Diagonal Relationship is seen between certain pairs of diagonally placed adjacent elements in the 2nd and 3rd periods of the periodic table. The pairs (Li & Mg, Be & Al, B & Si etc.) exhibit some similar properties; for ex: B and Si are both semiconductors, form halides that are hydrolyzed in water and have acidic oxides. Such a relation occurs because crossing and descending the periodic table have opposing sides. On crossing a period of the periodic table, the size of the atoms decreases, and on moving down the group the size of the atoms increases. Similarly, on moving along the period the elements become progressively more covalent and less reducing and more electronegative in nature, whereas on descending the group the elements become more ionic, more basic and less electronegative, and elements with similar properties which have similar chemistry are often found - the atomic size, electronegativity, properties of compounds (and so on) of the diagonal members are similar.