Making Order Out Of Chaos-Mendleev's Periodic Table

Periodic Classification of Class 10

While Dobereiner initiated the study of periodic relationship, it was Mendeleev who was responsible for publishing the periodic law for the first time. The first breakthrough in the classification of elements was provided by Mendeleev. He was regarded as the main contributor to the early development of the periodic table. In his periodic table elements were arranged on the basis of their fundamental properties i.e. atomic mass.

A periodic table may be defined as an arrangement which classifies all the known elements on the basis of their properties in such a way that similar elements are placed in the same vertical column while dissimilar elements are placed in different columns.

When he started his work on classification of elements, only 63 elements were known. He carefully examined the relationship between the atomic mass of the element and their chemical and physical properties. Among the chemical properties he concentrated on the compounds formed by the elements with hydrogen and oxygen as they are very reactive and formed compound with most of the elements. Therefore, the formula of oxides and hydrides formed by the elements were taken as the basis of classification of elements.

Mendeleev took 63 cards, each card representing an element, where he wrote down the properties of that element as well as the formulae of its hydride and oxide. He then separated and arranged the elements with similar properties and pinned the cards on the wall one after the other in a vertical column. When he observed these cards after arranging he found that most of the elements automatically got arranged in order of their increasing atomic masses. It was also observed that elements with similar properties occur after a certain interval or In other words there was recurrence of elements with similar physical and chemical properties after a regular interval. On this basis, he formulated a periodic law which states that

“the physical and chemical properties of elements are a periodic function of their atomic masses”.

Periodic function means that the properties of the elements get repeated after certain regular intervals. His law can also be stated as

“when elements are arranged in order of their increasing atomic masses, elements with similar properties are repeated after a certain regular intervals”.

This repetition of properties is called periodicity of properties.

MENDELEEV’S PERIODIC TABLE

Mendeleev arranged the known 63 elements in increasing order of their atomic masses in horizontal rows called periods in such a way that elements with similar properties fall under the same vertical column called groups.

Mendeleev’s Periodic Table

 Group I II III IV V VI VII VIII Oxide Hydride R2O RH RO RH2 R2O3 RH3 RO2 RH4 R2O5 RH3 RO3 RH2 R2O7 RH RO4 Periods ↓ A B A B A B A B A B A B A B Transition series 1 H 1.008 2. Li 6.939 Be 9.012 B 10.81 C 12.011 N 14.007 O 15.999 F 18.998 3. Na 22.99 Mg 24.31 AI 29.98 Si 28.09 P 30.974 S 32.06 Cl 35.453 4.First series: second series: K 39.102 Cu 63.54 Ca 40.08 Zn 65.37 Sc 44.96 Ga 69.72 Ti 47.90 Ge 72.59 V 50.94 As 74.92 Cr 50.20 Se 78.96 Mn 54.94 Br 79.909 Fe Co Ni 55.85 58.93 58.71 5.First series: second series: Rb 85.47 Ag 107.87 Sr 87.62 Cd 112.40 Y 88.91 In 114.82 Zr 91.22 Sn 118.69 Nb 92.91 Sb 121.75 Mo 95.94 Te 127.60 Tc 99 I 126.90 Ru Rh Pd 101.07 102.91 106.4 6.First series: second series: Cs 132.90 Au 196.97 Ba 137.34 Hg 200.59 La 138.91 Tl 204.37 Hf 178.49 Pb 207.19 Ta 180.95 Bi 208.98 W 183.85 Os Ir Pt 190.2 192.2 195.09

CHARACTERISTIC FEATURES OF MENDELEEV’S PERIODIC TABLE:

Mendeleev’s periodic table consists of

Eight vertical columns: Groups

They are designated as I, II, III, IV, V, VI, VII, and VIII. Except group VIII, each group is further sub-divided into two sub groups namely A and B. The elements which lie on the left hand side of the column constitute sub group A while the elements on the right hand side of that column constitute sub-group B.

Elements of sub-group A elements are known as normal or representative elements while sub group B elements are known as Transition elements. This sub division is made due to difference in their properties.

 Normal/Representative element Transition elements I A I B II A II B III A III B IV A IV B V A V B VI A VI B VII A VII B

Group VIII contain 9 transition elements arranged in a group of 3 elements and these lie in the 4th, 5th and 6th period.

Seven horizontal rows: Periods

In Mendeleev’s periodic table there were 7 horizontal columns present which we call as periods and are numbered from 1 to 7.

Nobel gases were not known at the time of Mendeleev. So there was no group for these gases but in 1900 when Nobel gases were discovered new group called zero group was introduced.

ACHIEVEMENTS OF MENDELEEV’S PERIODIC TABLE:

• Systematic study of the elements. The Mendeleev’s periodic table simplified and systematized the study of the elements and their compounds since their properties could now be studied as groups or families rather than individuals. Thus, knowing the properties of one element in a group, the properties of the other elements in the group can be easily predicted.
• Prediction of new elements. Mendeleev’s left some blank spaces or gaps. These gaps represented unknown elements. Further, Mendeleev predicted the properties of these unknown elements on the basis of their positions. He predicted the properties of gallium (ekaaluminium) and germanium (ekasilicon) which were unknown at that time.
• Correction of doubtful atomic weights. With the help of Mendeleev periodic table correction in doubtful atomic weights of some elements had been done. For example beryllium was assigned an atomic weight of 13.5 on the basis of its equivalent weight (4.5) and valency (wrongly calculated as 3).

Limitations of Mendeleev’s Periodic Table:

Inspite of many advantages, the Mendeleev’s periodic table has certain defects also. Some of these are given below.

All those Mendeleef’s classification of elements solved many problems like prediction of new elements, correction atomic masses and the properties of new elements to be discovered later but failed explaining some problems like

•  Position of Hydrogen atom in periodic table: Hydrogen is placed in group IB. However, it resembles the elements of group IA (alkali metals) as well as the elements of group VIIA (halogens). Therefore, the position of hydrogen in the periodic table is not correctly defined.
• Anomalous pairs of elements : Argon (Ar, atomic mass 39.9) is placed before potassium (K, atomic mass 39.0). Similarly cobalt (co, atomic mass 58.9) is placed before nickel (Ni, atomic mass 58.7) and tellurium (Te, atomic mass 127.6) is placed before iodine (I, atomic mass 126.9).
• Position of isotopes : Hydrogen isotopes with atomic masses 1, 2 and 3 should be placed at three places. However isotopes have not been given separate places in the periodic table.

Note : Isotopes are the atoms of the same element having different atomic masses.

4. Some similar elements are separated and dissimilar elements are grouped together.

Copper and mercury resembled in their properties but they had been placed in different groups. At the same time, elements of group IA such as Li, Na and K were grouped with copper (Cu), silver (Ag) and gold (Au) though their properties are quite different.

5.Cause of periodicity: Mendeleev did not explain the cause of periodicity among the elements.