4. (a) In the period from about 1800 to about 1865, the atomic weights of many elements were accurately measured. Why was this important to Mendeleev’s formulation of the periodic table? (b) What property of the atom did Moseley associate with the wavelength of X-rays emitted from an element in his experiments? In what ways did this affect the meaning of the periodic table?

Question

4.	(a) In the period from about 1800 to about 1865, the atomic weights of many elements were accurately measured. Why was this important to Mendeleev’s formulation of the periodic table? (b) What property of the atom did Moseley associate with the wavelength of X-rays emitted from an element in his experiments? In what ways did this affect the meaning of the periodic table?

anonymous · Answer

(a) With more accurate atomic weights grouping the elements by their chemical reactivity also ends up arranging them mostly according to atomic weight, which leads to an "aha!" feeling that you are on to something important.  (b) Atomic number.  (c) It was realized that the best arrangement of elements, according to their chemical reactivity, is to list them by atomic number Z, and not atomic weight.  Atomic weight A is almost right, but there are a few exceptions where an element with greater Z actually has lower A.

Moseley's was well-timed work, because he did it right around the time Bohr came up with a theory to interpret the results, and relate them back to the newly-discovered Rutherford nuclear model of the atom.  Moseley's work wrapped up the experimental proof of the nuclear model, and that the Bohr shell model applied beyond hydrogen (although it would take another 20 years before Bohr's work was fully fledged out into a quantum mechanics that could explain atomic spectroscopy quantitatively beyond hydrogen).

Moseley's work was widely appreciated at the time, and there was broad speculation that he would be in line for the 1916 Nobel Prize on Chemistry, an extraordinary achievement given that he was only 25 years old when he did this work.  However, when the First World War broke out Moseley volunteered to serve in the British Army, and he was killed in the ill-fated invasion of Gallipoli in August of 1915, at age 27.  

Isaac Asimov, who was himself both a trained scientist as well as prolific and successful science fiction author, once mused, only half-jokingly, that in view of what more Harry Mosely might have accomplished, his death might have been the single most costly death of the FIrst World War.  Supposedly Moseley's death prompted the British government to adopt a new policy that forbade the enlistment of its best scientists.