http://www.ck12.org Chapter 6. The Periodic Table
FIGURE 6.2
Mendeleev’s Periodic Tablechemical properties. However, even with the use of placeholders, there were some elements that did not quite fit
the pattern. For example, Mendeleev listed tellurium before iodine even though its atomic mass is higher, because
he knew that the properties of iodine were much more similar to those of fluorine (F), chlorine (Cl), and bromine
(Br) than they were to oxygen (O), sulfur (S), and selenium (Se). He simply assumed that there was an error in the
determination of one or both of the atomic masses. This turned out not to be the case, but Mendeleev was indeed
correct to group these two elements as he did.
Recall that Rutherford’s experiments leading to the discovery of the nucleus occurred in 1911, long after Mendeleev’s
periodic table was developed. Just two years later, in 1913, English physicist Henry Moseley (1887-1915) examined
the x-ray spectra of a number of chemical elements. His results led to the definition of atomic number as the number
of protons contained in the nucleus of each atom. He then realized that the elements of the periodic table should be
arranged in order of increasing atomic number instead of increasing atomic mass.
When ordered by atomic number, the discrepancies within Mendeleev’s table disappeared. Tellurium has an atomic
number of 52, while iodine has an atomic number of 53. Even though tellurium does indeed have a greater average
atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are
credited with formulating the modernperiodic law, which states that when elements are arranged in order of
increasing atomic number, there is a periodic repetition of their chemical and physical properties. We will discuss
some of these chemical and physical properties later on in this chapter. The result is the periodic table as we know
it today.