Periodic Table Comment

Homeschool advantage contains the following fact:

In the Periodic Table the elements are placed in increasing order of their Atomic Number.

One of our older students submitted the following comment:

"I wrote atomic weight on this answer. I believe I am
correct. If I am not please let me know but I will check it
out myself."

The statement "The atomic weight increases with the atomic number" is not universally true. In fact the history of chemistry shows a great deal of argument about the proper order of elements in the periodic table. The situation is greatly complicated by the existence of isotopes, which are versions of the same element (i.e. atoms having same number of protons but varying numbers of neutrons). The isotopes have the same atomic number, but different atomic weights. Also, the different isotopes have different abundances so that one isotope might be more abundant than another. The accepted atomic weight shown in the periodic table is basically a weighted average of the atomic weights of the various naturally-occurring isotopes times their respective abundances.

The result is that sometimes, order by atomic weights is not the same as the order by atomic number. For example, if you look on this periodic table of the elements, you will see that Argon (Ar) and Potassium are in order according to atomic number but backwards according to atomic weight:

Element Symbol Atomic # Atomic Wt.

Argon Ar 18 39.948

Potassium K 19 39.098

Atomic number can be exactly measured using several different techniques and always comes out the same. Atomic weight, on the other hand, is a function of the purity of the sample and sometimes even where it came from in the world.

Reality is always so much messier than we would like it to be.