Now we know what the values mean, but what does the equation say?

    If you have a little bit of algebra in your background, you can see that energy equals mass multiplied by the speed of light squared. This is the formula that tells you how much energy could be released from a substance if it is converted entirely from matter to energy.  That means, that the maximum amount of energy you could get from a substance if you converted it entirely to energy would be equal to the mass of the substance times the speed of light squared, or 9x10¹⁶ joules per every kilogram of mass. That number may not look very big to you, but in expanded form, it is 90,000,000,000,000,000.  This amount of energy would be the same as the amount of energy that would be released if you exploded 22 megatons, or 22 million tons, of TNT.
    Nuclear bombs are not that efficient. In reality, only a small percentage of the mass is converted to energy, but it is still enough to release a tremendous amount of energy. Today’s more powerful bombs can be over 100 megatons.   back

Uranium₂₃₅ and Uranium₂₃₈:
Ok, so we know that we can convert mass to energy, but how do we do it?

    The first thing we need is some uranium₂₃₅.  This is not as easy as it might sound.  Uranium₂₃₅ only occurs in less than one percent of natural uranium₂₃₈, which it needs to be extracted from.   Because U₂₃₅ and U₂₃₈ atoms are almost exactly the same in chemical composition, it is almost impossible to separate the two different kinds of materials.  Many steps must be taken to separate these two materials.  Once we have a pure sample of U₂₃₅, the rest is just physics.  When a neutron bombards an atom of U₂₃₅, it splits into atoms of Cesium and Rubidium, releasing a large amount of energy and three additional neutrons.  In a nuclear reactor, the amount of uranium present to be split is controlled.  For every uranium atom that splits, one other uranium fissions.  This causes a sustained reaction and is how nuclear power is produced.  These neutrons, if not controlled, can then cause more than one U₂₃₅ atoms to split per every U₂₃₅ atom that fission right before it, causing a chain reaction.  View the animation to get a better idea here (115k, requires flash 3 or higher).

Since U₂₃₅ and U₂₃₈ are almost exactly the same, why can't we just use U₂₃₈ to get a reaction?

When you bombard U₂₃₅ with neutrons, it takes on a neutron and becomes U₂₃₆. U₂₃₆ is unstable and will break down into Cesium and Rubidium, causing the release of 3 neutrons and much energy, as described above.   U₂₃₈ on the other hand will turn into U₂₃₉ which is too stable to break down and release any amount of energy.

Why do we need to use neutrons to bombard the atom instead of the other particles such as protons or electrons?

An atom is composed of electrons, protons and neutrons.  The protons and neutrons make up of the atom nucleus and the electrons orbit that nucleus.   For an atom to fission, the nucleus must split.  To do this, we can shoot a particle at it causing it to disintegrate.  We've all played with magnets before and we know that if we face two of the same sides of a magnet together, they repel.  This is because both ends are either positively or negatively charged and same charges repel each other.  Atomic particles work very much the same way. 

1. If we shoot a negatively charged electron at the atom, it well get close to the atom and then get pushed away by the electrons circling the atom. 
2.  If we shoot a positively charged proton at the atom, the proton will make it past the electrons circling the atom but will get pushed away by the protons located on the atom nucleus. 
3.  The only particle not affected by charge is the neutron which can penetrate all the way to the nucleus and cause it to fission.   back

Chain Reaction:
What is a chain reaction and how does it occur?

A chain reaction means that one thing leads to another and starts a chain of events.  An example of a chain reaction is the domino effect.  When the first domino falls, it causes the next one to fall, which causes the third one to fall and so forth.  Well, remember that in the breakdown of U₂₃₆ we had 3 neutrons released.  These neutrons fly off at an enormous velocity and can hit other atoms of U₂₃₅.  If the neutrons merge with the other atoms of U₂₃₅, the cycle starts over and energy, plus 3 more neutrons are released from each resulting breakdown of U₂₃₆.  Each breakdown only takes up a fraction of a second and it doesn't take long until many U₂₃₆ atoms have disintegrated.  The very rapid release of energy from all this fission causes a gigantic nuclear explosion (animation).   back

Critical Mass:
How much U₂₃₅ do we need to make a bomb?

The amount of uranium need to create a working bomb is directly related to critical mass.  Let’s talk about critical mass.  If you had a small sphere of pure fissile material, such as uranium₂₃₅, about the size of a golf ball, it would not sustain a chain reaction. Too many neutrons escape through the surface area, and in turn do not disintegrate more uranium so the chain reaction ceases.   This is called a sub-critical amount.
    In a mass of uranium₂₃₅ about the size of a baseball, there are more neutrons hitting the atoms of the fissile material than are escaping through the surface area, thus sustaining the chain reaction.  The minimum amount of fissile material required to maintain the chain reaction is known as the critical mass.   Increasing the size of the sphere produces a supercritical assembly, in which an explosion can occur.

Wouldn’t a bomb just explode if you had a supercritical mass of U₂₃₅ exposed to neutrons?

We can avoid this problem by creating two subcritical amounts of fissile material then assembling them in the bomb apart from each other. They do not become critical until an explosion is set off to fire one of the subcritical masses at the other one. The force of the impact welds the two pieces together. Together, these create a critical mass and an explosion occurs in about a millionth of a second.   back