Periodic Classification of Elements Class 10 Notes
e.g., Atomic masses of Li, Na and K are respectively 7, 23 and 39, thus the mean of atomic masses of I St and 3rd element is
- Thus, Newland’s classification was not accepted.
Merits of Mendeleev’s Periodic Table:
- No fixed position for hydrogen
- No place for isotopes
- No regular trend in atomic mass.
- Co was placed before Ni.
Mendeleev’speriodic table :
Achievements of Mendeleev’s periodic table:
- The arrangement of elements in groups and periods made the study of elements quite systematic in the sense that if the properties of one element in a particular group are known, those of the others can be easily predicted.
Prediction of new elements and their properties :
- Many gaps were left in this table for undiscovered elements. However, the properties of these elements could be predicted in advance from their expected position. This helped in the discovery of these elements. The elements silicon, gallium, and germanium were discovered in this manner.
Correction of doubtful atomic masses :
- Mendeleev corrected the atomic masses of certain elements with the help of their expected positions and properties.
Limitations of Mendeleev’s classification :
- He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resemble both with alkali metals as well as with halogens.
- The isotopes of the same element will be given different positions if the atomic number is taken as basis, which will disturb the symmetry of the periodic table
- The atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements.
Modern Periodic Law :
Cause of periodicity :
Periodicity may be defined as the repetition of the similar properties of the elements placed in a group and separated by certain definite gap of atomic numbers. The cause of periodicity is the resemblance in properties of the elements is the repetition of the same valence shell electronic configuration.
Modern Periodic Table:
Moseley proposed this modern periodic table and according to which “the physical and chemical properties of elements are periodic function of their atomic number and not the horizontal rows called “periods”. The groups have been numbered 1, 2, 3 …. 18 from left to right.
- The elements belonging to a particular group make a family and are usually named after the first member. In a group, all the elements contain the same number of valence electrons.
- In a period all the elements contain the same number of shells, but as we move from left to right the number of valence shell electrons increases by one unit.
- Atomic Number – denoted by Z and equal to the no. of protons in the nucleus of an atom.
- Modern periodic table contains 18 vertical columns known as groups and horizontal rows known as periods.
- Elements in a group have the same number of valence electrons
- No. of the shells increases as we go down the group.
- Elements in a period have same number of shells.
- Each period marks a new electronic shell getting filled.
- No. of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.
- The maximum no. of electrons that can be accommodated in a shell depends on the formula where n is the no. of the given shell. e.g. K shell – 2 × (1) = 2 elements in the first period L shell – 2 ×(2) = 8 elements in the second period.
- The position of the element in the periodic table tells about its reactivity.
Trends in Modern Periodic Table: Given by Henry Moseley in 1913.
- Valency: Valency may be defined as the combining capacity of the atom of an element with atoms of other elements in order to acquire the stable configuration (i.e. 8 electrons in the valence shell. In some special cases it is 2 electrons).
- Atomic size: It refers to the distance between the center of nucleus of an isolated atom to its outermost shell containing electrons. The atomic radius decreases on moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
- In a group atomic size decreases from top to bottom due to an increase in a number of shells.
- Metallic and non-metallic properties: In a period from left to right metallic nature decreases while the non-metallic character increases. In a group metallic character increases from top to bottom while non-metallic character decrease.
- Electronegativity: The relative tendency of an atom to attract the shared electron pair of electrons towards itself is called electronegativity. In a period from left to right, the value of electronegativity increases while in a group from top to bottom the value of electronegativity decreases.