Mendeleev's periodic law and periodic table

Later, Mendeleev arranged the sixty-three elements known at that time in the increasing order of the atomic masses, in the form of a table called the Periodic Table. The periodic table further classified the elements by arranging the elements with similar properties together and separating the elements with dissimilar properties from one another.

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	-	-