Atomic number and the modern periodic law:
After arranging elements in the Periodic Table, it was natural to give each element a number indicating only its position on the series based on increasing atomic weight. No real physical significance was associated with the concept of atomic number when it first came into use, but after Rutherford proposed the atomic model with a nucleus in the centre it was estimated that the charge on the nucleus was very close to one-half the atomic weight:
For many element one-half of atomic weight is equal to Atomic Number e.g., helium with an atomic weight of 4 is second in the table, carbon with an atomic weight of 12 is sixth in the table, oxygen, 16, is eighth ; and sulphur , 32, sixteenth .In the second and third periods this rule holds good.
The verification of this hypothesis came from the work of Moseley. He analysed X-rays produced when cathode rays strike metal target. When metallic element are used as target for cathode rays (electron beams) they emit an X-ray spectrum Figure 1.1 show a diagram of the apparatus used by Moseley, and fig 1.2 shows a typical X-rays spectrum produced by a metal target.
The X-rays are generated when the cathode ray collides with an electron in the K shell knocking the electron out of atom. The vacancy in the K shell is filled when electrons in higher energy level fall into the K shell. This causes the emission of X radiation fig 1.3
The energy of an electron in an atom depends (The atomic number). As the nuclear charge increases, the force holding the electron also increases. Hence, the energy and frequency of the X-rays emitted by metal targets must depend on the atomic number of the target element. Mostly found that the frequency and hence the energy at bright lines of the X-rays spectrum increases in a regular day as the atomic number increases. A plot atomic number versus the square roots of the frequency of either line is liner for 38 metallic elements investigated by moseley. Following the publication of moseley’s work in 1913, it became obvious that the properties of the elements or periodic functions of their atomic numbers.
The order of the elements according to atomic number is almost identical with the order according to atomic weight but there are a few important differences. The atomic weight but there are a few important differences. The atomic weight guide would put potassium before argon, nickel before cobalt, and iodine before tellurium.
K(39.10), Ar (39.95) ………… Ni (58.69), Co (58.93)
l (126.90), Te(127.60).
The atomic number for these elements are in the reverse order. Argon should appear in the Periodic Table as it does because atomic number of Ar is 18 and that of K, Similarly, Co(27) is put before Ni(28) and Te(52) before I(53).
In order to have similar elements in the same group Mendeleev found it necessary to transpose certain pairs of elements arbitrarily in his Periodic Table based on atomic weights. When the elements are placed in order of atomic number, no transpositions are necessary. The modern Periodic Law states the properties of the elements are periodic functions of their atomic numbers.