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He atom It is the smallest particle of an element that retains the chemical characteristics of the element itself. It consists of a nucleus formed by protons (positive particles) and neutrons (neutral), surrounded by one or more electron orbits (negative particles).

Under conditions of stability the number of electrons is equal to that of protons, so that the atom is electronically neutral.

The chemical characteristics of an element depend on the number and arrangement of electrons in the various electron orbits, so that it can be said that the atom is substantially composed of empty spaces. The mass of the atom resides almost all in the nucleus: each electron is only 1 / 1,840 with respect to the mass of a proton or a neutron (protons and neutrons have the same mass). The number of protons in the nucleus is called the atomic number; that of neutrons and protons together, mass number.

When two atoms have the same number of protons, and therefore belong to the same chemical element, but a different number of neutrons, and therefore a different number of mass, they are called isotopes.

The elements existing in nature are 92, from hydrogen to uranium and therefore the maximum number of protons found in a nucleus is 92. However, atoms of artificial elements with more than 100 protons have been constructed in the laboratory the nucleus.

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