Chapter 8 Atoms & Nuclei

Thomson’s model of atom- Every atom consists of charged sphere in which electrons are embedded like seeds in water melon
##### Its drawbacks:-

couldn’t explain large angle scattering & the origin of spectral series.

##### Rutherford’s model of atom-

i) Every atom consists of a tiny central core, called the atomic nucleus, in which the entire positive charge and almost entire mass of the atom are concentrated.

ii) The size of nucleus is of the order of 10-15, which is very small as compared to the size of the atom which is of the order of 10-10m.

iii) The atomic nucleus is surrounded by certain number of electrons. As atom on the whole is electrically neutral, the total negative charge of electrons surrounding the nucleus is equal to total positive charge on the nucleus.

iv) These electrons revolve around the nucleus in various circular orbits as do the planets around the sun. The centripetal force required by electron for revolution is provided by the electrostatic force of attraction between the electrons and the nucleus.

##### Limitations :-

couldn’t explain the stability of the nucleus & the emission of line spectra of fixed frequencies.

Distance of closest approach of the alpha particle in the α particle scattering experiment
Impact parameter of the alpha particle
Bohr’s model of atom Limitations-applicable only for hydrogen like atoms & couldn’t explain the splitting of spectral lines. (not consider electro static force among the electrons)
Orbit radius of the electron around the nucleus
Energy of the electron in the nth orbit of hydrogen atom
Angular momentum of electron in any orbit is integral multiple of h/2ð L = mvr = nh/2π, n=1,2,3,…
Wave number í
No of protons in a nucleus
Mass Number (A) Number of neutrons No. of nucleons(protons + neutrons) in a nucleus A-Z
Nuclear density Ñ= 3m/4ðR03
Isotopes Same Z & different A Ex, 1H2,1H3,1H1, & C12,C14,C16
Isobars Same A & different Z [ 18Ar40,20Co40] & (1H3, 2H3)
Isotones Same no. of neutrons Mass of neutrons – 1H3, 2He4
Mas defect Δ m Total Mass of nucleons – mass of nucleus
Binding energy Eb E= m c2 ( m= mass of reactants – mass of products) 1 a.m.u.= 931.5 Mev
Radioactive decay law dN/dt=-ëN -dW/dt= R= Activity, unit Bq.
N =N0e-λt
OR
N=N0( ½)n , n = t/t1/2
Half life
Mean life τ= 1/λ
Nuclear fission Splitting of a heavy nucleus into lighter elements.This process is made use of in Nuclear reactor & Atom bomb
Nuclear fusion Fusing Fusing of lighter nuclei to form a heavy nucleus.This process takes place in Stars & Hydrogen bomb. Controlled Thermonuclear Fusion In a fusion reactor-high particle density is requiredhigh plasma temperature of 109Ka long confinement time is required

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