Mendeleev stated the law of chemical periodicity as: "The physical and chemical properties of elements are periodic functions of their atomic mass."
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.
Contributions of Mendeleev's Periodic Table
(i) Systematic study of elements
Mendeleev's Periodic table simplified the study of elements. It became useful in studying and remembering the properties of a large number of elements, in a simpler way. This is because the elements showing similar properties
belonged to the same group.(ii) Prediction of new elements
While arranging the elements, in increasing order of atomic mass, Mendeleev 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 the elements predicted and later
discovered
| Property | Eka-boron | Scandoum |
|---|---|---|
| Atomic weight Oxide Specific gravity Sulphate | 44 Eb2O3 3.5 Eb2(SO4)3 | 43.79 Sc2O3 3.864 Sc2(SO4)3 |
| Property | Eka-aluminium | Gallium |
|---|---|---|
| Atomic weight Specific gravity Melting point Formula of oxide Solubility in acid and alkali | 58 5.9 Low Ea203 Dissolves slowly in both acid and alkali | 69.9 5.94 303. 15°K Ga203 Dissolves slowly in both acid and alkali |
| Property | Eka-silicon | Germanium |
|---|---|---|
| Atomic weight Specific Gravity Melting point Valency Reaction with acid and alkali | 72 5.5 High 4 Slightly attacked by acids, resists attack by alkali | 72.32 5.47 958°C 4 Dissolves neither by hydrochloric acid nor sodium hydroxide |
(iii) 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 e.g., atomic mass of beryllium was corrected from 13.5 to 9. Atomic masses of indium, gold, platinum were also corrected.
Mendeleev's Periodic Table
| Grp:1 | Grp:2 | Grp:3 | Grp:4 | Grp:5 | Grp:6 | Grp:7 | Grp:8 |
|---|---|---|---|---|---|---|---|
| H=1 | - | - | - | - | - | - | - |
| Li=7 | Be=9.4 | B=11 | C=12 | N=14 | O=16 | F=19 | - |
| Na=23 | Mg=24 | Al=27.3 | Si=28 | P=31 | S=32 | Cl=35.5 | - |
| K=39 | Ca=40 | - | Ti=48 | V=51 | Cr=52 | Mn=55 | Fe=56 Co=58.9 Ni=58.7 cu=63 |
| - | Zn=65 | - | - | As=75 | se=78 | Br=80 | - |
| Rb=85 | Sr=87 | Yt=88 | Zr=90 | Nb=94 | Mo=96 | - | Ru=104 Rh=104 pb=106 Ag=108 |
| - | cd=102 | In=113 | Sn=118 | Sb=122 | Te=127.6 | I=126.9 | - |
| Cs=133 | Ba=137 | Di=138 | Ce=140 | - | - | - | - |
| - | - | Er=178 | La=180 | Ta=82 | W=184 | - | Os=195 Ir=197 Pt=198 Au=199 |
| - | Hg=200 | Tl=204 | pb=207 | Bi=208 | - | - | - |
| - | - | - | Th=231 | - | U=240 | - | - |
Defects of Mendeleev's Periodic Table
(i) Position of hydrogen
The position of hydrogen is not correctly defined. It is still not certain whether to place hydrogen in group I A or VII A.
(ii) 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 e.g., argon was placed before potassium whereas its atomic mass is more than potassium.
(iii) Position of 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.
(iv) Grouping of elements
Some similar elements are separated and dissimilar elements are grouped together e.g., copper and mercury have similar properties but are placed in different groups. On the other hand elements of group IA such as lithium, sodium and potassium were grouped with dissimilar elements such as copper, silver and gold.
(v) Cause of periodicity
Mendeleev's table was unable to explain the cause of periodicity among elements.
(vi) Position of 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.
