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Best Answers by best Chemistry Teacher in Patna, IITian Rishi Sir
Development of Periodic Table by best Chemistry Teacher in Patna , IITian Rishi Sir
Development of Modern Periodic Table by best Chemistry Teacher in Patna , IITian Rishi Sir :
According to Best Chemistry Classes in Patna, AtoZ CHEMISTRY there are following important steps in development of Periodic Table.
(a) Dobereiner’s Triads: He arranged similar elements in the groups of three elements called as triads, in which the atomic mass of the central element was merely the arithmetic mean of atomic masses of other two elements or all the three elements possessed nearly the same atomic masses.
Li Na K
7 23 39 = 23
Fe Co Ni
55.85 58.93 58.71 nearly same atomic masses
It was restricted to few elements, therefore discarded.
(b) Newland’s Law of Octave: He was the first to correlate the chemical properties of the elements with their atomic masses.
According to him if the elements are arranged in the order of their increasing atomic masses the eighth element starting from given one is similar in properties to the first one.
This arrangement of elements is called as Newland’s Octave.
Li Be B C N O F
Na Mg Al Si P S Cl
This classification worked quite well for the lighter elements but it failed in case of heavier elements and therefore, discarded
(c) Lother Meyer’s Classification: He determined the atomic volumes by dividing atomic masses with their densities in solid states.
He plotted a graph between atomic masses against their respective atomic volumes for a number of elements. He found the following observations.
l Elements with similar properties occupied similar positions on the curve.
l Alkali metals having larger atomic volumes occupied the crests.
l Transitions elements occupied the troughs.
l The halogens occupied the ascending portions of the curve before the inert gases.
l Alkaline earth metals occupied the positions at about the mid points of the descending portions of the curve.
On the basis of these observations he concluded that the atomic volumes (a physical property) of the elements are the periodic functions of their atomic masses.
It was discarded as it lacks practical utility.
(d) Mendeleev’s Periodic Table:
Mendeleev’s Periodic’s Law
According to him the physical and chemical properties of the elements are the periodic functions of their atomic masses.
He arranged then known elements in order of their increasing atomic masses considering the facts that elements with similar properties should fall in the same vertical columns and leaving out blank spaces where necessary.
This table was divided into nine vertical columns called groups and seven horizontal rows called periods.
The groups were numbered as I, II, III, IV, V, VI, VII, VIII and Zero group
Merits of Mendeleev Periodic table:
l It has simplified and systematised the study of elements and their compounds.
l It has helped in predicting the discovery of new elements on the basis of the blank spaces given in its periodic table.
Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group. Eka-aluminium and Eka-silicon names were given for gallium and germanium (not discovered at the time of Mendeleev). Later on it was found that properties predicted by Mendeleev for these elements and those found experimentally were almost similar.
Density / (g/cm3)
Melting point (K)
Formula of oxide
Formula of chloride
l Atomic weights of elements were corrected. Atomic weight of Be was calculated to be 3 × 4.5 = 13.5 by considering its valency 3, was correctly calculated considering its valency 2 (2 × 4.5 = 9)
Demerits in Mendeleev’s Periodic Table :
l Position of hydrogen is uncertain. It has been placed in lA and VIIA groups because of its resemblance with both the groups.
l No separate positions were given to isotopes.
l Anomalous positions of lanthanides and actinides in periodic table.
l Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For example Ar(39.94) is placed before K(39.08) and Te (127.6) is placed before I (126.9).
l Similar elements were placed in different groups e.g. Cu in IB and Hg in IIB and similarly the elements with different properties were placed in same groups e.g. alkali metals in IA and coinage metals in IB.
l It didn’t explained the cause of periodicity.
(e) Long form of the Periodic Table or Moseley’s Periodic Table or Modern Periodic Table :
(i) In the long form of periodic table there is contribution of Ramsey, Werner, Bohr and Bury.
(ii)This table is also referred to as Bohr’s table since it follows Bohr’s scheme of the arrangements of elements into four types based on electronic configuration of elements The modern periodic table consits of horizontal rows (periods) and vertical column (groups).
(i) Moseley did an experiment in which he bombarded high speed electrons on
different Metal surfaces and obtained X-rays(electromagnetic rays).
He observed regularities in the characteristic X-ray spectra of the elements and found that plot vs. Z (atomic number) is straight line while vs. A (atomic weight) is not, and = a(Z – b), where a and b are constants that are same for all elements and n is frequency of X-rays. Thus he concluded that atomic number is more fundamental property than atomic weight.
(ii) Moseley concluded that the physical and chemical properties of the
elements are periodic function of their atomic number.
Modern Periodic Law
The physical and chemical properties of elements are periodic function of their
atomic number. So the elements are arranged in order of increasing atomic
number, the elements with similar properties comes after regular intervals.
The repetition of the properties of elements after regular intervals when the elements are arranged in the order of increasing atomic number is called periodicity.
(a) In a period, the ultimate orbit remain same, but the number of electrons gradually increases.
(b) In a group, the number of electrons in the ultimate orbit remains same, but the values of n increases.
Cause of Periodicty
The periodic repetition of the properties of the elements is due to the recurrence of similar valence shell electronic configuration after certain regular intervals. For example, alkail metals have same electronic configuration ns1, therefore, have similar properties.
In the periodic table, elements with similar properties occur at intervals of 2, 8, 8, 18, 18 and 32. These numbers are called as magic numbers.
The modern periodic table consists of horizontal rows (periods) and vertical column (groups).
There are seven periods numbered as 1, 2, 3, 4, 5, 6 and 7.
l Each period consists of a series of elements having same valence shell.
l Each period corresponds to a particular principal quantum number of the valence shell present in it.
l Each period starts with an alkali metal having outermost electronic configuration as ns1.
l Each period ends with a noble gas with outermost electronic configuration ns2np6 except helium having outermost electronic configuration as 1s2.
l Each period starts with the filling of new shell.
l The number of elements in each period is twice the number of atomic orbitals available in shell that is being filled. For illustration–
m Ist period shortest period having only two elements. Filling of electrons takes place in the first shell, for which, n = 1, l = 0 (s-subshell) and m = 0.
Only one orbital (1s) is available and thus it contains only two elements.
m 3rd period (short period) having only eight elements. Filling of electrons takes place in the third shell. For which,
n = 3, l = 0, 1, 2 and number of orbitals 1 3 5
(3s) (3p) (3d)
Total number of orbitals 9
But the energy of 3d orbitals are higher than 4s orbitals. Therefore, four orbitals (one 3s and three 3p orbitals) corresponding to n = 3 are filled before filling in 4s orbital (next energy level). Hence 3rd period contains eight elements not eighteen elements.
There are eighteen groups numbered as 1, 2, 3, 4, 5, ……….. 13, 14, 15, 16, 17, 18.
Group consists of a series of elements having similar valence shell electronic configuration.
Number of elements
(1)st n = 1
Very short period
(2)nd n = 2
(3)rd n = 3
(4)th n = 4
(5)th n = 5
(6)th n = 6
Very long period
(7)th n = 7
Very long period