CBSE Class 10 Chemistry Periodic Classification of Elements Notes Set 23

Early Attempts at Classification

1. Dobereiner's triads (1817): groups of three similar elements in which the middle atomic weight is about the average of the other two (Li, Na, K).
2. Newlands' law of octaves (1865): when elements are arranged by increasing atomic weight, every eighth element repeats the properties of the first. It failed beyond calcium.
3. Mendeleev's periodic law (1869): the properties of elements are a periodic function of their atomic weights.

Mendeleev's Table

Mendeleev arranged elements in horizontal rows (periods) and vertical columns (groups). He left gaps for undiscovered elements and predicted the properties of eka-aluminium and eka-silicon, later discovered as gallium and germanium. His table also helped correct doubtful atomic weights. Its defects were the position of hydrogen, the placement of isotopes and a few pairs, such as argon and potassium, being in the wrong order of atomic weight.

Modern Periodic Law

Moseley showed that atomic number is the fundamental property. The modern law states that the properties of elements are a periodic function of their atomic numbers. The long form has 7 periods and 18 groups. Elements of a group have the same number of valence electrons; elements of a period have the same number of shells.

Blocks of the Periodic Table

1. s-block: Groups 1 and 2 (alkali and alkaline earth metals).
2. p-block: Groups 13 to 18 including halogens and noble gases.
3. d-block: Groups 3 to 12, the transition elements.
4. f-block: lanthanides and actinides placed at the bottom.

Periodic Trends

Across a period (left to right): atomic size decreases, metallic character decreases, non-metallic character and electronegativity increase, valency first rises and then falls.
Down a group: atomic size increases, metallic character increases, electronegativity decreases and the tendency to lose electrons increases.
Metalloids such as boron, silicon, germanium, arsenic, antimony and tellurium lie along the border between metals and non-metals and show properties of both.