Hybrid-density functional calculations on methyl group substituted elemental hydrides of the form H3C-EHn (E is any main blocker d block transition element and n is 0, 1, 2, 3, or 4 depending on the position of E in the periodic table) give atomic radii (RA)as the: C-E bond distance minus half Of the C-C bond:distance of ethane. RA values show good linear correlations with the experimental covalent radii, Slater's empirical set of atomic radii, and experimental carbon-based atomic radii particularly,for the main block elements. Except a few cases, the sum of any two R-A values reproduces very well the corresponding mean single bond distances in molecules. Further, R-A values provide-the calculation of an expected density (Ds ') of the element, which shows:a good linear correlation with solid-state density of elements (Ds) when they are metals; suggesting similarity in their atomic packing. The percentage of free space (Vf(ree)) in the solid-state calculated using Ds ' and Ds suggests that all:the metals have V-free below 66, all the metalloids have V-free between 66 and 71, and all the nonmetals have V-free above 71.