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"Atom" derives from the Greek "A-tomos" that means "not divisible", This name was given in the past because this particle was considered the smallest unit of matter that couldn't be divided further in smallest particles.

Today, we know this is only partially true; an atom is the smallest particle of matter keeping the chemical properties of a date element, but it can be divided in much smaller particles that are common to all matter. We can obtain this atomic splitting as we bombard atoms with massive sub-atomic particles that I'm going to describe soon.

It has been shown, at the end of XIX century, that every atom is formed by a "nucleus" that contains particles with a positive electric charge, the "protons" (p+) and others without electric charge but with the same mass, the "neutrons" (n).

The number of p+ is, in most cases, slightly different from that of n that have the main role in keeping tied together in the nucleus the p+ that, otherwise, would reject one another, having equal charges. Nearly the whole mass of the atom is concentrated in the p+ and n within the nucleus.

The force that keeps united the atomic nucleus is the "strong nuclear interaction".

This force becomes less and less effective as the atomic nuclei increase their dimensions and, beyond those of the Bismuth atom, it can't prevent atoms anymore from disintegrating, more or less quickly. These elements, in fact, are unstable and tend to decay, releasing particles and radiations, as better treated below.

Around the nucleus, the "electrons" (e-) turn around the nucleus and their number is always equal to that of p+ in every atom that can be defined at the fundamental state, without any positive or negative charge.

e- have a negative charge of 1.602*10^(-19) Coulomb, equal but opposite to that of p+, but with a mass of 9.1095*10^(-31) Coulomb that is 1835-1840 times minor the mass of n and p+ .

Every chemical element has its typical number of p+ (equal to that of e-), named "atomic number" (Z). The number of n, instead, can vary for each element; this determines a certain difference in the mass of the nuclei of the same element that can be found in nature as a constant mixture of some "isotopes". These can be stable or unstable; in the last case, they have a tendency to disintegrate and produce "radioactivity", formed by:

- "Alpha-particles": in the practice, they are He++ nuclei, formed by 2 p+ and 2 n.

- "Beta-particles"; these are simply e- or also positrons (e+), with same mass and opposite charge.

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