. By firing neutrons into a nucleus to change an atom, can you make gold using other cheaper metals?
Yes. However, trying to build gold with nuclear reactions is an expensive way to make the precious metal. Furthermore, you would probably end up with radioactive gold because at least some of the nuclei you made would have the wrong numbers of neutrons in them and would be unstable.
. Can (or has) the nucleus be seen through microscopes?
Not exactly. A microscope "sees" an object by sending waves at that object and then looking at the waves it reflects or transmits. For example, a common light microscope sends light waves at an object and allows you to observe the transmitted or reflected light.
Unfortunately, light waves can't resolve details smaller than about 1/2 their wavelength. With a light microscope, the smallest objects you can make out are about 1/4 of a micron wide. To see still smaller objects, you must use something with a shorter wavelength than light. Because of quantum physics, even seemingly particulate objects such as electrons have a wave character and a wavelength, and fast moving electrons have much shorter wavelengths than light. Electron microscopes can resolve details down to about 1/2 the electron wavelength (in principle) and that brings their resolution down toward the level of individual atoms.
But to see a nucleus, which is much smaller than an atom, you need particles with even smaller wavelengths than are available in electron microscopes. The electrons in particle accelerators have such small wavelengths that they can resolve features as tiny as nuclei. However, the particles making up nuclei are always moving so that the "images" formed by accelerators are blurry. Nonetheless, it's possible to learn much about the structure of nuclei from these accelerator experiments. In fact, people now look at features even smaller than nuclei. They are presently looking at the individual nucleons (protons and neutrons) that make up nuclei and even at the quarks that make up those nucleons.
. How do you do work on an atom so that the nuclear force can overcome the repulsive force.
Building large nuclei is a curiosity in modern science, not a sensible scheme for synthesizing elements. Most of the heavy elements in our world were made during a supernova explosion sometime before the formation of our present solar system about 5 billion years ago. During the supernova explosion, the temperatures became so high that nuclei of all sorts crashed into one another violently and many heavy nuclei were created. The work needed to build these giant nuclei came from the heat of this horrendous explosion.
. How do you keep the nuclear bomb stable until you're ready to use it? (For example, on the way to Hiroshima)
The nuclear material will only explode if it is assembled to the point of critical mass. If that assembly is done slowly, the material will overheat and melt, perhaps causing a minor explosion but creating more of a radiation hazard than a nuclear detonation. Only if the assembly is done rapidly to well over the critical mass will the bomb explode. To keep a nuclear weapon "safe", the bomb makers ensure that the assembly cannot occur prematurely. They probably remove the triggers for the high explosives or block the paths through which the nuclear material must move. In most cases, even an accidental triggering of the high explosives use in assembly wouldn't cause the bomb to explode because all of the high explosives must be triggered at the same time for the assembly to work properly. If only part of the explosives ignited, the bomb would fizzle (very loudly).
. If we were ever to have a nuclear war, would we have to live underground?
The long-term effects of nuclear war would come primarily from the release of radioactive isotopes into our environment. Large nuclei, such as that of uranium 235, have many more neutrons than protons. These neutrons "dilute" the repelling protons and made these large nuclei less unstable. But once a large nucleus shatters into fragments of medium size, these fragments acquire electrons and become "normal" atoms with medium sized nuclei. Unfortunately, these medium sized nuclei need fewer neutrons than they wind up with and they are generally unstable. While they resemble normal atoms chemically, they contain unstable cores and eventually decay. The decays release energy and this energy can do chemical damage to surrounding material. If the atom has been incorporated into a biological system (e.g. a person), it can do chemical damage to that biological system, perhaps causing cancer or genetic damage. To avoid this insidious damage, people would have to stay away from the fallout chemicals. That would be a difficult task, even underground.
. What is the difference between the nuclear bomb and the H-bomb?
The fission bomb (uranium or plutonium bomb) derives its energy from the shattering of large nuclei; those in uranium or plutonium. The H-bomb (hydrogen, thermonuclear, or fusion bomb) derives most of its energy from the fusion or coalescence of small nuclei; those in hydrogen. The H-bomb releases more energy per kilogram than the fission bombs and can be made larger than the fission bombs. However, triggering a hydrogen bomb requires the enormous temperatures of a fission bomb.
. When a plane drops a nuclear bomb, what sets the detonation process into effect?
The altitude at which the bomb explodes affects its results. Near or on the ground, the blast would cause incredible local damage, but less long-range damage. Above the ground, the blast would cause less local damage, but more long-range damage. So the bomb-makers build altitude sensing equipment into the bomb; probably a pressure sensing or radar-based altimeter. When the bomb has determined that it is at the right height, it triggers. High explosives assemble the critical mass as quickly as possible (typically by crushing the central sphere with carefully shaped high explosive charges). Once the fissionable material exceeds its critical mass, the chain reaction starts and the bomb explodes.
. Why can't you make nuclear weapons with any old element?
Only a few elements/isotopes are fissionable, meaning that only a few elements/isotopes have nuclei that shatter when struck by a neutron. Moreover, only a few of this fissionable nuclei release more neutrons than they take to fission. Of naturally occurring isotopes, only Uranium 235 is suitable for nuclear weapons. Plutonium 239 is also suitable, but it must be made artificially in a nuclear reactor.
. What would happen if you saturated the uranium side of a fusion bomb with cobalt? I think it would destroy our planet.
A fusion bomb, also known as a thermonuclear or hydrogen bomb, releases enormous numbers of fast-moving neutrons. Neutrons are uncharged subatomic particles that are found in the nuclei of all atoms except the normal hydrogen atom. A normal cobalt nucleus contains 32 neutrons and is known as cobalt 59 (for its 59 nuclear particles: 32 neutrons and 27 protons). When a neutron collides with a cobalt 59 nucleus, there is a substantial probability that the cobalt 59 nucleus will capture it and become cobalt 60 (for its 60 nuclear particles: 33 neutrons and 27 protons). Cobalt 60 is radioactive—it falls apart spontaneously with a 50% probability each 5.26 years. When a cobalt 60 nucleus decays, it begins by emitting an electron and an antineutrino to becomes nickel 60 (for its 60 nuclear particles: 32 neutrons and 28 protons). But this nickel 60 has extra energy in it and it soon emits two high-energy gamma rays (electromagnetic particles, with more energy than x-rays) to become normal nickel 60, a common form of the nickel atom. A fusion bomb containing cobalt 59 could be expected to make lots of cobalt 60, which would then undergo this radioactive decay over the next few decades, releasing gamma rays as it does.
So a fusion bomb containing cobalt would release a large amount of cobalt 60 into the environment. This would certainly give the bomb long lasting radioactive fallout that would make it much more damaging to the environment than a pure fusion bomb would be. Whether it would destroy the planet, I can't say. The bomb's explosion wouldn't be any more destructive, but its long-term toxic effect to animals and plants certainly would be.
. How does a hydrogen bomb work? How does it differ from the atomic bomb besides the simple difference of fusion and fission? — KS, Lake Oswego, OR
A hydrogen bomb uses the heat from a fission bomb (a uranium or plutonium bomb, sometimes called an atomic bomb) to cause hydrogen nuclei to collide and fuse, thereby releasing enormous amounts of energy. While a fission bomb can initiate its nuclear reactions at room temperature, fusion reactions won't begin until the nuclei involved have been heated to enormous temperatures. That's because the nuclei are all positively charged and repel one another strongly up until the moment they stick. Only at enormous temperatures (typically hundreds of millions of degrees) will the nuclei collide hard enough to stick and release their nuclear energy. A typical hydrogen bomb (also called a fusion bomb or thermonuclear bomb) uses a fission trigger to initiate fusion in a mixture of deuterium and tritium, the heavy isotopes of hydrogen. These neutron-rich isotopes fuse much more easily than normal hydrogen. Because deuterium and tritium are both gases, and because tritium is unstable and gradually decays into the light isotope of helium, some hydrogen bombs form the tritium during the explosion by exposing lithium nuclei to neutrons from the fission trigger. Thus the "fuel" for many thermonuclear bombs is actually lithium deuteride, which becomes a mixture of tritium and deuterium during the explosion and then becomes various helium nuclei through fusion.