Atoms and matter

All matter that surrounds us is made up of atoms. Every atom consists of protons and neutrons, which together form the nucleus, surrounded by a cloud of negatively-charged electrons.
Within the atom, the nucleus is made up of positively charged protons and by neutrons that lack electric charge and are therefore neutral (as their name indicates). Atoms are electrically neutral because the number of protons is equal to the number of electrons. The total number of protons in the nucleus (and therefore of electrons in the outer cloud) determines the identity of a chemical element: for example, the chemical element with 8 protons is oxygen, the one with 26 protons is iron, the one with 79 protons is gold and the one with 92 protons is uranium.
Isotopes
A chemical element can have, besides the fixed number of protons that characterise it, a varying number of neutrons: in this case there will be different isotopes of the same element. For example, iron present in nature has four isotopes with 26 protons but with 28, 30, 31 and 32 neutrons respectively. Isotopes of the same element can be distinguished by their mass numbers (neutrons + protons): hence you can find iron-54, iron-56 etc.
There are about 90 elements that occur in nature (ranging from the lightest, hydrogen, to the heaviest, uranium) and nearly 270 isotopes. Among these elements, about twenty have only one stable isotope (for example, sodium, cobalt, arsenic and gold) while others have at least two stable isotopes (for example, chlorine has two, zinc has five and lead has ten). In addition to the isotopes present in nature (natural isotopes) nowadays there are a great number of artificial, man-made isotopes, such as cobalt-60 (27 protons, 33 neutrons), used in radiotherapy or plutonium-239 (94 protons, 145 neutrons) used as a nuclear fuel.
A question of stability
Almost all natural isotopes are stable contrary to artificial isotopes that are unstable, i.e. they tend to arrange themselves spontaneously in new nuclear structures that are energetically more favourable.  The transformation of an isotope into another is called disintegration or decay, and unstable isotopes are known as radioactive isotopes (radioisotopes or radionuclides). This process of spontaneous disintegration of atomic nuclei, during which ionising radiation is emitted, is called radioactivity. Ionising radiation is any particulate or electromagnetic radiation capable of modifying the structure of matter with which it interacts. In the case of biological tissues, this interaction can damage the cells. In the majority of cases the damage is repaired by the normal defence mechanisms in the organism but at times the affected cells could be impaired with negative consequences on the health of the individuals exposed. The extent of the damage also depends on the magnitude and length of exposure.
The time taken for an isotope to decay can be short or very long. The half-life of a radioactive isotope is defined as the time it takes for half of the atoms of a pure sample of the isotope to undergo decay into another element. The half-life is a measure of the stability of an isotope: the shorter the half life, the less stable the atom is. For example, the half-life of uranium-238 (92 protons and 146 neutrons), one of the isotopes that has been present in the Earth’s crust since its formation, is 4.47 billion years. At present, the amount of uranium-238 left after decay is about half the original quantity present on Earth, which has been estimated to be about 4.5 billion years old.

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