OF ELEMENTS Periodic Classificaiton of Elements . MODERN PERIODIC LAW Mendeleev’s periodic table had some discrepancies, which were difficult to overcome. For example, the atomic mass of argon ( . amu) is greater than that of potassium ( .
amu), but argon comes before potassium in the periodic table. If elements were arranged solely according to increasing atomic mass, argon would appear in the position occupied by potassium in our modern periodic table (see in Figure . ). No chemist would place argon, a gas with no tendency to react, in the same group as lithium and sodium,which are two highly reactive metals.
This kind of discrepancies suggested that some fundamental property other than atomic mass must be the basis of periodicity. The fundamental property turned out to be the number of protons in an atom’s nucleus, something that could not have been known by Mendeleev and his contemporaries. Henry Moseley, a British scientist in , discovered a new property of elements called atomic number, which provided a better basis for the periodic arrangement of the elements. It is a well-known fact that atomic number of an element is equal to the number of protons or the number of electrons present in the neutral atom of an element.
The periodic law was, therefore, modified to frame a modern periodic law, which states that “The physical and chemical properties of the elements are the periodic functions of their atomic numbers”. . MODERN PERIODIC TABLE With reference to the modern periodic law, the elements were arranged in the increasing order of their atomic numbers to form the modern periodic table. The modern periodic table is a tabular arrangement of elements in periods and groups, highlighting the regular repetition of properties of the elements.
Figure . shows the modern periodic table of elements discovered so far. As you have studied the features of the modern periodic table in standard IX, here let us confine to the study of the features of periods and groups. .
. Features of Periods The horizontal rows are called periods . There are seven periods