Astatine electron configuration

The halogens—fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)—are active nonmetals. Because of their chemical reactivity, they don t exist as free elements in nature. Their chemical behavior is characterized by a tendency to gain one electron to complete their valence-electron configuration and form a 1 — ion with a noble-gas configuration. Chlorine, for example, has the configuration [Ne]3s 3p. ... 

In Group 7A(17), fluorine and chlorine have the condensed electron configuration [noble gas] ns np, as do the other halogens (Br, I, At). Little is known about rare, radioactive astatine (At), but all the others are reactive nonmetals that occur as diatomic molecules, X2 (where X represents the halogen). All form ionic compounds with metals (KX, MgX2), covalent compounds with hydrogen (HX) that yield acidic solutions in water, and covalent compounds with carbon (CX4). 

From these two examples, we see that the characteristic valence electron configuration of a halogen is nP iip, where ii ranges from 2 in the case of fluorine to 6 in the case of astatine. 

From fhese fwo examples, we see fhaf fhe characferisfic outer-shell electron configuration of a halogen is ns ttfP, where // ranges from 2 in fhe case of fluorine fo 6 in fhe case of astatine. 

As noted in Section 7.8, the chemistry of astatine (At) is far less developed than that of the other halogen elements. (a) In the periodic table the atomic weight of At is written as (210). Why are the parentheses used How is this nomenclature related to the difficulty of studying astatine (b) Write the complete electron configuration... 

Under ordinary conditions, the elemental halogens (except astatine) exist as covalent, diatomic molecules, acquiring the electronic configuration of the next noble gas by sharing one pair of electrons between two atoms. Their appearance ranges from a pale yellow gas (F2), through a dark red liquid (Br2), to an almost black, crystalline solid (I2). Bromine is unique in being the only nonmetallic element that exists as a liquid under ordinary conditions. Iodine crystals do not melt at atmospheric pressures, but sublime directly into the gas phase. 

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