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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 rows and columns, highlighting the regular repetition of properties of the elements. Figure 8.1 shows the modern periodic table of 118 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.
The horizontal rows are called periods. There are seven periods in the periodic table.
First period (Atomic number 1 and 2): This is the shortest period. It contains only two elements (Hydrogen and Helium).
Second period (Atomic number 3 to 10): This is a short period. It contains eight elements (Lithium to Neon).
Third period (Atomic number 11 to 18): This is also a short period. It contains eight elements (Sodium to Argon).
Fourth period (Atomic number 19 to 36): This is a long period. It contains eighteen elements (Potassium to Krypton). This includes 8 normal elements and 10 transition elements.
Fifth period (Atomic number 37 to 54): This is also a long period. It contains 18 elements (Rubidium to Xenon). This includes 8 normal elements and 10 transition elements.
Sixth period (Atomic number 55 to 86): This is the longest period. It contains 32 elements (Caesium to Radon). This includes 8 normal elements, 10 transition elements and 14 inner transition elements (Lanthanides).
Seventh period (Atomic number 87 to 118): Like the sixth period, this period also accommodates 32 elements. Recently 4 elements have been included by IUPAC.
The vertical columns in the periodic table starting from top to bottom are called groups. There are 18 groups in the periodic table.
Based on the common characteristics of elements in each group, they can be grouped as various families.
The Lanthanides and Actinides, which form part of Group 3 are called inner transition elements.
Except 'group 0', all the elements present in each group have the same number of electrons in their valence shell and thus have the same valency. For example, all the elements of group 1 have one electron in their valence shells (1s1). So, the valency of all the alkali metals is ‘1’.
As the elements present in a group have identical valence shell electronic configurations, they possess similar chemical properties.
The physical properties of the elements in a group such as melting point, boiling point and density vary gradually.
The atoms of the 'group 0' elements have stable electronic configuration in their valence shells and hence they are unreactive.
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