Describe how elements of the periodic table are arranged relating to their atomic structure. (6 marks)

Elements in the periodic table are arranged by increasing atomic number (number of protons). Atomic number of elements steadily increases across a period (horizontal rows of the periodic table). Elements in the same period have the same number of electron shells. Elements in groups of the periodic table (vertical columns) have similar chemical properties. Atoms of elements in the same group have the same number of outer shell (valence) electrons. The group number is equal to the number of valence electrons.