4
CHAPTER
CLASSIFICATION OF ELEMENTS Chapter Outline 4.1 4.2 4.3 4.4 4.5 4.6
Genesis of Periodic Classification Modern Periodic Law and Present form of The Periodic Table Nomenclature of Elements with Atomic number >100 Electronic configuration of Elements and the Periodic Table Electronic Configuration and Types of Elements: s,p,d,f-blocks Periodic Trends in Properties of Elements
4.1 GENESIS OF PERIODIC CLASSIFICATION ■ By 1865, the number of known elements increased from 31 to 63; 88 occur naturally, while 26 are man-made, with atomic weight becoming a key property after Dalton’s atomic theory.
Dobereiner's Law of Triads ■ John Dobereiner identified triads, sets of three elements with similar chemical properties, where the middle element’s atomic weight is approximately the average of the other two. Dobereiner’s triads Element
Atomic weight
Element
Atomic weight
Element
Atomic weight
Li
7
Ca
40
Cl
35.5
Na
23
Sr
88
Br
80
K
39
Ba
137
I
127
Telluric Helix ■ In 1862, A.E.B. de Chancourtois arranged elements by increasing atomic weight in a spiral cylindrical table called the Telluric Helix. ■ It did not apply to all known elements.
Newland’s Law of Octaves ■ John Newlands arranged elements by increasing atomic weight and observed that every eighth element showed similarities, calling it the Law of Octaves. ■ Drawback: It worked only for lighter elements (up to calcium) and failed for heavier elements.