So Moseley set about to determine the wavelengths of the K radiation using recently discovered techniques by the father-and-son team of W.L Bragg and W.H. Someone, perhaps Barkla or Bohr or Moseley, realized that this meant the X-rays were characteristic of the nucleus. These X-rays were independent of the physical or chemical state the element was in. He had demonstrated that the elements emitted characteristic X-rays, called K and L rays. Why did he choose to study this area for what he needed? We can find the answer in the work of Charles Barkla. It turns out that the square root of the frequency moves by a constant value (let's call it "one unit") for each one unit move by the atomic number. He found it in the K line of the X-ray spectra of each element. Moseley needed some function of a nuclear property that increased in the same pattern, that is, by one for each element in turn.
The atomic number increased by steps of one (18, 19, 20, 21, and so on). Moseley's problem was to find a linear relationship between the atomic number and a measureable property of the nucleus. 39), 'if all elements be arranged in order of increasing atomic weights, the number of each element in that series must be equal to its intra-atomic charge.' " 78) the hypothesis was proposed that the atomic weight being equal to about twice the intra-atomic charge, 'to each possible intra-atomic charge corresponds a possible element,' or that ( Physik. "In a previous letter to NATURE (July 20, 1911, p. Within a few months of Rutherford's nucleus paper being published, the true, physical meaning of "atomic number" was suggested by A. Rutherford was not all that excited by Moseley wanting to study X-rays, but the energy and enthusiasm of the younger man soon wore Rutherford down. (3) In 1913, Bohr took up the question of where the negative electrons are (in the atom) and Moseley studied where the positive charges were.īy the way, Moseley was part of Rutherford's research group - having arrived in Manchester just weeks before Rutherford published his great nucleus paper - when he started his atomic number work. (2) In 1911 Rutherford announced his atomic model: (a) a nucleus - a dense concentration of positive charge with (b) electrons orbiting the nucleus in an unspecified manner. Thomson in 1903, had electrons as negative particles with mass, while the positive charge was spread out through the space of the atom. Keep in mind that the electron (the first sub-atomic particle discovered) was not discovered until 1897. This holds true for cobalt and nickel, although it has long been known that they occupy an anomalous relative position in the periodic classification of the elements according to atomic weights."Įxactly where the positive protons (and the negative electrons) were in the atom took time to be worked out. By examination of the wave-length of the characteristic X rays emitted by twelve elements varying in atomic weight between calcium (40) and zinc (65.4), he has shown that the variation of wave-length can be simply explained by supposing that the charge on the nucleus increases from element to element by exactly one unit. "Recently Moseley has supplied very valuable evidence that this rule also holds for a number of the lighter elements. Rutherford (in 1914) described Moseley's discovery thus: He found that certain lines in the X-ray spectrum of each element moved the same amount each time you increased the atomic number by one.
This was the discovery made by Henry Gwyn-Jefferies Moseley. Today, we know that the atomic number gives the number of protons (positive charges) in the nucleus. Thread 2 - Research on X-Rays before Moseley Thread 1 - Atomic Weights, then Atomic Numbers Moseley's Discovery - The Modern Concept of Atomic Number