Periodic Classification of Elements
Early attempts of classification of elements:-
Johann Wolfgang Dobereiner was a German chemist. His effort is considered as one of the earliest attempts to classify the elements into groups.
He found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle; is the arithmetic mean of rest of the two. On this basis, he arranged three elements in one group which is known as 'Triad'. This arrangement of elements is known as Dobereiner's Triads.
Limitation of Dobereiner's Triads:-
Dobereiner could find only three such triads (group of three elements) and he could not even put all the elements known at that time in his triads.
The rules of Dobereiner's triads could not be applied to the elements which had very low or high atomic mass. Such as; if F, Cl and Br are put together in a triad, in increasing order of their atomic masses, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
After the advancement of techniques of measuring atomic mass more correctly Dobereiner's Law became obsolete.
Newlands' Law of Octaves:-
Newlands found that every eighth element has similar physical and chemical properties when they are arranged in increasing order of their relative masses. This law is known as Newlands' Law of Octaves which states that "any given element will exhibit analoogus behaviour to the eighth element following it in the table". This means every eight element has the similar chemical and physical properties. For example; Sodium is the eighth element from Lithium and both have similar properties.
The arrangement of elements in Newlands' Octave resembles the musical notes. In musical notes, every eighth note produces similar sound. Because of this; Newland's classification of elements was popularly known as just Octaves.
Limitation of Newlands' Octaves:-
Newlands' Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves.
Newlands' Octaves was valid for lighter elements only.
It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time.
More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
Iron; which has similar property as cobalt and nickel, was placed far from them.
Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses.
Mendeleev's Periodic Table:
Dmitri Ivanovich Mendeleev, a Russian scientist arranged the elements in increasing order of their relative atomic masses. He was honoured with Noble prize in 1906 for his Periodic Table.
Mendeleev's Periodic Law states that the properties of elements are the periodic function of their relative atomic masses.
Mendeleev arranged all 63 elements; which were discovered till his time; in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev's Periodic Table. He divided the table in eight columns and seven rows. The columns are known as groups and rows are known as periods.
Explanation of Mendeleev's Periodic Table:-
Elements are arranged in the periodic table in the increasing order of their relative atomic masses.
Mendeleev divided his periodic table in eight groups and seven periods.
Groups from I to VII are meant for normal elements and group VIII is for transition elements.
Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.
Periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
Elements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group.
Two general formulae; one for oxides and second for hydrides; have been given for the elements of each group in the periodic table. For example: R2O for oxides and RH for hydrides, of the elements; of 1st group.
Using the given general formulae; the formula of oxides and hydrides can be written for the elements of each group. For example; hydrogen, sodium, potassium, etc. belong to the first group. The general formula of oxides for the elements of 1st group is R2O. Accordingly they form H2O, Na2O, K2O, etc.
Merits of Mendeleev's Periodic Table:-
Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group.
For example; titanium has been placed in IVth group, leaving a blank space adjacent to it in IIIrd group. Similarly, arsenic has been placed in Vth group; leaving two adjacent spaces blank. These spaces have been occupied by scandium, gallium and germanium after their subsequent discovery.
Prediction for the elements to be discovered in future:
Mendeleev predicted the discovery of some elements and named them as eka-boron, eka-aluminium and eka-silicon. He gave the name of these elements prefixing the word 'eka' to the name of the preceding elements.
Scandium, Gallium and Germanium have been discovered later and took the place of eka-carbon, eka-aluminium and eka-silicon, respectively in the gap left in the Mendeleev's Periodic table; as their properties were exactly similar to the predicted elements.
Position of Noble gases which were discovered later:-
Noble gases were discovered much later after Mendeleev. After the discovery of noble gas, they were placed in a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements in the Mendeleev's Periodic Table. Noble gases are chemically un-reactive and present in very low concentration in the atmosphere.
Limitation of Mendeleev's Periodic Table
Position of Hydrogen:-
Hydrogen has been placed in 1st group with alkali metals, since hydrogen makes compound in the same way as alkali metals do.
On the other hand, hydrogen exists as diatomic molecule; similar to halogen and hydrogen makes covalent compounds also as halogens do.
Thus, on the basis of properties of hydrogen similar to halogen, hydrogen may be placed with halogens but Mendeleev did not explain about this anomaly.
Position of Isotopes:-
Elements having same atomic number but different atomic masses are known as isotopes. Although isotopes were discovered after Mendeleev, but it became a challenge to accommodate those isotopes in Mendeleev's Periodic Table without disturbing the order of elements.
Wrong Order of Elements:-
Mendeleev placed many elements in wrong order of their increasing atomic masses in order to place elements having similar properties in similar group.
The atomic mass of nickel is less than that of cobalt; in spite of that cobalt is placed before nickel.
The atomic mass of Chromium is 50.20 and the atomic mass of vanadium is 50.94. In spite of this, chromium is placed after vanadium.
In spite of above limitations and anomalies, the Mendeleev's Periodic Table was one of the wonderful discoveries.
Modern Periodic Table:-
Law of Modern Periodic Table states that properties of elements are the periodic function of their atomic numbers. In the modern periodic table, elements are arranged in order of their increasing atomic numbers.
Explanation of Modern Periodic Table:-
Elements are arranged in order of their increasing atomic numbers.
The vertical columns are known as groups and horizontal columns are known as periods; in the modern periodic table.
There are 18 groups and 7 periods in the modern periodic table.
Elements having same number of valence electrons are placed in the same group. For example; elements having valence electrons equal to 1 are placed in the 1st group, elements having valence electrons equal to 3 are placed in the 13th group, elements having valence electrons equal to 2 are placed in 2nd group except helium which is placed in 18th group, since it is an inert gas.
Elements having same number of shells are placed in the same period.
Groups in Modern Periodic Table:-
1st group:- Alkali metals are placed in the 1st group in the modern periodic table. Hydrogen is also placed in the 1st group although hydrogen is not an alkali metal.
2nd group:- Alkaline earth metals are placed in the 2nd group in the modern periodic table.
Elements placed in 1st and 2nd groups in the modern periodic table are collectively known as light metals.
3rd to 12th group:- Transition elements are placed from 3rd to 12th group in the modern periodic table.
13th group:- Metals are placed in the 13th group; except boron which is a metalloid.
14th group:- Carbon, silicon, germanium, tin and lead are placed in this group. Among them, carbon is a non-metal, silicon and germanium are metalloids and tin and lead are metals.
15th group:- Nitrogen, phosphorous, arsenic, antimony and bismuth are place in the 15th group; among which nitrogen and phosphorous are non-metals, arsenic and antimony are metalloids and bismuth is a metal.
16th group:- Oxygen, sulphur, selenium, tellurium and polonium are placed in this group, among which oxygen, sulphur and selenium are non-metals, tellurium is metalloid and polonium is a metal.
17th group:- Non-metals are placed in the 17th group. Since, halogens are placed in this group hence this group is also known as group of halogen.
18th group:- Noble gases are placed in the 18th group. This group is also known as zero group.
Periods in Modern Periodic Table:
At present there are seven periods in the Modern Periodic Table.
1st period: This is known as very short period as there are only two elements, i.e. hydrogen and helium.
2nd and 3rd period: There are total 8 elements in each of the 2nd and 3rd periods. These periods are known as short periods.
4th and 5th period: There are total 18 elements in each of the 4th and 5th periods. These periods are known as long periods.
6th period: There are total 32 elements in 6th period. This period is known as very long period.
7th period: This period is known as incomplete period. Blank spaces in this period are supposed to be filled by the elements discovered in future.
Position of elements in the Modern Periodic Table:
Elements are placed in groups according to their valence electrons and placed in periods according to the number of shells present in them.
Helium has valence electrons equal to 2, but it is placed in group number 18 because it is a noble gas and has completely filled outermost shell.
Hydrogen: Atomic number 1
Valence electron = 1
Number of shell = 1
Position in periodic table 1st group 1st period
Sodium: Atomic number 11
Valence electron = 1
Number of shell = 3
Position in periodic table 1st group 3rd period.
Carbon: Atomic number 6
Valence electron = 4
Number of shell = 2
Position in periodic table 14th group 2nd period.
Neon: Atomic number 10
Valence electron = 8 i.e. equal to 0 (zero)
Number of shell = 2
Position in periodic table 18th group 2nd period.
Trends in Modern Periodic Table:
Number of valence electron remains the same while moving from top to bottom in a group.
Number of valence electron increases while moving from left to right in a period.
Valency remains the same on moving from top to bottom in a group.
Valency first increases upto 4 on moving from left to right in a group and then decreases upto zero.
Atomic size increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
Atomic radius increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
Metallic character of elements:
Metallic character decreases on moving from left to right in a period.
Tendency to lose electron:
Tendency to lose electrons decreases on moving from left to right in a period and tendency to lose electrons increases on moving from top to bottom in a group.
Tendency to lose electron is also known as electropositive character, thus electropositive character decreases on moving from left to right in a period and increases on moving from top to bottom in a group.
Tendency to gain electron:
Tendency to gain electrons increases on moving from left to right in a period. Tendency to gain electron is called electro-negativity, thus electro-negativity of elements increases on moving from left to right in a period.
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