-235 ( 143 neutrons ) .