Iodine covalent radius

The single bond covalent radius of carbon may be taken to be about 0.77 A, just half of the C—C bond length in diamond. Similarly, the covalent radii of chlorine and iodine may be set at 0.99 A and 1.33 A, respectively, one half of the interatomic distances in the Ch and I2 molecules. 

Why does a weaker X-H bond make larger The covalent radii of iodine and fluorine can be compared to make an initial comparison. Covalent radii are estimated from homonuclear bond lengths, which are pertinent to covalent molecules such as HF and HI. The covalent radius of I (135 pm) is much greater than that of F (71 pm), so the bond distance between H and I will... 

How will the lengths of the H—X and C—X bonds change from fluorine to iodine They will increase as the covalent radius of the halogen atom increases. 

Physical Properties. The absorption of x-rays by iodine has been studied and the iodine crystal stmcture deterrnined (12,13). Iodine crystallizes in the orthorhombic system and has a unit cell of eight atoms arranged as a symmetrical bipyramid. The cell constants at 18°C (14) are given in Table 1, along with other physical properties. Prom the interatomic distances of many iodine compounds, the calculated effective radius of the covalently bound iodine atom is 184 pm (15). 

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

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