Re: why do uranium atoms split up.

Hello Ashal,

A uranium nucleus consists of 92 protons and about 145 neutrons, stuck together in a very small space (10^-14 meters: 0.00000000000001 meters) across. There are also electrons orbiting around the nucleus, but they are so far away (10^-10 to 10^-12 meters) that they don't have much effect on the nucleus.

Protons always have a positive electric charge. Since "like charges repel", according to Coulomb's Law, all of these protons are shoving one another apart. You have probably experienced this effect - "electrostatic repulsion" - at home: on dry, static-y day, your hair may stick up due to "static electricity". This basically means that your hair has picked up a positive charge, so each strand is exerting a force on the other strands, shoving them apart. If you have ever tried to sweep up Styrofoam balls or packing peanuts, you may have noticed that the peanuts repel each other (since they all pick up the same sign of electrical charge) and stick to walls, clothing, and dustpans (if the object has the opposite charge)

The protons in a uranium nucleus want to fly apart because of this very same electrostatic repulsion. However, the nucleus is also held together very tightly by the "strong nuclear force", a powerful short-range attraction between all protons and neutrons. In most nuclei, the stable ones that don't break up (helium-4 or oxygen-16 or iron-56, etc.), the nuclear force is much stronger than the electrostatic repulsion. In uranium-238 and many other radioactive nuclei, the nuclear force is just a little bit weaker than the repulsive force; the nucleus will come apart, but only after waiting a long time, or after getting hit with a neutron. In some rare nuclei (like roentgenium, meitnerium, or ununpentium) which physicists manufacture in accelerator labs, the nuclear force is much weaker than the electrostatic repulsion, so the nuclei fall apart in only a millisecond or two.

There are two different ways that a uranium nucleus can break up. A small piece (a helium nucleus, two protons and two neutrons) can break up and fly away. This is called alpha decay. Alternatively, the whole nucleus can break more or less in half; this is called fission. Fission does not happen very often under normal circumstances, but it is very likely to happen if a uranium-235 nucleus is hit with a neutron. Neutron-induced fission is the process that powers nuclear power plants and (unfortunately) nuclear weapons.

Hope this helps!

-Ben Monreal