Periodic Classification of Elements | Class 10 CBSE | Web Notes | Part 1 - Early Attempts of classification

• Elements are substances that contain only one type of atom.
• Currently, 118 elements are known, of which 94 are naturally occurring.

MAKING ORDER OUT OF CHAOS – EARLY ATTEMPTS AT THE CLASSIFICATION OF ELEMENTS

• Elements were initially grouped as metals and non-metals.

Döbereiner’s Triads

• In 1817, German chemist Johann Wolfgang Döbereiner identified groups of three elements, called triads.
• In a triad, when elements are arranged by increasing atomic mass, the atomic mass of the middle element is roughly the average of the other two.
• Examples of triads:

Triad 1	Atomic Mass
Li	6.9
Na	23.0
K	39.0

Triad 2	Atomic Mass
Ca	40.1
Sr	87.6
Ba	137.3

Triad 3	Atomic Mass
Cl	35.5
Br	79.9
I	126.9

• However, only three triads were identified, making this classification system limited. For example, the elements N (14.0), P (31.0), and As (74.9) do not form a triad, as the average atomic mass of N and As (44.45) does not match P’s atomic mass.

Newlands’ Law of Octaves

• In 1866, English scientist John Newlands arranged elements in order of increasing atomic masses, from hydrogen to thorium (the 56^th element).
• He observed that every 8^th element had properties similar to the first, resembling musical octaves, hence named the Law of Octaves.
• Examples: Sodium (8^th after lithium) resembles lithium; beryllium and magnesium resemble each other.
• A part of Newlands’ Octaves:

Notes of Music	sa (do)	re (re)	ga (mi)	ma (fa)	pa (so)	da (la)	ni (ti)
	H	Li	Be	B	C	N	O
	F	Na	Mg	Al	Si	P	S
	Cl	K	Ca	Cr	Ti	Mn	Fe
	Co & Ni	Cu	Zn	Y	In	As	Se
	Br	Rb	Sr	Ce & La	Zr	—	—

• Drawbacks of Newlands’ Octaves:
• It was applicable only up to calcium. After calcium, every 8^th element did not share similar properties with the first.
• Newlands assumed only 56 elements existed. New elements discovered later did not obey the Law of Octaves. To fit elements, he placed two elements in the same slot (e.g., cobalt and nickel) and grouped unlike elements together (e.g., cobalt and nickel with fluorine, chlorine, and bromine).

Iron, which resembles cobalt and nickel, was placed far away.

• The discovery of noble gases made the Law of Octaves irrelevant, as it worked well only with lighter elements.

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1. Early Attempts of Classification 2. Mendeleev's Periodic Table 3. Modern Periodic Table

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