AN ELEMENT is a substance that consists of atoms of one type only. The 92 elements that occur naturally, and the 17 elements created artificially, are often arranged into a chart called the periodic table. Each element is defined by its atomic number—the number of protons in the nucleus of each of its atoms (it is also the number of electrons present). Atomic number increases along each row
(period) and down each column (group). The shape of the table is determined by the way in which electrons arrange themselves around the nucleus: the positioning of elements in order of increasing atomic number brings together atoms with a similar pattern of orbiting electrons (orbitals). These appear in blocks. Electrons occupy shells of a certain energy. Periods are ordered according to the filling of successive shells with electrons, while groups reflect the number of electrons in the outer shell (valency electrons). These outer electrons are important—they decide the chemical properties of the atom. Elements that appear in the same group have similar properties because they have the same number of electrons in their outer shell. Elements in Group 0 have “filled shells,” where the outer
shell holds its maximum number of electrons, and are stable. Atoms of Group I elements have just one electron in their outer shell. This makes them unstable—and ready to react with other substances.
