Question

In 1869 there were 60 known elements. Mendeleev arranged the elements in order of their atomic mass (atomic weight). He realised that elements with similar properties occurred at regular intervals.

(a) Suggest why one of the groups that is on today's periodic table was not in Mendeleev's periodic system.​

Answer 1

One of the groups that is on today's periodic table that was not in Mendeleev's periodic system is the noble gases. This is because the noble gases were not discovered until much later, in the late 1800s and early 1900s. The first noble gas, helium, was discovered in 1868, just a year before Mendeleev published his periodic table. The other noble gases, including neon, argon, krypton, xenon, and radon, were discovered in the following decades. Because these elements were not known at the time, they could not be included in Mendeleev's periodic table.