The diagonal relationship in Be and Al is due 'to similar ionic size and charge/radius ratio. Generally, on moving from left to right across a period, the ionic charge increases to maximum and then decreases, while the ionic size decreases, causing an increase in its polarising power (Fajan's rule). On the other hand, on moving down a group the ionic charge remains the same while ionic size increases. Therefore, polarising power decreases. On moving diagonally, these two effects partly balance each other and therefore, there is no marked change in their properties.