The Anomalous Behavior of Period 2 Members

A consistent feature within the main groups is that, as a result of their small atomic size and small number of outer-level orbitals, all the Period 2 members display some anomalous (unrepresentative) behavior within their groups. 

In Group 1A(1), Li is the only member that forms a simple oxide and nitride, Li20 and Li3N, with O2 and N2 in air, and only Li forms molecular compounds with organic halides  

Because of the high charge density of Li , many lithium salts have significant covalent character. Thus, halides of Li are more soluble in polar organic solvents than the halides of Na and K. 

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Why the Alkali Metals Have Unusual Physical Properties 427 Why the Alkali Metals Are So Reactive 427 The Anomalous Behavior of Period 2 Members 429... 

Looking down Group 3A(13), we see a wide range of chemical behavior. Boron, the anomalous member from Period 2, is the first metalloid we ve encountered so far and the only one in the group. It is much less reactive at room temperature than the other members and forms covalent bonds exclusively. Although aluminum acts like a metal physically, its halides exist in the gas phase as covalent dimers— molecules formed by joining two identical smaller molecules (Figure 14.3)—and its oxide is amphoteric rather than basic. Most of the other 3A compounds are ionic, but they have more covalent character than similar 2A compounds because the 3A cations can polarize nearby electron clouds more effectively. 

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