History of the Periodic Table

John Dobereiner was a German scientist who developed the Law of Triads. Each triad was a group of three elements similar to each other; for example, alkali formers were grouped together, and so were salt formers. In 1829, he noticed that bromine, an element that fell about halfway between chlorine and iodine in atomic weight, also seemed halfway between them in its other properties. Other “triads” that he discovered were: calcium, strontium, and barium, and sulfur, selenium, and tellurium. Soon, the lines were blurred, causing confusion, and these triads were treated like coincidences.

He discerned that the second seven elements repeated the properties of the first seven closely, and he called them octaves. His law wasn’t taken too seriously, because the rest of the elements didn’t clearly follow it.

In 1864, he published The Modern Theory of Chemistry in which he published the use of atomic weights to group elements. In the work, he arranged 28 elements into 6 families that had similar chemical and physical traits. His contribution was the use of valance, or combining power of an atom of a specific element. He wasn’t as acknowledged as Mendeleev for his contribution because he didn’t leave gaps in his periodic table for the addition of elements.

Mendeleev realized that the periodic pattern didn't apply to heavier elements. He decided to try to keep the pattern by saving spaces for elements that weren't discovered yet. The gaps he focused on most were the gaps between aluminum and indium, silicon and tin, and born and yttrium. He called the elements that were unknown eka-aluminum, eka-silicon, and eka-boron, and predicted their characteristics. He published a paper where he demonstrated all known elements on a single periodic table. Like Meyer, he trained under Robert Bunsen and used similar techniques to complete the table.

Through studying nitrogen, found a gas heavier that didn’t react with anything. He named it Argon, which means “lazy one” and was the first inert element discovered.

He discovered neon, krypton, and xenon with Lord Rayleigh in 1864. He also isolated helium which had been observed in the spectrum of the sun but had not been found on earth. He ran Nitrogen across liquid Magnesium which resulted in a small amount of unreactive gas that later came to be known as Argon. In 1910 he made and characterized radon.

In 1913, he used X-ray to order the elements. Each element has a unique emission pattern when X-rayed. Moseley used this to show that atomic number, not atomic weight was most important in grouping and ordering the elements. The problems with Mendeleev's periodic table disappeared when atoms were positioned from lowest to highest atomic number.

He was the author of the concept of heavy elements. He determined how heavy elements that were rare in the earth fit in the periodic table. Both the Lanthanide series and the Actinide series are under the Aluminum column and have similar properties.