22.THE ATOMIC STRUCTURE-Discovery of The Nucleus.

We continue our discussion from the earlier topic and delve still deeper into the subatomic domain.The Plum Pudding Model, hypothesized by Sir J.J. Thompson failed to correctly explain the structure of atom but his efforts didn’t entirely go in vain – thanks to his student Ernest Rutherford – a genius who unleashed the power of an atom by discovering its true structure.

Ernest Rutherford – The father of Nuclear Physics.

Ernest Rutherford, was born on 30th August 1887 , in New Zealand.he grew up in a small town in New Zealand on a farm!He studied in New Zealand and in 1894 , he got a scholarship , which enabled him to go to Trinity College , Cambridge as a research student , under J.J.Thompson!Later he went to Mc  Gill University, Montreal, in Canada, as a Professor of Physics.

J.J. Thomson wrote supporting Rutherford for the MacDonald Chair at McGill University:“I have never had a student with more enthusiasm or ability for original research than Mr Rutherford and I am sure that if elected he would establish a distinguished school of Physics at Montreal.”

While in  McGill University in Canada, .He found out that the α – particle is formed due to the disintegration of a parent nucleus into a daughter element.In short, he discovered nuclear transmutation.He won a Nobel prize in Chemistry in 1908 for this discovery.

Rutherford left Mc Gill university in 1907 as he thought that Europe was a better place to conduct his research.He stated –

“I shall be glad to be nearer the scientific centre as I always feel America as well as Canada is on the periphery of the circle.”

So,he came to Victoria University of Manchester,England.As a Nobel laureate, Rutherford determined that the  α – particles are helium nuclei (He2+) – helium atoms devoid of their electrons.Thus they are positively charged.And the β – particles are electrons (e) and are thus negatively charged.In 1919, he accepted an invitation to succeed his mentor sir J.J.Thompson ,as Cavendish Professor of Physics at Cambridge.

THE Gold Foil / Rutherford-Geiger-Marsden EXPERIMENT

Following are the events that lead to the Gold Foil Experiment – 221

Around the year 1907, began an important series of researches under the guidance of Rutherford in Manchester.Rutherford and Hans Geiger were deflecting  α- particles in electric and magnetic fields , to study the charge on them.The experimental setup had a ZnS screen , which would produce scintilla of light when the  α- particles hit it. They observed that , there was a change in the deflection pattern of the  α-particles , when a mica sheet was introduced in its path.In this case, the spots on the photographic plate, which were indications of  α-particles hitting the screen became blurred .Each time the mica plate was inserted they saw this phenomenon.This was happening because, the atoms of the mica sheet were kind of scattering the  α-particles.Rutherford and his group of scientists started studying this phenomenon to find out exactly what was the reason for what they were observing.They inserted plates of various metals (Al,Pb etc)in the path of α-particles.The best known experiment they did was with a gold foil – The Gold Foil /Rutherford-Geiger-Marsden experiment.

Rutherford and his two associates – Hans Geiger and Ernest Marsden(Marsden was about 20 years old ,who was a college dropout and was recruited by Rutherford to work in his lab)were using the α- particles to check the validity of the plum pudding model and to find out the reasons behind their observations. They aimed a beam of α- particles at a piece of gold foil that was approximately 600 nm thick.

They choose gold because –

1) It is a noble metal (i.e a metal that does not corrode , is not very reactive and is not easily affected by acids) and it does not oxide in air.

2)Gold is highly malleable – it can be drawn into very very thin sheets(of the order of 600 nm/0.6µm thick )

The source of  α- particles  was Radium)Z= 88),Polonium(Z=84),Thorium(Z=90) [Z= Atomic number].These elements  are highly radioactive,which disintegrate to form α- particles. These elements were stored in a lead(Pb) container because lead has high density and high atomic number.Thus, it protects the environment from the radioactive radiations acting as a shield.They had  particles zooming out of this source at 7.68 MeV per particle.

The Gold Foil Experiment.

To detect the α- particles that pass through the gold foil , they placed a circular fluorescent screen coated with zinc sulphide, ZnS.This screen was developed by Geiger.Each time an α- particle hits the screen, a scintilla (spark) of light would be produced and a microscope placed above the screen would magnify it , thus making it easy for one of the two experimentalist(Geiger / Marsden) to observe where the α- particles landed on the screen.If the structure of an atom was in accordance with the model proposed by J.J.Thompson, all α- particles should have passed through the gold foil and hit the screen directly behind the foil.To observe the light produced upon the contact of α- particles with the screen , the experiment was conducted in total darkness.The experimenter would sit alone in total dark and manually count the sparks and see their positions !

Months passed but Geiger and Marsden didnot see anything unusual. One day , Geiger bumped into his mentor Rutherford in the corridor outside the lab and reported that they still hadn’t seen anything unusual.Rutherford asked Geiger to check if they see any scintilla of light on the same side as that of the experimenter.He suggested that they place the screen on the same side as that of the source and check if they see anything! Now, Rutherford was a man of intuition! His students followed his instructions and did exactly what he asked them to do.

What did they observe?

1)Most α- particles behaved exactly as expected. They passed directly through the foil,without any deviation  and hit the screen behind the foil.

2)A few α- particles were deflected at small angles(by 1 or 2 degrees).

3)Very few  were deflected by an angle of 90 degrees.

4)For days after the suggestion by Rutherford to check if some particles were bouncing back Geiger and Marsden saw absolutely nothing.One afternoon however they saw a scintilla of light on the screen on the same side of the source.  Geiger reported to Rutherford in great excitement that –  ‘We have been able to get some of the α-particles coming backwards…’Rutherford quoted –

“It was quite the most incredible event that has ever happened to me in my life. It was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”. (Rutherford, 1938, p. 68)

This was the greatest discovery made that very very few  (1 in 20000) α-particles were deflected at an angle of 360 degrees  from the parent beam! This particle fired right back or bounced back  at the experimenter! This phenomenon ofback scattering’ lead to the discovery of nucleus!

The Plum pudding  atomic model could not explain these observations.So, Rutherford proposed his own nuclear Model.He however gave credit to Geiger too, quoting –

“Geiger is a good man and worked like a slave. I could never have found time for the drudgery before we got things going in good style. Finally all went well, but the scattering is the devil. Our tube worked like a charm and we could easily get a throw of 50 mm. for each particle. … Geiger is a demon at the work of counting scintillations and could count at intervals for a whole night without disturbing his equanimity. I damned vigorously and retired after two minutes”. (Quoted in Eve, p. 180.)

Rutherford’s / Planetary/Nuclear  Atomic Model

Rutherford’s Atomic Model .

Based on his experiments, Rutherford concluded that –

♦ An atom must be extremely hollow and must have empty space within it as most α- particles passed through it without any deviation.

♦ The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.

♦If the α- particles got close to the positive centre they would be deflected by small angles due to electrostatic repulsion.

♦ As very few are deflected violently , α- particles must be encountering a huge positive charge from which they get repelled completely and so they fling back at the source.But as the frequency of these particles(deflected at 360 deg) is low, this positive charge must occupy a very little volume as compared to the volume of the atom.He thus inferred that an atom consists of a very heavy positively charged body at its centre called the nucleus, which contains protons.Thus,this atomic model is also called Nuclear model.Nucleus occupies a small fraction of volume of the atom and is the place where all the weight of the atom is concentrated(Electrons have negligible mass as compared to that of protons and neutrons).

♦ The electrons revolve around the nucleus with huge velocity in a circular orbit , just as  the planets revolve around the sun. Thus, this model is also called the planetary model.

♦ Marsden calculated for Rutherford the diameter of the nucleus.He found out that,
Radius of the nucleus/Radius of the atom ≈ 1/10000
The diameter of a nucleus is around 10-15 m.
The diameter of a nucleus is around 10-10 m.

Hence, there is lots of empty space in the atom, which explains why most α- particles go zooming through the gold foil without getting deflected.To understand this ratio, assume that one takes all the empty space from all atoms in a human body out.Then the human will be reduced to just a speck of dust , although his weight would remain the same.If the empty space from all atoms in all the human bodies in this world is taken out, the remaining mass would be the size of an apple!Other way to imagine this is the nucleus is just a spec of a dust particle in a atom which is as huge as a cathedral! 

Limitations of Rutherford’s planetary atomic model –

Rutherford’s atomic theory received a lot of condemnation from the scientific world.The reason for this strong negative reaction was that the theory could not explain the following points –

♦ The electrons moving around the nucleus are negatively charged. So, due to Coulombic/electrostatic forces of attraction,the electrons should get attracted to the nucleus and there should be a NUCLEAR COLLAPSE ! However, this never happens.

224 According to classical mechanics and electromagnetic theory, any moving ,accelerated charged particle emits electromagnetic radiation(EMR) and thus lose energy. So,the electrons should spiral into the nucleus –  BAM !! or just stop due to ENERGY DEFICIET !! Although in reality an atom is very stable and electrons keep moving around the nucleus.

♦ Another drawbacks of Rutherford’s model was that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.He assumed that the electrons could occupy any position around the nucleus.This arrangement should give us a continuous spectrum.However, atomic spectrum is a line spectra, with discrete lines.Thus,this  theory was unable to explain the atomic spectra too.

So, what happens next? Is the Rutherford model correct? Does it need amendment or corrections? Who would decode this mystery? We shall see the answers to these questions in my post.Till then,

Be a perpetual student of life and keep learning…

Good day !

References and Further Reading –

5.MIT 3.091SC Introduction to Solid State Chemistry, Fall 2010 by Professor Donald Sadoway,Lecture 3.
6.History of atom (BBC) documentary.

Image source –

2.By Unknown – http://wal.nbed.nb.ca/sciencesettechnologies/pierrebrideau/geiger.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=34187140
3.By S P Andrew Ltd. – http://mp.natlib.govt.nz/detail/?id=27055&l=en, Public Domain, https://commons.wikimedia.org/w/index.php?curid=34174345

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