John Dalton gave the concept that :

Chronology

Cathode Rays & Discovery of Electrons

Alpha, Beta, Gamma Rays

Thomson's Atomic Model

Rutherford's Atomic Model

Bohr model – electron arrangement

Neutron discovered – Chadwick (1932)

Isotopes & radioactivity

The line diagram of electric discharge tube is given :

Atoms must have at least two types of particles

Some should have +ve charge, some –ve

Number of particles having +ve charge should be equal to number of particles of –vecharge

Atom is electrically neutral

+ve charge ---- protons

-ve charge ---- electronsNeutrons

are still not discovered

Atom is like a spherical ball

Positive electric charge is uniformly spread on

its total volume

Protons possessing +ve electric charge and

Electrons possessing –ve electric charge are

arranged at definite places

If the atom is like a sphere (Thomson model)

then all Alpha rays should return back after

striking foil.

Rutherford observed that most rays passed

straight through and only very few got deviated

or got thrown back

Proportion of Alpha rays returning and those

passing through was 1 : 1200

So, Mr. Thomson were wrong!!!

Atom is hollow

Centre – heavy, very small, positively charged nucleus

Electrons – negligible mass, negatively charged are arranged around nucleus

to summarize…..

i) 1st orbit (K orbit) – 2 electrons

ii) 2nd orbit (L orbit) – 8 electrons

iii) 3rd orbit (M orbit) – 18 electrons

iv) 4th orbit (N orbit) – 32 electrons

2nd, 3rd and 4th orbit have sub-orbits

called Orbitals

First electrons fill 1st orbit then go

stepwise to 2nd, 3rd and 4th orbits

Do not possess any electric charge

It is neutral

Mass almost equal to mass of

proton

1838 times more mass than

electron

If the number of protons remain same (atomic

number) but number of neutrons vary than the

element will have different masses

O16 , O17, O18

Isotopes of Uranium

U235

92U232

92 U238

92

* The isotopes of elements having high atomic masses

possess the property of radioactivity

* Not all high mass elements are radioactive

* Eg. Lead is not radioactive

* As atomic number increases, number of protons also

increase

* If no. of neutrons also increase the atom becomes

unstable

Determining age of old trees

Age of fossils

Radiometric dating

Medical treatments

Industries

Treatment of diseases like

cancer, etc.