This is the notes for Mendeleev and modern periodic table.
Periodic Table
Mendeleev’s periodic table:
In 1869, Dmitri Ivanovich Mendeleev classified the elements in the form of a table which is known as Mendeleev's periodic table and put forward the law known as Mendeleev periodic law which states that, “the physical and chemical properties of the elements are periodic functions of their atomic weights”. This means that, if the elements are arranged in order of their increasing atomic weights, similar properties are repeated after a regular interval.
Features of Mendeleev's periodic table:
- It consists of 9 vertical columns called groups. They are numbered as I, II, III, IV, V, VI, VII, VIII and zero groups. Except for group VIII and zero, each group from I to VII subdivided into sub-group A and B.
- It consists of 7 horizontal rows called periods. They are numbered as 1, 2, 3, 4, 5, 6, and 7. The periods are divided into short periods and long periods.
Advantages of Mendeleev periodic table
1. Systematic study of chemistry of elements: It simplified and made the study of the elements systematic by classifying the elements into groups and periods. The properties of other elements in the group can be easily predicted by knowing the properties of one element in a group.
2. Prediction of properties of undiscovered elements: Mendeleev kept some blank spaces in his periodic table for the undiscovered elements and also predicted the properties of those undiscovered elements along with their position in the periodic table. For example: Scandium, Gallium and Germanium which were unknown at that time were named by Mendeleev as Eka- Boron, Eka- Aluminium and Eka-Silicon respectively. Later, when these elements were discovered, their properties were found to be similar to those as predicted by Mendeleev.
3. Correction of doubtful atomic mass : Mendeleev’s periodic table helped in correcting the doubtful atomic masses of some elements like Beryllium, Gold, Indium and Platinum.
For example: Be was assigned with atomic weight 13.5 on the basis of equivalent weight 4.5 and valency wrongly calculated as 3.
atomic weight = equivalent weight X valency
According to this atomic weight Be should be placed between Carbon (12) and nitrogen (14). But there is no vacant place between C and N and properties of Be did not justify that position. Therefore, Mendeleev assigned the valency of Be 2, so that its atomic mass becomes 9 and was placed between Lithium (7) and Boron (11).
atomic weight = equivalent weight X valency
4.5 X 2 = 9 (correct)
Anomalies and defects in the Mendeleev’s periodic table
1. Position of hydrogen: Hydrogen is placed in group IA along with alkali metals without any justification. But hydrogen resembles the halogens group (VIIA) in many respects. Thus, the position of hydrogen is anomalous or controversial.
Group I Na-e- →Na+ H-e- →H+ e.g. HCl
Group VII Cl+e- →Cl- H+e- →H- e.g. NaH
2. Position of isotopes: According to Mendeleev periodic law, isotopes of the same element should have separate positions in the periodic table but no separate positions were given for the isotopes. For example: Three isotopes of hydrogen; protium (atomic mass 1), deuterium (atomic mass 2) and tritium (atomic mass 3) were not given separate positions.
3. Position of lanthanides and actinides: The Lanthanides and actinides have not given the proper position within the main frame of the periodic table but they are placed into separate rows at the bottom of the periodic table without any explanation.
4. Anomalous pairs: Certain elements having higher atomic weights have been placed before those having lower atomic weights. The following are such anomalous pairs: Ar→ 39.9 has been placed before K→ 39.1 Co→ 58.9 has been placed before Ni→ 58.7
5. Similar elements are separated and dissimilar elements are kept together: In the Mendeleev periodic table chemically similar pairs of elements like copper and mercury, silver and thallium, gold and Platinum, etc. have been placed in separate groups. Whereas coinage metals like copper, silver and gold having no similarity with alkali metals are grouped together in group I.
Modern Periodic Table (Henry Moseley, 1992)
Modern periodic law states that, “the physical and chemical properties of the elements are periodic function of their atomic number.” This means that, if the elements are arranged in the order of increasing atomic number, the elements with similar property reoccur after regular intervals.
Periodicity and cause of periodicity
• The repetition of elements with similar properties after certain regular intervals when these are arranged in the order of increasing atomic number is called periodicity.
• We know that, the electrons present in the valence shell take part in chemical reaction which controls the properties of the atoms. If the arrangement of electrons in the valence shell of the atoms is same, then their properties should be similar.
• Acc. to modern periodic law, when the elements are arranged in the order of their increasing atomic no., the elements having similar valence shell electronic configuration gets repeated at regular intervals.
• Thus, periodicity arises due to similarity in the valence shell electronic configuration.
• For e.g. Alkali metals (group IA: Li, Na, K, Rb, Cs, Fr) have 1 electron in their valence shell with the outermost electronic configuration ns1 and therefore their chemical properties are similar.
• Similarly, Halogens (group VIIA: F, Cl, Br, I, At) have 7 electrons in their valence shell with the outermost electronic configuration ns2np5 and therefore their chemical properties are similar.
Long form/ Modern periodic table
The most commonly and widely used long form of periodic table is based on modern periodic law and electronic configuration of atoms. It is also called Bohr’s periodic table since it follows Bohr’s scheme for the arrangement of elements based on their electronic configuration.
Main features of long form/modern periodic table:
The long form of periodic table consists of :
a) 7 horizontal rows called periods
b) 18 vertical columns called groups
Study of periods:
i. First period (n=1); very short period, contains 2 elements; H and He
ii. Second period (n=2); short period, contains 8 elements; Li to Ne
iii. Third period (n=3); short period, contains 8 elements; Na to Ar
iv. Fourth period (n=4); long period, contains 18 elements; K to Kr
v. Fifth period (n=5); long period, contains 18 elements; Rb to Xe
vi. Sixth period (n=6); very long period, contains 32 elements; Cs to Rn
vii. Seventh period (n=7); very long period; incomplete period with capacity of 32 elements; elements are radioactive in nature.
Study of groups:
There are 18 vertical columns called groups. Each group consists of series of elements having the same valence shell electronic configuration. These 18 vertical columns which are divided into IA, IIA, IIIA, IVA, VIA, VIIA; IB, IIB, IIIB, IVB, VB, VIB, VIIB; VIII and zero [ Acc. to Bohr ]. Also, according to IUPAC, the long form of periodic table has been numbered from left to right as group 1 to group 18.
Advantages of Modern Periodic Table
1. Position of isotopes: Since the classification is based on the atomic no., the position of isotopes is fully justified.
2. Removal of Mendeleev’s misfit points: The position of Ar and K, Co and Ni, was not according to Mendeleev’s periodic law. These defects/anomalies have been removed when elements are arranged on the basis of atomic number. For e.g. Ar (18) preceeds K(19) Co (27) preceeds Ni(28)
3. Separate position for subgroup: The modern periodic table has given separate position for subgroup A and subgroup B. The anomalies due to grouping of chemically dissimilar elements and separation of chemically similar elements is removed. For e.g. alkali metals (IA) and coinage metals (IB) do not fall in the same group.
4. Beauty of periodic law: The arrangement of elements according to the increasing atomic number is more scientific and systematic. In such arrangements, periodicity occurs after certain interval so that similar elements fall in the same group. The position of elements is governed by the electronic configuration which determines their property.
5. Division of elements into s, p, d and f blocks: The division of elements into s, p, d and f blocks according to their electronic configuration has made their study easier.
Hope this will help you a lot.
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