The elements on the periodic table were generally arranged in order of increasing atomic weight. However, in a number of places chemists chose to swap the positions of certain adjacent elements so that they appeared in a group with other elements with similar properties. For instance, tellurium is placed before iodine even though tellurium is heavier (127.6 vs 126.9) so that iodine can be in the same column as the other halogens. In 1913, Henry Moseley discovered that atoms of each element, when excited, emit X-rays at a frequency proportional to the element's position on the adjusted periodic table, which validated these adjustments.