Conversion atom to another

One child has claimed to have find a solution to all physical problems. On asking for details, he said that all periodic elements has common components, i.e. electrons, protons, neutrons. The child has suggested a solution: convert atom to another by adding electron. This way one can get substance like $\ce{H2O, Au, He}$ in abundance. How can it be done?

Answer

While what you suggest may sound nice on paper, it has some serious problems.

1. Getting the elements. Let's say that we're talking about purifying water to remove toxic elements such as Hg or Cd. Extracting the elements out of the water is a feat by itself, for example using reverse osmosis methods. This is a method used for seawater desalination - to turn them from salt water filled with all kinds of elements into drinkable water. This process is very expensive, and a very polluting one as well. Desalination just the amount of water you need for drinking water is complicated, so desalinating an entire reservoir is simply not going to happen.


2. Let's say you did somehow manage to extract the element in question. Now you need nuclear reactions to transmute one element to another. Not all are possible. For some elements, like Tc or Am this is the only way you can produce them. However, you are going to end up with radioactive nuclear waste.

To sum it up, even if it was possible, you would need so much power and to do it and you will produce some much pollution that it's simply not worth it. Just going and mining the gold will be orders of magnitude cheaper (and probably cleaner) than producing it using nuclear reactions.

If you do manage to somehow extract the the polluting elements, you usually do something else with them (aka recycling) and you do not attempt being an alchemist. Another example is soils contaminated with lead. The solution is to just dig it up, put it somewhere where it is not hazardous to anyone and replace it with clean soil.