These comprehensive RBSE Class 10 Science Notes Chapter 5 Periodic Classification of Elements will give a brief overview of all the concepts.
RBSE Class 10 Science Chapter 5 Notes Periodic Classification of Elements
→ In the Periodic Table, elements are classified on the basis of similarities in their properties.
→ Triads of Dobereiner-In 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each. So he called these groups “triads’. Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. So the property of middle element is also in between other two elements. Example-Li, Na and K.
→ Newlands Law of Octaves-He arranged the elements in the increasing order of atomic masses and found that every eighth element had properties similar to that of first like music in which eighth note is similar to first note. This is called Newlands Law of Octaves.
→ With the discovery of noble gases, the Law of Octaves become irrelevant.
→ According to Mendeleev the elements were arranged on the basis of their fundamental property. The atomic mass, and also on the similarity of chemical properties.
→ Mendeleev’s Periodic Law-The properties of elements are the periodic function of their atomic masses.
→ Mendeleev’s arranged the elements in the increasing order of atomic masses and found the similarities in the properties of elements of same group.
→ Mendeleev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.
→ When Mendeleev started his work, 63 elements were known. He studied the relationship between the atomic masses of the elements and their physical and chemical properties.
→ Among chemical properties, Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen.
→ Mendeleev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
→ No fixed position can be given to hydrogen in the Mendeleev’s Periodic Table and there is no place for isotopes.
→ Moseley classified the elements on the basis of atomic numbers.
→ Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, in modern Periodic Table. Because the similarities in the properties of elements can easily be determined on the basis of atomic number.
→ Modern Periodic Law – The properties of elements are the Periodic Function of their atomic numbers.
→ Modern Periodic Table have 18 groups (vertical columns) and 7 Periods (horizontal rows).
→ The outer most electronic configuration of all the elements of every group is same. So properties of these elements are also same because chemical properties of elements depends on electronic configuration.
→ The properties of elements show periodicity in groups and periods.
→ The number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period.
→ The maximum number of electrons that can be accommodated in a shell (K, L, M) depends on the formula 2n2 where ‘n’ is the number of the given shell from the nucleus.
→ The distance between the centre of the nucleus and the last electron of an isolated atom is called atomic radius or atomic size.
→ In the Periodic Table the atomic size increases in a group in moving top to bottom because new shells are being added as we go down the group. This increases the distance between the last electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge. The atomic radius decreases in moving from left to right along a period. Because an increase in nuclear charge (Protons) nuclear attraction force also increases so this reduce the size of the atom.
→ In the Periodic table, the metals are towards the left-hand side while the non-metals are found on the right-hand side.
→ In the Modern Periodic Table, a zig-zag line separates metals from non-metals. The borderline elements are called metalloids or semi-metals.
→ Metallic character of elements decreases in a period and increases in a group.
→ Metalloids (sub metals) have the properties of metals and non metals both.
→ Metal oxides are generally basic and non metal oxides are acidic.
→ Metals are electropositive means they have the tendency to lose the electrons.
→ In a period effective nuclear charge on valance shell electrons increases so electron lossing tendency of elements decreases.
→ Effective nuclear charge decreases in a group on moving top to bottom. So electron lossing tendency of elements decreases.
→ Non metals are electronegative means they have the tendency to accept the electrons.