In 1869, Mendeleev classified the then known 56 elements on the basis of their physical and chemical properties in the increasing order of the atomic masses, in the form of a table. Mendeleev had observed that properties of the elements orderly recur in a cyclic fashion. He found that the elements with similar properties recur at regular intervals when the elements are arranged in the order of their increasing atomic masses. He concluded that 'the physical and chemical properties of the elements are periodic functions of their atomic masses'. This came to be known as the law of chemical periodicity and stated:
Based on this law all the known elements were arranged in the form of a table called the 'Periodic Table'. Elements with similar properties recurred at regular intervals and fell in certain groups or families. The elements in each group were similar to each other in many properties. The elements with dissimilar properties from one another were separated. Mendeleev's periodic table contains eight vertical columns of elements called 'groups' and seven horizontal rows called 'periods', Each group has two sub-groups A and B. The properties of elements of a sub-group resemble each other more markedly than the properties of those between the elements of the two sub-groups.
Achievements of Mendeleev's Periodic Table
Mendeleev's periodic table was one of the greatest achievements in chemistry with some of its important contributions as follows:
Systematic Study of Elements
Mendeleev's Periodic table simplified the study of elements. As the arrangements of elements showing similar properties were classified into groups, it was very useful in studying and remembering the properties of a large number of elements in a systematic way.
Prediction of New Elements
Based on the positions in the periodic table, Mendeleev could predict the properties of some undiscovered elements. He left three blanks for elements that were not discovered at that time. He was able to predict the properties of these unknown elements more or less accurately. He named them eka-boron, eka-aluminium and eka-silicon. He named them so, as they were just below boron, aluminium and silicon in the respective sub-groups. Eka-boron was later named as scandium, eka-aluminium as gallium and eka-silicon as germanium.
A Comparative Study of the Properties of Elements Predicted and Later Discovered
| Property | Eka-boron | Scandium |
| Atomic weight | 44 | 43.79 |
| Oxide | Eb2O3 | Sc2O3 |
| Specific gravity | 3.5 | 3.864 |
| Sulphate | Eb2(SO4)3 | Sc2(SO4)3 |
| Property | Eka-aluminium | Gallium |
| Atomic weight | 68 | 69.9 |
| Specific gravity | 5.9 | 5.94 |
| Melting point | Low | 303.15 |
| Formula of oxide | Ea2O3 | Ga2O3 |
| Solubility in acid and alkali | Dissolves slowly in both acid and alkali | Dissolves slowly in both acid and alkali |
| Property | Eka-silicon | Germanium |
| Atomic weight | 72 | 72.32 |
| Specific gravity | 5.5 | 5.47 |
| Melting point | High | 958 |
| Valency | 4 | 4 |
| Reaction with acid and alkali | Slightly attacked by acids, resists attack by alkali | Dissolves neither by hydrochloric acid nor sodium hydroxide |
Correction of Atomic Masses
Mendeleev's periodic table helped in correcting the atomic masses of some of the elements, based on their positions in the periodic table. For example, atomic mass of beryllium was corrected from 13.5 to 9.0. Atomic masses of indium, gold and platinum were also corrected.
Mendeleev's Periodic Table
Period  |
Group I | Group II | Group III | Group IV | Group V | Group VI | Group VII | Group VIII |
| 1 | H=1 | - | - | - | - | - | - | - |
| 2 | Li=7 | Be=9 | B=11 | C=12 | N=14 | O=16 | F=19 | - |
| 3 | Na= 23 | Mg= 24 | Al= 27.3 | Si=28 | P=31 | S=32 | Cl=35.5 | |
| 4 First series | K=39 | Ca=40 | Sc=45 | Ti=48 | V=51 | Cr=52 | Mn=55 | Fe=56 Co=58.9 Ni=58.7 |
| 4 second series | Cu =63 | Zn= 65 | Ga=70 | Ge= 73 | As=75 | Se=79 | Br=80 | - |
| 5 First series | Rb= 85 | Sr=87 | Y=89 | Zr=91 | Nb=93 | Mo=96 | Tc=99 | Ru=102 Rh=103 Pd=106 |
| 5 second series | Ag= 108 | Cd= 112 | In= 115 | Sn= 118 | Sb=122 | Te=128 | I=127 | - |
| 6 First series | Cs= 133 | Ba= 137 | La= 138 | Hf= 178 | Ta=181 | W=184 | - | Os=190 Ir=193 Pd=106 |
| 6 second series | Au= 197 | Hg= 200 | Ti= 204 | Pb= 207 | Bi=208 | - |
Limitations of Mendeleev's Classification
In spite of the above advantages, Mendeleev's periodic table suffered defects as follows:Position of Hydrogen
The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).
Grouping of Some Elements
In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses, while some dissimilar elements were grouped together. Thus, the position of these elements was not justified. For example, cobalt (at. mass 58.9) was placed before nickel (at. mass 58.6); copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.
Anomalous Pair
In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.
Isotopes
Isotopes are atoms of the same element having different atomic mass but same atomic number. For e.g., there are three isotopes of hydrogen with atomic mass 1, 2, and 3. According to Mendeleev's periodic table these should be placed at three separate places. However isotopes have not been given separate places in the periodic table.
Lanthanides and Actinides
Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Mendeleev's periodic table.
Cause of Periodicity
Mendeleev's table was unable to explain the cause of periodicity among elements.
