Thread 2 - Research on X-Rays before Moseley

Moseley and Atomic Numbers

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Leading up to Moseley - Atomic Weights and Periodic Properties

Henry Gwyn Jeffreys Moseley was born on November 23, 1887 and would die in battle on August 10, 1915, before he turned 28. However, as long as our civilization stands, he will be remembered as the man who numbered the elements. That work, completed in a six-month span during 1913 and 1914 and published in the last two papers of his life was a tour de force of scientific accomplishment. Said Robert Milikan:

"In a research which is destined to rank as one of the dozen most brilliant in conception, skillful in execution, and illuminating in results in the history of science, a young man twenty-six years old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainity never dreamed of before. Had the European War had no other result than the snuffing out of this young life, that alone would make it one of the most hideous and most irreparable crimes in history."

A brief summary of atomic weights and periodic properties is in order. Before Moseley, periodic tables were created on the basis of increasing atomic weight (with two exceptions). Moseley showed that the correct ordering of the periodic table is on the basis of the atomic number (the number of positive charges in the nucleus).

As an aside, he also showed that there are no elements lighter than hydrogen (atomic number = 1) and that there is no possibility for elements between hydrogen and helium (atomic number = 2). Both possibilities had been advanced, with some proposals demanding three elements between H and He.

I. The Concept of Atomic Weight

Leucippus and Democritus (about 440 BC) are credited with the origin of the atom concept. It was Epicurus, slightly more than 100 years later, who added weight as a property of atoms.

The first tables of relative atomic weights were prepared by John Dalton about 1803. There was much discussion and controvery over the next several decades concerning atomic weights. Various authoritative chemists of the time prepared competing tables of atomic weights with many values the same, but a significant number of differences. Some issues were not be fully resolved until 1860, when wide-spread agreement about atomic weight values in the chemistry community started to come together.

II. Periodic Properties of Elements

Ten elements were known from pre-historical times. Phosphorus was discovered about 1665 and from then, up to 1800, 20 more elements were discovered. There was an explosion of element discovery starting around 1800, with 27 more elements being discovered by the 1840s.

Starting in 1816, but not fully developed unil nearly 1830, Johann Wolfgang Döbereiner was the first person to emphasize chemical similarities, pointing out "triads" of elements like lithium, sodium and potassium as well as chlorine, bromine and iodine. He published five triads as well as several "incomplete" triads.

John Alexander Reina Newlands, working after the reform of atomic weights in 1860, was the first to proclaim a pattern for ALL elements. His tables, done in 1864 and 1865, followed what he called the "Law of Octaves." This meant that, when ordered by atomic weights, every eighth element showed similar chemical properties. In his early tables, he left gaps for missing elements, but his final table of 1865 left no gaps whatsoever. Also, he put two elements into the same position several times. Finally, he allowed for no period longer than eight.

However, with Newlands, the "atomic number" first enters the scene. His table of 1865 shows no atomic weights and simply numbers the elements in order from 1 to 56.

Newlands' work was not favorably received. In March, 1866 he spoke on his work and one of his listeners, a man named Carey Foster with no other claim to fame, rose to facetiously ask if Newlands had ever attempted to classify the elements in alphabetical order.

III. The Modern Periodic Table

The modern periodic table was developed (discovered? invented?) by Dmitri Mendeleev during the years 1869-1871. (Some historians credit others as co-discoverers, but we will ignore them.) Mendeleev did not use the "atomic number" that Newlands had used. "Atomic number" remained a number without any physical meaning. It was simply the numbering of the elements after they had been placed in order by atomic weght.

Mendeleev had periods of eight like Newlands, but he also correctly allowed for longer periods in the transition and rare earth elements. He made a number of correct predictions for missing elements and he had Co/Ni and Te/I in their correct chemical order -- the reverse of the order based on increasing atomic weights. He ordered the elements on their atomic number except for the two pairs just noted, which he put in their correct chemical order, even though no one knew why.

It would be Moseley that finally gave the correct answer to why the elements were reversed from a strict ordering based on atomic weights. The elements were correctly ordered based on the atomic numbers.

Thread 2 - Research on X-Rays before Moseley

Moseley and Atomic Numbers

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