Alessandro C.- I.C. "Falcone_Cascino"-Italy-The atom

 The atom is the smallest fraction of an element capable of preserving its chemical and physical characteristics. Matter can have a property called electric charge; the latter can be of two forms,positive (+) and negative (-). Electric charges of the same sign repel each other, while charges of opposite sign attract each other. At the end of the nineteenth century it was discovered that atoms contain very small parts of cells with a negative electric charge, called electrons. But the atoms as a whole are neutral, that is, they have no electric charge: therefore they must also contain positive charges, which exactly compensate for the negative charges of the electrons. All positive charges are collected in a small volume in the center of the atoms, called the nucleus, around which electrons orbit. The nucleus of atom are made up of two types of particles: protons, each of which has a positive electric charge, and neutrons, which have no electric charge. The structure of the atom is similar to that of an onion: it is in fact divided into various concentric shells, where the electrons are placed. In the shell closest to the nucleus there can be up to 2 electrons, in all the others up to 8. In the periodic table all the chemical elements of the same group have the same chemical characteristics, which are determined by the number of electrons present in the outermost shell of the atom.