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

PropertyEka-boronScandoum
Atomic weight
Oxide
Specific gravity
Sulphate
44
Eb
2O3
3.5
Eb
2(SO4)3
43.79
Sc
2O3
3.864
Sc
2(SO4)3

Property Eka-aluminium Gallium
Atomic weight
Specific gravity
Melting point
Formula of oxide
Solubility in acid
and alkali
58
5.9
Low
Ea
203
Dissolves slowly in both
acid and alkali
69.9
5.94
303. 15°K
Ga
203
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 - -