By Lower S. K.
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Additional resources for Atoms, Electrons and the Periodic Table (1997)(en)(36s)
Distances on the atomic scale have traditionally been expressed in Ångstrom units (1 Å = 10–8 cm), but nowadays the picometer is preferred; 1 pm = 10–12 m = 10–10 cm = 100 Å. The radii of atoms and ions are typically in the range 70-400 pm. Page 30 Chem1 Atoms, electrons, and the Periodic Table Periodic trends in atomic radii. We would expect the size of an atom to depend mainly on the principal quantum number of the highest occupied orbital; in other words, on the “number of occupied electron shells”.
This is particularly evident in the first few rows of the periodic table, in which small effects tend to be magnified anyway because an added electron produces a large percentage increase in the number of electrons in the atom. He H –21 73 Li Be B C N Na Mg Al Si P 53 43 134 72 60 27 122 –7 –19 –19 O F Ne S Cl Ar 141 328 –29 200 349 –35 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 48 51 64 –10 Cs Sr 47 18 8 16 64 112 118 –47 29 116 78 195 325 –39 Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te 30 41 86 72 53 101 110 54 Rb Ba La Hf Ta W Re Os 45 79 31 Fr Ra Ac Db Jl Ir I Xe 126 –32 29 116 103 190 295 –41 Pt Au Hg Tl Pb Bi Po At Rn 14 106 101 205 223 –61 20 35 91 183 270 –41 Rf Bh Hn Mt 44 Figure 11 Electron affinities.
2. Covalent radius is half the distance between like atoms that are bonded together in a molecule. 3. van der Waals radius is the effective radius of adjacent atoms which are not chemically bonded in a solid, but are presumably in “contact”. An example would be the distance between the iodine atoms of adjacent I2 molecules in crystalline iodine. 4. Ionic radius is the effective radius of ions in solids such as NaCl. It is easy enough to measure the distance between adjacent rows of Na+ and Cl– ions in such a crystal, but there is no unambiguous way to decide what parts of this distance are attributable to each ion.