The nucleus is an extremely dense cluster of matter that is located at the center of an atom. It is made up of subatomic particles called protons and neutrons. Protons have a positive electrical charge and their number present in a nucleus is what determines an atom's chemical properties. All atoms of the same element therefore have the same number of protons. All uranium has 92 protons.
Neutrons are electrically neutral and have no bearing on chemical properties.
Different atoms of the same element can have different numbers of neutrons. Atoms having varying numbers of neutrons are known as Isotopes. Different isotopes of an element are identified by their atomic mass number. The atomic mass number is simply the total of all the protons and neutrons present in a nucleus. For example, uranium 238 has a mass number of 238, which is the total of its 92 protons and 146 neutrons.