11. Although the head element of each group is often exceptional
in its properties, it does often show a resemblance to the element one
place to its right in the period below, i.e. Period 3. Thus lithium resembles
magnesium both physically and chemically. Similarly beryllium
resembles aluminium and boron resembles silicon but the resemblances
of carbon to phosphorus and nitrogen to sulphur are less
marked. Oxygen, however, does resemble chlorine in many respects.
These are examples of what is sometimes called the diagonal
relationship in the periodic table.