Download Atomic models

DISCOVERY OF ELECTRON :
Cathode ray discharge tubes [ partially evacuated tubes – made of glass containing 2
thin pieces of metal (electrodes) , sealed in it ]. Electrical discharge through gases can
be observed only at very low pressures and high voltage. The pressure of different gases
could be adjusted by evacuation.
Results :
1) Cathode rays start from cathode and move towards anode.
2) These are not visible and can be detected by fluorescent/phosphorescent
materials (which glow when hit by them).
3) These rays travel in straight lines in absence of electrical and magnetic fields.
4) In the presence of electrical or magnetic field, they behave like negative charged
particles, suggesting that cathode rays consist of negatively charged particles
(called electrons).
5) The characteristic of cathode rays do not depend upon the material of electrodes
and the nature of the gas present in the cathode ray tube. Thus, we conclude that
electrons are basic constituents of all the atoms.
Atomic models
Observations –
1) Most of the
∝ –particles passes through the gold foil undeflected.
2) A small fraction of the
3) A very few
∝ –particles was deflected by small angles.
∝ –particles (
by nearly 180
°
1 in 20000) bounced back, i.e., deflected
.
Conclusions regarding the structure of atom –
1) Most of the space in the atom is vacant as most of the
∝ –particles passes
through the foil undeflected.
2) A few positively charged
∝ –particles were deflected. The deflection must be
due to some enormous repulsive force showing that the positive charge of the
atom is not spread throughout the atom as Thomson had presumed. The positive
charge has to be concentrated in a very small volume that repelled and deflected
the positively charged
∝ –particles.
3) Calculations by Rutherford showed that the radius of the atom is about 10 –10 m,
while that of nucleus is 10–15 m.
Rutherford’s Nuclear Model of Atom (was proposed after the discovery of protons)– based
on above observations and conclusions
1) The positive charge and most of the mass of the atom was densely concentrated
in extremely small region , which was called nucleus by Rutherford.
2) The nucleus is surrounded by electrons that move around the nucleus with very
high speed in circular paths called orbits.
3) Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks of Rutherford Model –
1) According to classical electromagnetic theory of Maxwell, charged particles when
accelerated should emit electromagnetic radiation. Hence, an electron in an
orbit will emit radiation, the energy carried by radiation comes from electronic
motion. The orbit thus continues to shrink. Calculations show that it should take
an electron only 10–8 s to spiral into the nucleus. But this does not happen.
Thus, Rutherford model cannot explain the stability of an atom.
2) It says nothing about the electronic structure of atoms , i.e. , how the electrons
are distributed around the nucleus and what are the energies of these electrons.
BOHR ’S MODEL FOR HYDROGEN SPECTRUM ( Neils Bohr , 1913 ) –
This explained quantitatively the general features of hydrogen atom structure and its
spectrum.
Postulates which forms the basis of Bohr’s model for hydrogen spectrum :
1) The electron in the hydrogen atom can move around the nucleus in a circular path
of fixed radius and energy. These paths are called orbits / stationary states /
allowed energy states , which are arranged concentrically around the nucleus.
2) The energy of an electron in the orbit does not change with time.
3) Out of infinite number of possible circular orbits around the nucleus, the electrons
can move only in those orbits whose angular momentum is integral multiple of
h
2π
[ and this is why only certain fixed orbits are allowed ]
+¿
h
me v r =n .
, n∈ Z ¿
2π
Bohr’s model of the hydrogen atom was an improvement over Rutherford’s nuclear
model, as it account for the stability and line spectra of hydrogen atom and hydrogen like
ions (He+ , Li2+ , Be3+ and B4+).
Limitations of Bohr’s model – It was too simple to account for the following points :
1) It fails to account for the finer details
2) Bohr’s theory was unable to explain the splitting of spectral lines in the presence
of magnetic field (Zeeman effect) or an electric field (Stark effect).
3) It could not explain the ability of atoms to form molecules by chemical bonds.
Heisenberg’s Uncertainty Principle (Werner Heisenberg , 1927) –
“It is impossible to determine simultaneously, the exact position and momentum(or
velocity) of an electron”.
This uncertainty principle is the consequence of dual behaviour of matter and radiation.
Mathematically ,
Δx × Δ p x
≥
h
4π
⇒
Δx × Δv x ≥
h
4 πm
where,
Significance of Uncertainty Principle :
Reasons for the Failure of the Bohr Model –
1) It ignores dual behaviour of matter.
2) It contradicts Heisenberg uncertainty principle.
QUANTUM MECHANICAL MODEL OF ATOM –
Quantum mechanics – theoretical science that deals with the study of microscopic
objects taking into account the dual behaviour of matter. It specifies the laws of motion
that these objects obey. This branch was developed independently in 1929 by Werner
Heisenberg and Erwin SchrÖdinger.