An Atom
The smallest indivisible particle of chemistry which has been studied for ages and still we haven't found its precise true nature yet... This is the basic and the most important concept of chemistry, It's importance and mysterious nature can be highlighted by the fact that even nobel-prize winning scientist (looking at you Thomson) couldn't figure out it's true nature...yet still became famous because of their wrong theories (ง •_•)ง
In this blog, we will be learning about one of the theories put forth by Rutherford, which helped discover some intriguing facts about these miniscule particles!
So let's start!
Rutherford's Alpha Scattering Experiment
When radioactive elements were discovered, Rutherford turned his attention towards the particle emitted by these compounds. While doing experiments related to this, he noticed some peculiar observations which were not according to his expected results, this made him formulate the ever-important theory about atomic structure ( You might be thinking -- So he starts with an experiment to figure out about radioactive elements but ends up finding something about the structure of atom? Yes, Scientist used to digress alot back in the days...)
The above diagram shows the set-up used by Rutherford to do his experiment. Radium is a radioactive material which gives out alpha, beta and gamma rays ( here, we will be discussing only about the alpha rays, which are positively charged). He took a gold sheet , which is the most malleable material, and beat it into a thin foil of about 0.00004 cm in thickness. Alpha particles on their own are not visible to naked eyes, but when they hit the Zinc Sulphide screen, sparks of light is observed. (In the above set-up, a lead box is used as lead is a good absorber of alpha particles, it helps to control the amount of alpha particles released.)
Rutherford observed that:-
1) Most of the alpha- particles went straight through the gold-foil without any deflection ( nearly 99.9 %)
2) A few of the particles got deflected through small angles after passing through the foil
3) About one in 20,000 did not pass through the gold foil at all but suffered large deflections ( more than 90 degrees) or even come back in the direction from which they had come (i.e 180 degrees)
Now, this last point is what rattled Rutherford's brain... You might be wondering ...Why?
Let me give an example to make visualisation easier, Imagine firing a bullet at a tissue paper, it hitting the paper but instead of rupturing a hole and passing through it bounces back right in your direction ( and you move so that the bullet doesn't hit you, I don't want you to die before we even get to the important stuff!)
Impossible right? That's exactly what happened in his experiment. Makes sense why he completely freaked out when he saw what he did , right?
and we all know what scientist would do if they find something they couldn't explain... Say that either it's supernatural and that he has lost his mind or investigate futher (^///^)
Fortunately, Rutherford decided to take the second option and investigate further
The conclusion he drew from the experiment are :-
1) There must be a very large empty space within the atom as most of the alpha particles went straight through the metal foin undeflected.
2) Whole of the positive charge is concentrated and the space occupied by this positive charge is very small in the atom. This is the reason why few alpha particles, when they come closer to this point, they suffer a force of repulsion (like charges repel) and deviate from the path.
This concentration of positive charge was named Nucleus.
3) A very few of the alpha particles suffered strong deflection or even returned on their path indicating that the nucleus is rigid and the alpha particles recoil due to direct collision.
On the basis of his experiments, Rutherford proposed his atomic model - which was also known as the nuclear model, he found out some interesting things --
*a) Nucleus has a radius of 10 ^ -13 cm ( Superscript -13)
b) Atom has a radius of 10 ^ -8 cm (Superscript -8)
Hence, the radius of an atom is 10 ^5 times the radius of Nucleus
Similarily the volume of an atom is 10 ^15 times the volume of Nucleus
*Distance of Closest of closest approach
Let's take a simple example. Imagine something you absolutely hate (or a week old sandwich with a nauseating smell you at no condition can tolerate ) .Now, if you are walking down a relatively narrow corridor and you find it lying in the middle of your way, you are most likely to change direction, turn back before your stomach gives away.
Now the alpha particle treats the nucleus of the other atom as a week old sandwich. If the nucleus is in the path of the alpha particle, it will stop, and immediately turn back ( alpha particles never cared about manners anyway). The minimum distance the alpha particle can approach the nucleus before getting repelled ( or loosing the contents of its imaginary stomach) is called the distance of closest approach.
To calculate the distance of closest approach, we would have to dabble a little into a few basic Physics concepts.
(If you haven't checked out my previous blog on Electricity and Magnetism, which covers few of the grassroot topics of physics, (also details on coulombs law) https://mohanapriyawrites.wixsite.com/studiedyes/post/electricity-and-magnetism Click on this link!)
Here, we will be using the Coloumb's law and concept of Potential energy
a) Coulomb's Law
Force of attraction or repulsion = K q1 q1 / r^2 ( for the exact meaning of each letter, check out the physics blog!)
b) Potential Energy of the charge
P.E = K q1 q2 /r
Total energy of alpha particle throughout its path is conserved.
Hence,
T.E initial = T.E final
K.E intial + P.E initial = K.E final + P.E final
As alpha particle and the nucleus of the atom are very very very ( even a million verys would be too large) small. The distance for the alpha particle to travel towards the nucleus would seem to large or infinite. Hence K q1 q2 /r = K q1 q2 / ∞
Any number by infinite = 0.
P.E intial = 0
At final position, the alpha particle stops then turns back to retrace it's path. At that moment , the velocity is 0. Hence the Kinetic Energy which is 1/2 mass * velocity ^2 is also zero
K.E intial = P.E final
K.E = K. q1 q2 / minimum distance of approach ( which will be the radius at the final position)
And we know that Kinetic Energy aka K.E = 1/2 * mass * velocity^2
Hence,
Minimum distance of approach is also equal to =
Phew, who knew that such a tiny particle can wreck so much havoc in our world (especially in our report cards!) I really hope, this blog has helped you understand the concept of Rutherford's experiment better. So the next time you see a action movie where the hero ( or heroine) is deflecting a bullet with his/her bare hands, hopefully you are reminded of this blog and remember how even the impossible is possible in the world of chemistry!
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