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The principles of classification of elements are:
The periodic table is divided into four main blocks (s, p, d, and f) depending upon the sub-shell to which the differentiating configurations of the atom (the valence electron, which differentiates it from the previous atom is called the differentiating electron).
10Ne= 1s2, 2s2 2p6
11Na= 1s2, 2s2 2p6, 3s1
This last electron in sodium is known as a differentiating element which enters to 3s orbital. Hence, sodium belongs to s-block.
Those elements whose configuration end with 's' configuration are known as s-block elements. The elements belonging to group IA (alkali metals) and IIA (alkaline earth metals) are s-block elements. For example:
11Na= 1s2, 2s2 2p6, 3s1 (alkali metals)
20Ca= 1s2, 2s2 2p6, 3s23p6, 4s2(alkaline earth metals)
Those elements whose configuration end with 'p' configuration are known as p-block elements. The elements of group IIIA, IVA, VA, VIA, VIIA, and 0 belong to the p-block. For example- non-metal and inert gas.
5B= 1s1, 2s2 2p1
17Cl= 1s2, 2s2 2p6, 3s2 3p5
10Ne=1s2, 2s2 2p6
18Ar= 1s2, 2s2 2p6, 3s2 3p6
Those elements whose configuration end with 'd' configuration are known as d-block elements. 'd' block elements occupy 10 columns in the periodic table. The elements of group IB, IIB, IVB, VB, VIB, VIIB, and VIII belong to d-block. In this block of elements, d-orbital is gradually filled up. For example, in the 3rdperiod, the element scandium has 1 electron in 3d-orbital, while zinc has 10 electrons in 3d-orbital. The elements belong to the 'd' block are also known as transitional elements.
21Sc= 1s2,2s2 2p6, 3s2 3p6, 4s2 3d1
26Fe= 1s2, 2s2 2p6, 3s2 3p6, 4s2 3d6
30Zn= 1s2, 2s2 2p6, 3s2 3p6, 4s2 3d10
Those elements whose configuration end with 'f' configuration are known as f-block elements. In f-block elements, the f-orbital is gradually filled up with 14 electrons. It consists of two series of 14 elements known as lanthanides and actinides. These elements have been placed at the bottom of the periodic table to avoid its unnecessary expansion. These elements are also known as inner transitional elements.
The s, p, d, and f block elements are shown schematically in the long periodic table.
The elements in the periodic table can also be classified into four different types of elements depending upon the number of incomplete shells in an atom. This type of classification is also known as Bohr's classification.
Noble gases include the elements He, Ne, Ar, Kr, Xe, and Rn. These elements belong to the 0-group of the periodic table. These elements have ns2 np6electronic configuration except He (1s2). Noble gases are inert, stable and they do not undergo chemical combination in normal condition. Their molecules are monoatomic.
The elements with only one shell, namely the valence shell, incompletely filled belong to representative elements. This includes both s and p-block elements except inert gases. Thus the elements of group IA, IIA, IIIA, IVA, and VIIA are representative elements. These elements exhibit the valency of the group and they form a bridge between the elements of subgroup A and B,
The elements with two outermost shells incompletely filled belong to transitional elements. They are characterised by incompletely filled inner'd' orbital with one or two electrons in the valence shell. The electronic configuration of outermost two shells will be (n-1) d0-10 , ns1-2where (n-1) is the penultimate shell. These elements belong to subgroups B of I to VII and group VIII. All the elements in d-block belong to transitional elements. The main characteristics of transitional elements are:
I) formation of coloured-ions
II) show variable valency
III) acts as catalyst
V) form complex ions
VI) alloy formation
VII) Ionization Potential (I.P) is intermediate in between 's' and 'p' block elements.
Except for Cu, Ag, and Au, all of them possess two elements in their outermost shell. The penultimate orbit is incomplete and one or two electrons can be drawn from it for valency purposes. This is the cause of the variable valency.
Elements with three incompletely filled outermost shells belong to inner transitional elements. These elements constitute f-block in the periodic table. The electronic configuration of theses elements is given as:
(n-2)(f-14), (n-1)s2, (n-1)p6, (n-1)d(0-10), ns2. There are two series of inner transitional elements. One corresponds to filling of 4f orbital, which is known as lanthanides series starting after lanthanum. The elements from cerium (At. No 58 )to lutetium (At. No 71) forms the lanthanide series. The second series corresponds to the filling of 5f orbital. They are called actinides series which start after actinium (At. No 89). The elements from thorium (At. No 90) to Lawrencium (At. No 103) form the actinide series. They have similar physical and chemical properties due to the presence of same valence electrons.
Certain elements of the second period exhibit similarity in properties as shown by the elements diagonally placed to them in the third row. Example -Li and Mg, Be and Al, Band Si as shown below:
This is called diagonal relationship. This is due to identical ionic ratio and polarizing power (i:e charge/size ratio of the pairs of these elements). Elements of the second period are known as bridge elements.
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).
The principles of classification of elements are: Classification based on differentiating electrons and Classification based on a number of incomplete shells.