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In 1913, Henry Moseley, an English physicist, by his experiment concluded that atomic number is a more fundamental property of an element and not its atomic mass. Thus, he defined Modern Periodic Law as the physical and chemical properties of the elements are the periodic functions of their atomic numbers.
Periodicity may be defined as the repetition of similar properties of the elements placed in a group and separated by certain gaps of atomic numbers. Physical and chemical properties of the elements must be related to the arrangement of electrons in their atoms. Since electrons present in the inner shells do not take part in chemical combinations, it must be the electrons in the outermost shell, which control the properties of the atoms. Thus, if the electrons in the outermost shell of the atoms are same, their properties will also be similar.
|Element||Atomic number||Electronic configuration|
From the table above, it is quite clear that all the atoms listed have one electron (ns1) in their valence shells. This means that the same valence shell electron configuration gets repeated in a group when elements are separated by certain definite gaps of atomic numbers (8, 8, 18, 18, 32) which are also known as magic numbers. Similar explanation can be offered for the periodicity of the elements present in any other group.
A number of tables have been suggested for the classification of elements in order to eliminate the defects of Mendeleev's periodic table. The widely accepted table is the extended or long form of periodic table, also known as Modern Periodic Table.
There are 18 vertical columns in the long form of the periodic table called groups. In the earlier arrangement, the groups retained the same names which were assigned to them in the Mendeleev's periodic table i:e IA, IIA, IIIB to VIIB, VIII, IB, IIB, IIIA to VIIA and zero.
Characteristics of groups
a) In a group, all the elements have the same general electronic configuration of the atom.
b) The elements in a group are separated by definite gaps of atomic number (2, 1, 18, and 32).
c) The atomic sizes of the elements in a group increase down the group due to the successive addition of the number of electron shells.
d) The elements in each group have generally similar chemical properties because they are related to the valence shell electronic configuration of the atoms.
e) The variation in physical properties of the elements such as melting point, boiling point, density, solubility e.t.c. follows a systematic pattern.
The horizontal rows in the long form of the periodic table are called periods. There are seven periods in the long form of the periodic table as in the original, Mendeleev's periodic table. Different periods contain a variable number of elements. In fact, there is a correlation between the period number and the number of principal shells occupied by the electrons. For example, 4thperiod contains the elements whose atoms have electrons in four different shells (i:e K, L, M, and N).
The first three periods of the periodic table from the top with 2, 8, and 18 elements, respectively are known as short periods. The next three periods with 18, 18, and 32 elements are called long periods while the seventh period is an incomplete long period. The periodic table shows at the bottom of table two horizontal rows each containing 14 elements. The elements in the first row are called lanthanides. These elements actually belong to the sixth period and the atoms of the elements are filled with electrons in 4f sub-shell. Similarly, the elements in the second row are called actinides. They belong to the seventh period and have the atoms which are filled with electrons in 5f sub-shell.
Table showing correlation between number of elements in periods and number of shells accommodating the electrons
|Period||Energy levels||Orbitals being filled||Number of elements||Type of period|
|2||n=2||2s 2p||8||Short period|
|3||n=3||3s 3p||8||Short period|
|4||n=4||4s 3d 4p||18||Long period|
|5||n=5||5s 4d 5p||18||Long period|
|6||n=6||6s 4f 5d 6p||32||Long period|
|7||n=7||7s 5f 6d 7p||32||Long period|
Characteristics of periods
a) For the atoms of the elements in the same period, the electrons are added successively in the same principal shell while moving from left to right.
b) The atomic-sized of the elements generally decrease from left to right in the periodic table.
c) In a period, on going from left to right, the metallic character of the elements decreases while their nonmetallic character increases.
d) As the electronic configuration changes along the period, the chemical properties of the elements also change.
The long form of the periodic table has several advantages over the original Mendeleev's table. Some of the superior characteristics are mentioned below:
1) The basis of the classification of the elements is the most fundamental property of the element i.e. the atomic number.
2) The positions of the elements in the periodic table are linked with the electronic configuration of their atoms.
3) Each group occupies an independent column in the extended periodic table. The idea of sub-groups has been discarded.
4) A single position for all the isotopes of an element is justified since the isotopes of an element have the same atomic number.
5) Anomalous position of some of the elements in the original periodic table has been corrected and is now justified according to the periodic law.
6) The classification of the elements into s, p, d, and f- blocks has been quite helpful in understanding their properties.
7) A complete separation between metals and non-metals has been achieved. The non-metals are present in the upper right corner of the periodic table.
8) Some justification has been offered for placing lanthanides and actinides at the bottom of the periodic table.
In spite of many advantages, the long form of periodic table has still some minor drawbacks as listed below:
a) The position of hydrogen : IUPAC periodic table has recommended group 1 for hydrogen. The position is, however, still debated with respect to its electronic structure.
b) The position of lanthanides and actinides : Lanthanides and actinides have not been incorporated in the main body of the periodic table.
c) The position of helium : Helium is placed in group 18 along with other noble gases which are p-block elements. However, with respect to electronic structure, helium is an s-block element.
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).
Modern Periodic Law is - The physical and chemical properties of the elements are the periodic functions of their atomic numbers.