Anomalous Properties

The first member of each main group frequently differs markedly from its heavier congeners because of a combination of distinctive features. These first elements have unusually small atomic and ionic sizes, high charge density, high electronegativity, and, importantly, the absence of accessible d-orbitals in their valence shell. The small size and high charge density give greater polarising power and a stronger tendency toward covalent bonding, while the lack of d-orbitals limits their maximum covalency. For example, boron cannot expand its octet, whereas heavier Group 13 elements can. The option of large atomic size is the opposite of the truth. The option that they are gases is not generally true for these elements. The option of more valence electrons is wrong, since group members share the same valence count. This anomalous first-member behaviour is an important NCERT periodicity theme. Understanding anomalous properties in this way ties directly into the wider study of classification of elements and periodicity in properties, where the same reasoning recurs across many problems. Plausibility check: the restricted covalency of boron and the covalent character of lithium compounds both follow directly from small size and missing d-orbitals, confirming the explanation.