As the atomic number increases along a row of the periodic table, additional electrons are added to the same, outermost shell. The radius of this shell gradually contracts as the attraction between the additional electrons and the nucleus increases.The chemical elements are arranged in order of increasing atomic number. The horizontal rows are called periods and the vertical columns are called groups. Elements in the same group have similar chemical properties. This is because they have the same number of outer electrons and the same valency.
