Before the beginning of the eighteenth century, when there were only 30 elements known, it was easier to study and remember their properties. In later years, when number of elements discovered were increased then it became difficult to study them.
So, scientist fell the need of a simple method to facilitate the study of the properties of various elements and their compounds. After numerous attempts they got success & elements were arranged in such manner that similar elements were grouped together and different elements were separated. This arrangement of elements is known as classification of elements which led to the formation of periodic table.
Periodic table may be defined as the arrangement of all the known elements according to their properties in such way that the elements of similar properties are grouped together in a tabular form.
Earlier attempts of classification of elements (development of periodic table):
Earlier attempts to classify the elements resulted in grouping as metals and non-metals. Later on they were classified on the basis of their atomic masses.
DOBEREINER TRIADS RULE:
In 1817, Johann Walfgang Dobereiner, a German chemist, arranged the elements in group of three elements and in a manner that the atomic mass of middle element was roughly the average of the atomic masses of the other two elements of the triad.
Average of the atomic masses of Lithium and Potassium is . Only three triads could be arranged in this manner at that time. They were:
Average of the atomic masses of first and third element
SHORT COMING OF DOBEREINER' S TRIAD'S RULE:
This classification was not found satisfactory as it could be applied to the limited number of elements. Now a days some more triads have been made they are:
For a Dobereiner's triad, all the three elements should belong to the same group and the difference in atomic number should be 8 or 18.
New lands law of octave: In 1866, J.A.R. Newlands correlated the chemical properties of the elements with the increasing order of atomic masses. i.e. to place the element having lowest atomic mass (H) at first position and end with the element having highest atomic mass. (Thorium which was 56th known element at that time).
Definition: When the elements are arranged in order of their increasing atomic masses, every eighth element has the properties similar to those of the first elements like the eighth note of an octave in music.
Thus according to this law, the physical & chemical properties are repeated after an interval of eight elements and this is similar to eight notes of an octave on a musical scale shown below:
Co and Ni
Ce and La
The properties of Lithium are similar to that of 8th element i.e. Na, Be is similar to Mg and so on.
1. Law of octaves was applicable only up to calcium. It worked well with lighter elements only.
2. At that time only 56 elements were existed in nature, but later several elements were discovered which cannot be kept in periodic table as per this law. Their properties were not in accordance with the law of octaves.
Law of octaves:
(i) In order to fit element in to his table, Newlands adjusted two elements in the same column. For example, cobalt and nickel were placed in the same position and in the same column as fluorine, chlorine and bromine.
(ii) Iron which resembles cobalt and nickel in properties were placed far away from these elements.
After the discovery of inert gases & included in the periodic table it becomes the eighth element from alkali so this law has to be dropped out.
Mendeleev's Periodic table:
In the year 1861, D Mitri Ivanovich Mendeleev arranged all the known elements (63 elements) in the form of a table in which elements were arranged in the increasing order of their atomic mass and also on the similarities of chemical properties.
The arrangement of elements was based on their physical and chemical properties and also on the formulae of the compounds they formed with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.
The table which classifies the elements in such a way that elements having similar properties are placed in same vertical column or group in known as periodic table.
The term periodic means repetition of elements having similar properties after a certain regular interval. The periodic table consists of vertical columns which are called as groups and horizontal rows called as periods.
Mendeleev's periodic table had six periods and eight groups as shown in the table, he arranged all the elements horizontally in the order of their increasing atomic masses and vertically according to their similarities in properties. Each group was further sub divided into two sub groups A & B.
Achievements of the Mendeleev's periodic table:
1. Systematic study of the elements: All the elements in general were arranged systematically in increasing order of their atomic masses. This arrangement helped to study the properties of various elements. If the nature of the element present in a group is known, it become easier to predict or guess the expected properties of other elements.
2. Prediction of new elements: Mendeleev predicted the properties of some unknown elements and left gaps for these elements to be filled as and when discovered. For eg. Scandium, Gallium and Germanium were not known at that time but Mendeleev already named these elements as eka-boron, eka-aluminium and eka-silicon.
When these elements were later on discovered, they were found to have more or less similar properties as predicted by Mendeleev.
3. Position of Noble gases: When noble gases were discovered they were placed in a new group without disturbing the existing order.
4. Correction of atomic masses: Atomic masses of several elements were corrected on the basis of periodic table. eg. Atomic mass of Beryllium was corrected from 135 to 9. Mendeleev predicted that atomic mass of gold is incorrect. Later on it was found to be 197. Similarly atomic masses of Indium, Uranium and Platinum were also corrected.
Drawbacks of Mandeleev's periodic table:
Position of Hydrogen is uncertain because it resembles with IA group alkali metals elements and VII A (halogens) group elements.
(i) Isotopes: Isotopes of an element have similar chemical properties but different atomic masses.
(ii) Position of isotopes: Since basis of periodic table was increasing atomic mass. So isotopes should be placed separately but no separate place was given to isotopes.
(iii) Anamolus pairs of certain elements: Certain elements were not arranged according to their increasing atomic mass. eg.
(a) Argon (Atomic mass 39.9) was placed before potassium (atomic mass 39.0)
(b) Cobalt (58.95) before Nickel (58.70)
(c) Tellurium (127.6) before Nickel (126.9)
(d) Thorium (232) before Protactimum (231)
(iv) Similar elements were placed in different groups. eg.
(a) Silver and thallium
(b) Barium and lead
(c) Copper and mercury
(d) Platinum and gold.
(v) Dissimilar elements were placed in same group eg. silver and gold were placed in a same group while there is little similarity in physical and chemical properties.
(vi) Cause of periodicity: Mendeleev did not explain the cause of periodicity in the physical and chemical properties of the elements.
(vii) Metals have not been separated from non-metals.
(viii) Position for elements of group: There is no proper position for the elements of group
(ix) Consisting of elements in three triads. These elements are placed outside the main structure of the periodic table.