PERIODIC CLASSIFICATION OF ELEMENTS(CBSE CLASS 10)

PERIODIC CLASSIFICATION OF ELEMENTS 

Dobereiner’s Triads  In a triad the atomic weight of middle element is always equals to Arithmetic mean of other two MERITS  The law of triads recognise the relationship between the atomic weight and properties of element DEMERITS  Not every element could be grouped in the triads

Newlands Law Of Octaves  If elements are arranged in increasing order of their atomic weight like the Octave so that 8 element shows similar properties like first element. MERITS  atomic weight are made the basis of classification periodicity of properties was recognised first time DEMERITS  the law of Octave failed because it does not shows the existence of transition elements

Mendeleev’s Periodic Law  The physical and chemical properties of elements are the periodic function of their atomic weight MERITS  correction of atomic weight are marked and undiscovered elements are predicted DEMERITS  Mendeleev could not assign a correct position to hydrogen in the periodic table as hydrogen resembles alkali metals as well as halogens. It was observed that at certain places, an element of higher atomic mass Cobalt (Co=58.93) has been placed before an element of lower atomic mass Nickel (Ni = 58.71).  the position of rare Earth metals elements and isotopes are ambiguous Some elements placed in the same sub - group varies in their properties. Manganese is placed with halogens which totally differ in the properties  the periodic table did not reflect the electronic configuration

Why Mendeleev's left some gaps in his periodic table 

Because at that time existence of some element has not been discovered mendellev  named them by prefixing Sanskrit numeral.

Modern Periodic Law  The physical and chemical properties of elements are the periodic function of their atomic number. MERITS  It is the most appropriate classification till date DEMERITS  the position of hydrogen is still unknown

TRENDS IN MODERN PERIODIC TABLE

1.  Atomic Size 

LEFT TO RIGHT

Number of cell remains same but electron goes on increasing to keep element  near the nucleus atomic size shrink

TOP TO BOTTOM 

as we go top to bottom number of cell keep on increasing  which results into increasing atomic size of atom

(2) Metallic And Nonmetallic Character 

LEFT TO RIGHT

metallic character decreases and non metallic character increases

REASON

Effective nuclear charge acting on the valence shell electrons increases across a period the tendency to lose electrons will decreases

TOP TO BOTTOM 

REASON 

the outermost electron are further away from the nucleus therefore they can be lost easily

Electropostive

X-e-------->++

• Metals are electronpostive in nature.
• They are formed by loss of electron.

Electronegative

X+e-------->X-

• Non metals are electronegative in nature
• They are formed by gain of electron.

Valence Electron 

Electron present in outermost shell are known as valence electron

Valency 

it is the combining capacity of an atom with other atom

example

carbon has a valency so it combined with other four atom with itself or other atom