Moseley’s Experiment

Moseley formed the opinion that some physical attribute of the atom must increase by (a) regular fixed amount, from one element to the next, rising through the periodic table. He postulated that this could only be the atom’s nuclear charge.
According to this hypothesis, the number N, that is the element’s ordinal
position in the periodic table, is equal to the number of natural units of
positive electricity carried by the nuclei of the element, i.e., N=Z . The number
Z is now called the atomic number of the element; it is equal to the number of
protons in the element’s nuclei.
Prior to Moseley’s investigation, the elements were arranged in the
periodic table in the ascending order of their atomic weights and on the basis
of their chemical properties. As a result of Moseley’s researches, which
provided the first direct means of determining an element’s atomic number,
inaccuracies in the periodic table were discovered and corrected. For example,
the positions of the transition metals cobalt (Z = 27) and nickel (Z = 28), that
had been previously determined by the ascending order of their atomic
weights, Ni = 58.71 and Co = 58.93, were switched. Similarly, empty positions
were revealed in the table, corresponding to the yet undiscovered elements of
atomic number 43, 61, 72 and 75.
The origin of the characteristic X-ray radiation is readily explained by the
Bohr model of the atom. Let us assume that the electrons orbiting the nucleus
in many electron atoms are arranged in shells, each electron having its specific
slot in a shell. We will designate the innermost shell as the K shell it
corresponds to the principal quantum number (n=1). The second shell is
designated the L shell; it corresponds to the principal quantum number (n=2).
The third shell is designated the M-shell; it corresponds to the principal
quantum number (n=3), and so on. According to this model, all the electrons
in a particular shell have the same energy and the closer a shell is to the
nucleus the greater the energy binding its electrons to the atom. The electrons
in the K shell are all in the energy level E1, those in the L shell in the level E2,
and so on (see Fig. 6.7).