Periodic Classification of Elements

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-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.