Table of Contents –
- Mendeleev’s periodic law and Important terminology
- Arrangements of Mendeleev’s Periodic Table
- Importance of Mendeleev’s periodic Table
- Limitations of Mendeleev’s periodic Table
- Frequently asked questions (FAQ’s)
- Quiz – Check your understanding
Mendeleev’s periodic law and Important terminology –
A most important contribution on arrangement of elements after Newland was given by a Russian Chemist called Dmitri Mendeleev. Before discussing Mendeleev’s periodic table, we should learn the meaning of following terms –
Periodic Table – It is a chart or table of elements where elements having similar physical and chemical characteristics are placed in a similar vertical column (group).
Periodic – The elements having similar properties are repeated after certain intervals or periods.
Group – Vertical rows of elements present in periodic table is called group.
Period – Horizontal row of elements present in periodic table is known as period.
Mendeleev arranged the then known all 63 elements in the order of increasing atomic masses and similarities in their physical and chemical properties. Among chemical properties,
he specially concentrated on the compounds formed by elements with oxygen (oxides) and hydrogen (hydrides). The formulae of hydrides and oxides formed by an element were considered as one of the basic properties of an element for arrangement in Mendeleev’s periodic table.
Further, the Mendeleev’s periodic law states –
“The properties of elements are the periodic function of their atomic masses.”
♦ Oxygen and hydrogen are very reactive and form compounds with most elements. If general formula of oxides and hydrides of elements are same, then they show similar chemical properties.
Read first –
Arrangements of Mendeleev’s Periodic Table –
It consists of –
(1) The horizontal rows called periods . The first two periods are similar to Newland’s octaves.
(2) Eight vertical columns called groups.
♦ In a group , the elements have very similar properties and exhibit a clear trend in properties down the group.
♦ First seven groups were divided into two subgroups called ‘A’ and ‘B’, on the basis of similarities in the properties and formulae of oxides and hydrides . The elements which had kept in the left hand side of a group( group A) are called normal or representative elements. The elements which were present in the right hand side (group B) are called transition elements. The subdivision is made on the basis of difference in their properties.
♦ Group VIII had nine transition elements present in three sets, each containing three elements lied in the 4th, 5th and 6th period.
(1) Systematic Study of the elements –
Mendeleev’s periodic table was developed in accordance to-
(a) increasing atomic masses
(b) grouping of similar elements together
By the way, the study of elements became quite simpler, as if properties of one element in a particular group were known, properties of others could easily be assumed. Thus, there was a regular gradation in the properties of elements in groups and periods.
(2) Prediction of new elements and their properties –
In Mendeleev’s periodic table, some vacant spaces were left with strong prediction for their discovery later on. For e.g.
The predicted properties including atomic masses of the then discovered elements were found almost similar to the actual properties.
(3) Accommodation of Nobel gases –
Nobel gases were discovered later and were added in a new group called zero group which was not present in original Mendeleev’s periodic table. So, that original periodic table was not disturbed.
For e.g. Co (at. Mass = 58.9 higher) was placed before Ni (at. Mass = 58.7 lower) so that Co could be in the same column as Rhodium which closely resembles it in properties.
(4) Correction of doubtful atomic masses –
Mendeleev’s periodic table helped in correction of atomic weights of certain elements such as Beryllium (Be, group IIA) , gold (Au, group IB) and platinum (Pt, group VIII), based on their position in periodic table.
Limitations of Mendeleev’s periodic Table –
(1) Anamolous position of Hydrogen
(2) Position of Isotopes could not be explained
Isotopes were discovered later. They were atoms of same elements having similar chemical properties, but different atomic masses.
(3) Atomic masses do not increase in a regular manner of two successive elements.
So, it was not possible to predict the number of missing elements between two known elements, especially when we consider the heavier elements.
(4) Anomalous position of some pairs of elements
In some cases, Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses . Some elements having higher atomic masses had been placed before an element having lower atomic masses.
- Co(58.9) and Ni(58.7), Co was placed before Ni.
- Te (127.6) and I(126.9) Te was placed before I.
Thus, the position of these elements was not justified.
(5) Some similar elements were separated and dissimilar elements were grouped together.
For e.g., Cobalt and Mercury; Gold and Platinum; Lead and Barium; Silver and thallium; Manganese and halogens were placed separately although similar in properties. Co was placed in group I
although it did not resemble the elements in the group.
Thus, Mendeleev’s Periodic Table provided the basis of modern periodic classification.
QUIZ – Check your understanding