Period 2 elements anomalous behavior

Why the Alkali Metals Have Unusual Physical Properties Why the Alkali Metals Are So Reactive Anomalous Behavior of the Period 2 Elements... 

About 25 years ago it was rare that any scientist would speak about 4f electrons being involved in bonding, since everyone knew that the 4f electrons were localized in the ion core and were well shielded by the 5p and 5s electrons, especially in the case of the lanthanide metals. However, a few scientists led by Matthias and Gschneidner believed that some of the properties of the lanthanide elements were sufficiently anomalous with respect to the rest of the periodic table that just the (5d6s) valence electrons could not account for these behaviors. Slowly the evidence built up, such that today virtually everyone believes that there is some hybridization of the 4f electrons with the outer 5d and 6s electrons, especially in the light lanthanides (see, e.g., Freeman et al. 1987). The work on intermediate valence cerium and more recently the heavy-fermion studies (see section 4.4.4) convinced even the most conservative scientists that 4f electrons must be involved. Reference to the early papers on this subject can be found in the article by Gschneidner (1971). 

Boron. Boron is structurally the most bizarre element in the periodic table. Simple bonding rules that are applicable to the other elements have to be bent considerably in order to accommodate the behavior of boron. Boron should be a metal, but it is a semiconductor with unique structures and anomalous physical properties. Combined with metals it participates in metallic bonding but boron atoms are simultaneously covalently bonded to other boron atoms in the higher metal borides. Similar to carbon it does not form mononuclear ions, its halides are molecules, and its other compounds with nonmetals are solids. 

Because the Period 2 elements have a small atomic size and only four outer-level orbitals, they exhibit some behavior that is anomalous within their groups. 

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