Differences between Mendeleev's and Moseley's periodic law?

## EXPERT ANSWER

Mendeeleve’s periodic table “ The properties of the elements are a periodic functon of their atomic weights”. Following are the properties

- The elements, if arranged according to their atomic weights, exhibit an evident stepwise variationof properties.
- Chemically analogous elements have either similar atomic weights (Pt, Ir, Os), or weights which increase by equal increments (K, Rb, Cs).
- The arrangement according to atomic weight corresponds to the
*valence*of the element and to a certain extent the difference in chemical behavior, for example Li, Be, B, C, N, O, F. - The elements distributed most widely in nature have
*small*atomic weights, and all such elements are marked by the distinctness of their behavior. They are, therefore, the*representative*elements; and so the lightest element H is rightly chosen as the most representative. - The
*magnitude*of the atomic weight determines the properties of the element. Therefore, in the study of compounds, not only the quantities and properties of the elements and their reciprocal behavior is to be taken into consideration, but also the atomic weight of the elements. One can predict the discovery of many*new*elements, for example analogues of Si and Al with atomic weights of 65-75. - A few atomic weights will probably require correction; for example Te cannot have the atomic weight 128, but rather 123-126.

** Mosely Law: ** Henry Moseley discovered that the wavelength (energy) of an X-ray depended upon the nuclear charge of an atom. Moseley's Law describes the relationship between atomic number and wavelength of a spectral line:

The constant σ (sigma) is equal to 1 for the K-lines and 7.4 for the more shielded L-lines. For energy this expression is approximately equivalent to:

Moseley's determination of this relationship provided a simple test of the order of the elements according to Z. Where Z denote atomic number of an element.

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