Atomic Structure of Uranium

The Nucleus

• The nucleus, at the very center of the atom, contains protons and neutrons. These are collectively known as nucleons. Virtually all the mass of the atom is concentrated in the nucleus.

Atomic Number

• The atomic number of uranium is 92. The atomic number is the number of protons in the nucleus of the atom.

Atomic Weight

• The number of protons in a nucleus is equal to the atomic number and is always the same as the number of electrons in orbit about that nucleus. Electrons have negligible weight, but the atomic weight includes them and both the protons and also the neutrons, which help hold the nucleus together. A neutron is slightly more massive than a proton and has no electrical charge. The atomic weight of uranium is 238.02891.

Isotopes

• Isotopes are atoms which have the same atomic number but different mass, or atomic weight, numbers. They have the same number of protons but different numbers of neutrons. Naturally occurring uranium consists of three isotopes: uranium 234, 235 and 238. All three isotopes are radioactive, but only one of them, uranium 235, is fissionable and can be used in the generation of nuclear power.

Electron Shells

• The electrons orbit at considerable distances from the nucleus. They are arranged in a series of different energy levels, each of which is capable of holding only a certain number of electrons. Uranium has seven energy levels to contain its 92 electrons. The first level is the one closest to the nucleus and it holds two electrons. The second energy level holds eight electrons, and the third holds eighteen electrons. Beyond that:
  Fourth energy level: 32
  Fifth energy level: 21
  Sixth energy level: 9
  Seventh energy level: 2
  Electrons will always gravitate to the lowest possible energy level (nearest the nucleus) if there is space available in the shell to do so.