Periodic Table Key

The Periodic Table

"If all the elements are arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by the law of periodicity."
Dmitri Mendeleev, Principles of Chemistry, Vol. 2, 1902, P. F. Collier, p17.

"We have here a proof that there is in the atom a fundamental quantity, which increases by regular steps as one passes from one element to the next. This quantity can only be the charge on the central positive nucleus, of the existence of which we already have definite proof."
Henry Moseley, Philosophical Magazine, Vol. 26, 1913, p1030.

"The chemistry of an atom depends only on the number of electrons, which equals the number of protons and is called the atomic number. Chemistry is simply numbers, an idea Pythagoras would have liked. If you are an atom with one proton, you are hydrogen; two, helium;....."
Carl Sagan, Cosmos, 1980, Random House, p223. Photo: NASA.

The Periodic Table

The periodic table we use today is based on the one devised and published by Dmitri Mendeleev in 1869.

Mendeleev found he could arrange the 65 elements then known in a grid or table so that each element had:

1. A higher atomic weight than the one on its left. For example, magnesium (atomic weight 24.3) is placed to the right of sodium (atomic weight 23.0):

The True Basis of the Periodic Table

In 1913, chemistry and physics were topsy-turvy. Some big hitters - including Mendeleev - were talking seriously about elements lighter than hydrogen and elements between hydrogen and helium. Visualizing the atom was a free-for-all, and Mendeleev's justification for a periodic table based on atomic weights was falling apart at the seams.

This is the story of how Henry Moseley brought light to the darkness.

2. Similar chemical properties to other elements in the same column - in other words similar chemical reactions. Magnesium, for example, is placed in the alkali earths' column, with other elements whose reactions are similar:

Mendeleev realized that the table in front of him lay at the very heart of chemistry. And more than that, Mendeleev saw that his table was incomplete - there were spaces where elements should be, but no-one had discovered them.

Just as Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium on paper. He called this new element eka-silicon, after observing a gap in the periodic table between silicon and tin:

Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) on paper, because he predicted their existence and their properties before their actual discoveries.

Periodic Table Image

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Although Mendeleev had made a crucial breakthrough, he made little further progress. With the benefit of hindsight, we know that Mendeleev's periodic table was underpinned by false reasoning. Mendeleev believed, incorrectly, that chemical properties were determined by atomic weight. Of course, this was perfectly reasonable when we consider scientific knowledge in 1869.

In 1869 the electron itself had not been discovered - that didn't happen for another 27 years.

In fact, it took all of 44 years for the correct explanation of the regular patterns in Mendeleev's periodic table to be found...

Read More... The Periodic Table continued