In the modern periodic table, each box contains four data. Besides the element name and symbol, the atomic weight is at the bottom, and the atomic number is at the top. The elements are arranged in order of increasing atomic number in horizontal rows called periods.

The elements appear to be arranged in order of increasing atomic weight. But there are several exceptions.

Consider the proper placements of tellurium (Te) and iodine (I) in the periodic table. Te has the heavier atomic weight. The chemical properties of tellurium are like those of sulfur (located two rows above it). Iodine resembles bromine (located one row above it) because these elements are nonmetallic halogens that form compounds like those of chlorine. Therefore, the order in the table cannot be based solely on atomic weight.

The atomic number, which appears above each element symbol, represents the meaningful order in the periodic table. When an element is referred to by an integer, this number means the atomic number, not the atomic weight. Thus, element 27 is cobalt (whose atomic number is 27), not aluminum (whose atomic weight is 27).