History of periodic table | Lavoisier Classification, Prout's Hypothesis, Dobereiner trade, Newland octave rule, Lothar Mayer's Curve, Mendeleev's periodic table

1.0 Introduction :

The arrangement of all the known elements according to their properties in such a way that the elements which similar properties are grouped together in a tabular form.

Development of periodic table

(A) Lavoisier Classification :

(i) Lavoisier classified the elements simply in metals. Metals are the one which have the tendency of gaining the electrons.

Na → Na⁺ + e ‐ and  K → K⁺ + e-

Non-metals are the one which have the tendency of gaining the electrons.

F  + e- → F⁻  and   Cl + e ⁻→ Cl ⁻

(ii) Drawback or limitation :

•  As the number of elements increases, this classification become insufficient for the study of elements.
• There are few elements which have the properties of both meals as well as non-metals and they are called metalloids. Lavoisier could not decide where yo place the metalloids.

(B) Prout's Hypothesis :

He simply assumed that all the elements are made up of hydrogen, so we can say that

Atomic weight of elements = n × (Atomic weight of one hydrogen atom)

¤ Atomic weight of H = 1
Where n = number of hydrogen atom = 1, 2, 3,...

Drawback or limitation :

• Every element can not be formed by Hydrogen.
• Atomic weight of all elements were not found as the whole numbers.

   Ex. Chlorine (atomic weight 35.5) and Strontium (atomic weight 87.6)

(C) DOBEREINER TRIAD RULE [1817] :

(i) He made groups of theee elements having similar chemical properties known as TRIAD.
(ii) In Dobereiner triad, atomic weight of middle element is nearly equal to the average atomic weight of first and third element.

Ex. 1. Cl = 35.5 , Br = 80.0 , I = 127

X =( 35.5 + 127 ) ÷ 2 = 81.2

2. Ca = 40 , Sr = 87.6 , Ba = 137

X = (40 + 135) ÷ 2 = 88.5

3. Li = 7 , Na = 23 , K = 39

X = (7 + 39) ÷ 2 = 23

¤ Where x = average atomic weight

(iii) Other examples - (K, Rb, Cs), (P, As, Sb), (S, Se, Te)

Drawback or limitation :

All the known elements could not be arranged as triads. It is not applicable for d and f-block elements.

(D) NEWLAND OCTAVE RULE [1865]

(i) He arranged the elements in the increasing order of their atomic mass and observe that properties of every 8th element was similar to the 1st element. (like in the case of musical vowels notation)

(ii) At that time jnert gases were not known.

(iii) The properties of 'Li' is similar to 8th element that is 'Na' are similar to 'Mg' and so on.

Drawback or limitation :

• This rule is valid only upto Ca because after Ca due to presence of d-block element there is a difference of 18 elements instead of 8 elements.
• After the discovery of Inert gand including them in the periodic table, it has become the 8th element from Alkali metal so, this law had to be dropped out.

(E) LOTHAR MEYER'S CURVE [1869] :

(i) He plotted a curve between atomic weight and atomic volume of different elements.
(ii) The following observation can be made from the curve -

• Most electropositive elements that is Alkaki metals (Li, Na, K, Rb, Cs) occupy the peak positions on the curve.
• Less electropositive that is Alkaljne Earth metals (Be, Mg, Ca, Sr, Ba) occupy the descending position on the curve.
• Metalloids (Si, Ge, As, Sb, Te, Po, At) and transition metals occupy bottom part on the curve.

Conclusion : Kn the basis of this curve Lother Meyer proposed that the physical properties of the elements are periodic functions of their atomic weight and this has become the base of Mendeleev's Periodic Table.

Periodic Function : When the elements are arranged in the increasing order of their atomic weight, elements having similsr properties gets repeated after a regular interval.

(F) MENDELEEV'S PERIODIC TABLE [1869]:

(i) Mendeleev's periodic law: The physical and chemical properties of elements are the periodic function of their atomic weight.
(ii) Characteristics of Mendeleev's periodic table:

• It is based on atomic weight
• 63 elements were known, noble gases were not discovered
• He was the first scientist to classify the elements in a systematic manner i.e. in horizontal rows and in vertical columns.
• Horizontal rows are called periods and there were 7 periods in Mendeleevs Periodic table
• Vertical columns are called groups and there were 8 group in Mendeleev's Periodic table
• Each group upto VIll is divided into A & B subgroups. 'A' sub group element are called normal or representative elements and 'B' sub group elements are called transition elements
• The VIl group consisted of 9 elements in three rows (Transitional metals group)
• The elements belonging to same group exhibit similar properties

(iii) Merits or advantages of Mendeleev's periodic table :

• Study of elements: First time all known elements were classified in groups according to their similar properties. So study of the properties of elements become easier
• Prediction of new elements : It gave encouragement to the discovery of new elements as some gaps were left in it.

Sc (Scandium)
Ga (Gallium)
Ge (Germanium)
Tc (Technetium)

These were the elements for whom position and properties were well defined by Mendeleev even
before their discoveries and he left the blank spaces for them in his table.

Ex. Blank space at atomic weight 72 in silicon group was called Eka silicon (means properties like
silicon) and element discovered later was named Germanium.
Similarly other elements discovered after mendeleev's periodic table.

Eka Aluminium -- Galium(Ga)
Eka Silicon -- Germanium(Ge)
Eka Boron -- Scandium(Sc)
Eka Mangenese -- Technetium(Tc)

• Correction of doubtful atomic weights : Coection were done in atomic weight of some elements.

 Atomic weight = Valency × Equivalent weight.

Initially, it was found that equivalent weight of Be is 4.5 and it is trivalent (V = 3), so the weight of
Be was 13.5 and there is no space in Mendeleev's table for this element. So, after correction, it was found that 'Be' is actually bi-valent (V = 2). So, the weight of Be became 2 x 4.5 = 9 and there was a space between Li and B for this element in Mendeleev's table.

Corrections were done in atomic weight of elements are- U, Be, In, Au, Pt.

(iv) Demerits of Mendeleev's periodic table:

• Position of hydrogen : Hydrogen resembles both, the alkali metals (i A) and the halogens (Vii A) in properties so Mendeleev could not decide where to place it.
• Position of isotopes : As atomic wt. of isotopes differs, they should have placed in different position in Mendeleev's periodic table. But there were no such places for isotopes in Mendeleev's periodic table.
• Anomalous pairs of elements : There were some pair of elements which did not follow the increasing order of atomic weights.

Ex. Ar and Co were placed before K and Ni respectively in the periodic table, but having highe
atomic weights.

[Ar = 39.9 K = 39.1]
[Co = 58.9 Ni = 58.6]
[Te = 127.5 I = 127]
[Th = 232 Pa = 231]

• Like elements were placed in different groups : There were some elements like Platinum (Pt) and Gold (Au) which have similar properties but were placed in different groups in Mendeleev's table.

Pt(VIII)
Au(I B)

• Unlike elements were placed in same group :

Cu, Ag and Au placed in first group along with Na, K etc. While they differ in their properties (Only similar in having ns' electronic configuration).

(f) It was not clear that 'lanthanides and Actinides' are related with IIIA group or IIIB group.
(g) Cause of periodicity : Why physical & chemical properties repeated in a group.