Classification and Blocks in Periodic Table

Classification and Blocks in Periodic Table:

Topics covered:
Earlier classifications
Dobereiner’s Triads
Newland’s law of octaves
Mendeleev’s Periodic Classification
Modern Periodic Table or Moseley’s Periodic Law 

With the discovery of a large number of elements, it became difficult to study the elements individually, so the classification of elements was done to make the study easier.

 Earlier Classification:

Dobereiner’s Triads: In triads, the atomic mass of the middle element is approximately the average of the other two elements. This is known as the Law of Triads. This classification was applicable to very few elements and so it was rejected. For example:

New Lands Law of Octaves: He arranged the elements in increasing order of their atomic weights and showed that the properties of every eight elements were similar to those of the first one. The relationship is just like the resemblance of first and eighth musical notes. He named this the Law of Octaves. But his classification was rejected since the law of octaves was applicable to elements up to calcium. The properties of the eighth element become not similar for the rest of the element.                                                                                   

Mendeleev’s Periodic Law: Dimitri Mendeleev classified the elements in the increasing order of their atomic weights. He founded that the properties of elements repeat after a regular interval. Based on this observation, he proposed a Periodic Law which states that “The properties of elements are the periodic functions of their atomic weights.” That is, when elements are arranged in the increasing order of their atomic weights, their properties repeat after a regular interval.                                                                                             

       Mendeleev arranged elements in horizontal rows called Periods and vertical Columns called Groups. When Mendeleev proposed his periodic table, some of the elements were not discovered. He left some vacant places (gaps) for them in the periodic table and predicted some of their properties. For e.g. both Gallium and Germanium were not discovered at that time. He named these elements as Eka-Aluminium and Eka-Silicon respectively and predicted their properties.

Merits of Mendeleev’s periodic table:
(i) It was the first comprehensive classification of elements.
(ii) He corrected the wrong atomic weights of some elements and placed them in the correct position in the periodic table.
(iii) He left vacant places for undiscovered elements and predicted some of their properties.
(iv) Elements with similar properties are placed in the same group.

Drawbacks of Mendeleev’s periodic table:
(i) Elements with dissimilar properties are found in the same group.
(ii) He could not give an exact position for hydrogen.
(iii) He could not give an exact position for Lanthanoids and Actinoids and also for isotopes.
(iv) Mendeleev’s periodic table did not strictly obey the increasing order of atomic weights.

Classification and Blocks in Periodic Table

Modern Periodic Law: Moseley performed experiments and studied the frequencies of the x-rays emitted from the elements. With these experiments, he concluded that the atomic number is a more fundamental property of an element than its atomic mass.
Based on this observation, he modified Mendeleev’s periodic law as the physical and chemical properties of elements are the periodic functions of their atomic numbers. This is known as Modern Periodic law.

Long-form of Periodic Table (Bohr’s table):
(i) It is divided into two categories   
Vertical columns – Groups       
Horizontal rows – Periods
(ii) There are 18 groups
(iii) There are 7 periods
There are 7 periods in the Modern periodic table. The period number corresponds to the highest principal quantum number of the elements.

The first period contains 2 elements (H and He). Here the subshell filled is 1s. This period is called a very short period.

The second period contains 8 elements (Li to Ne). Here the subshells filled are 2s and 2p. The third period also contains 8 elements (Na to Ar). Here the subshells filled are 3s and 3p. These 2 periods are called short periods.

The fourth period contains 18 elements (K to Kr). Here the subshells filled are 4s, 3d, and 4p. The fifth period also contains 18 elements (Rb to Xe). Here the subshells filled are 5s, 4d, and 5p. These 2 periods are called long periods.

The sixth period contains 32 elements (Cs to Rn). Here the subshells filled are 6s, 4f, 5d, and 6p. This period is the longest period in the periodic table and is called the Monster period.

The seventh period is an incomplete period. It can also accommodate 32 elements. Here the subshells filled are 7s, 5f, 6d, and 7p.

The 14 elements each of the sixth and seventh periods are placed in separate rows below the main body of the periodic table. These are together called inner transition elements. The 14 elements of the sixth period (from cerium to lutetium) are called Lanthanides or Lanthanones or Lanthanoids or rare-earth metals. The 14 elements of the seventh period (from thorium to lawrencium) are called Actinides or Actinones or Actinoids.

Main Group Elements/ Representative Elements:
The s- and p- block elements are called main group elements or representative elements.
s-block: Elements in which the last electron enters the s orbital of their outermost shell. Since there is only one orbital in the s block which can accommodate two electrons so there are two groups in the s block. Group 1 (alkali metals), group 2 (alkaline earth metals). They are all reactive metals with low ionization enthalpies. They lose their outermost electrons readily to form +1 and +2 ions. Their metallic character and reactivity increase down the group. They mainly form ionic compounds (except Li and Be).
General electronic configuration: ns1-2 , n = 1-7

p-block: Elements in which the last electron enters any one of the three p orbitals. Since p subshell has three orbitals that can accommodate six electrons so there are 6 groups in the periodic table. They are also called Representative elements.
Group 13 – 18. Their non-metallic character increases from left to right in a period and metallic character increases from top to bottom in a group.
General Electronic Configuration: ns2 np1-6, n = 2-7
Group 16: Chalcogens,
Group 17: Halogens
Group 18: Noble gases

Elements of s and p block collectively are called representative elements
d-block: Elements in which the last electron enters any one of the five d orbitals of their respective penultimate shells. Since d subshell has five d orbitals, which can accommodate ten electrons so there are 10 groups in the periodic table. they show a transition (change) from the most electropositive s-block elements to the least electropositive p-block elements. So they are called Transition Elements. They are all metals, form colored compounds or ions, show variable oxidation states and valencies, paramagnetism, and catalytic properties.
Group 3 – 12
General Electronic Configuration: (n-1)d1-10 ns1-2 , n = 4-7.
Since properties of these elements are midway between s and p block. They are called Transition Elements. An incomplete d-subshell in the atom or its stable ion is a pre – Condition for Transition Elements.

f-Block elements (Inner-transition Series): Lanthanoids characterized by the filling of 4 f-orbitals, are the elements following lanthanum from 58Ce to 71 Actinoids characterized by filling of 5f-orbitals, are the elements the following actinium from 70Th to 103Lr. Characteristic outer electronic configuration is (n-2) f1-14 (n-1) d0-1 ns2.                  Classification and Blocks in Periodic Table

Leave a Reply

Your email address will not be published.