Uranium

Uranium atoms were made during a supernova. Their nuclei are very large and unstable.

Uranium (U) is 19 times as dense as water. However it is a relatively weak metal, too weak to be useful for construction. All Uranium atoms contain 92 protons -- that is why we call them Uranium. However they can contain various numbers of neutrons. There are always more neutrons than protons. This is because neutrons help to stick the protons together. Without the neutrons the positive electric charge on all these protons would make them fly apart.

The main value of uranium is its radioactivity. On Earth almost all of the metal consists of uranium-238. This has 92 protons and a total of 238 protons and neutrons. So how many neutrons does it have? Uranium-238 can absorb a neutron, forming uranium-239, and this eventually decays into plutonium-239 -- a material of great importance in nuclear power and nuclear weapons.

The two large planets, Jupiter and Saturn, were the first to form. They are mostly gas. These massive planets probably had a great influence on how the rest of the solar system evolved. For some reason they were in almost perfect circular orbits. This is unusual. In the few other planetary systems studied so far, most of the giant planets are in oval (elliptical) orbits.