What are the three exceptions in the periodic table?

Answer 1

This question is a little bit cryptic. I think I can decipher what it means though. When Mendeleev introduced his periodic law, he expressed it in something like these terms:

If the elements are arranged in order of increasing atomic weight, there is a periodic recurrence of elements with similar properties.

Now in Mendeleev's time -- around 1870 -- not all of the natural elements were known, and even for those that were, not all atomic weights were known accurately enough.

When Mendeleev produced his second periodic table in 1871-- a table of elements arranged so that the periodic recurrence of similar elements could easily be seen, he had to make some "adjustments" to the atomic weight values that were generally accepted at the time. In most cases he was right to say that the accepted atomic weights were wrong, but in one case he was quite wrong about an atomic weight. The atomic weight of tellurium, a rare element that belongs in the sulfur family proved to be significantly larger than that of iodine, which belongs in the chlorine family. It simply did not fit with the periodic lawas Mendeleev understood it.

By the year 1900, this anomaly had been joined by two others: nickel and cobalt, and argon and potassium.

In 1913 an English chemist/physicist, Moseley, discovered the atomic number, and the modern version of the periodic law says that

If the elements are arranged in order of increasing atomic number, there is a periodic recurrence of elements with similar properties.

Usually, an element with larger atomic number has a larger atomic weight, but there are just these three examples where this is not the case.

• Element 18, argon, has atomic weight 39.95, but element 19, potassium has atomic weight 39.10

• Element 27, cobalt, has atomic weight 58.93, but element 28, nickel, has atomic weight 58.71

• Element 52, tellurium, has atomic weight 127.60, but element 53, iodine, has atomic weight 126.90

I think that these are probably the "three exceptions" that your question is referring to.