10. In noting changes of properties down the typical element
groups I-VII of the periodic table, it soon becomes apparent that
frequently the top or head element in each group does not fall into
line with the other elements below it. This is clearly seen when we
consider the melting points and boiling points of elements and their
compounds (p. 17), and when we come to look at the properties of
the individual groups in detail we shall see that the head element and
its compounds are often exceptional in both physical and chemical
properties. It will be sufficient to note here that all the head elements
in Period 2, namely lithium, beryllium, boron, carbon, nitrogen,
oxygen and fluorine, have one characteristic in common—they cannot
expand their electron shells. The elements of Periods 3 onwards
have vacant d orbitals, and we shall see that these can be used to
increase the valency of the elements concerned—but in Period 2 the
valency is limited.
Unlike 'typical element' groups the 'transition metal' groups do
not have head elements.