The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 11 elements. Many atomic weights are not constants of nature, but depend upon the physical, chemical, and nuclear history of the material. The standard atomic weights of 10 elements having two or more stable isotopes have been changed to reflect this variability of atomic-weight values in natural terrestrial materials. To emphasize the fact that these standard atomic weights are not constants of nature, each atomic-weight value is expressed as an interval. The interval is used together with the symbol [a; b] to denote the set of atomic-weight values, A_r(E), of element E in normal materials for which a ≤ A_r(E) ≤ b. The symbols a and b denote the bounds of the interval [a; b]. The revised atomic weight of hydrogen, A_r(H), is [1.007 84; 1.008 11] from 1.007 94(7); lithium, A_r(Li), is [6.938; 6.997] from 6.941(2); boron, A_r(B), is [10.806; 10.821] from 10.811(7); carbon, A_r(C), is [12.0096; 12.0116] from 12.0107(8); nitrogen, A_r(N), is [14.006 43; 14.007 28] from 14.0067(2); oxygen, A_r(O), is [15.999 03; 15.999 77] from 15.9994(3); silicon, A_r(Si), is [28.084; 28.086] from 28.0855(3); sulfur, A_r(S), is [32.059; 32.076] from 32.065(2); chlorine, A_r(Cl), is [35.446; 35.457] from 35.453(2); and thallium, A_r(Tl), is [204.382; 204.385] from 204.3833(2). This fundamental change in the presentation of the atomic weights represents an important advance in our knowledge of the natural world and underscores the significance and contributions of chemistry to the well-being of humankind in the International Year of Chemistry 2011. The standard atomic weight of germanium, A_r(Ge), was also changed to 72.63(1) from 72.64(1).