The fact to the atomic theories as we know it today is the product of hundreds, if not thousands of different insights. Today we are going to look into some of the most prolific experiment and theories of science.
Rutherford’s Scattering Experiment
In 1911, Rutherford performed an experiment in which he bombarded a thin foil of a heavy metal like gold with a team of fast moving alpha particles which is double the ionized ions (42He)2+each having two unit of positive charge and 4 unit of mass which is denoted by amu for you (4amu). These were obtained from radium placed in the cavity of block of lead and made into fine beam with a slit. He observed the scattering of the rays after hitting the foil by placing a circular zinc sulfide screen around the metal foil wherever alpha particle struck the screen a flash of light is produced at that point on the screen. That is the heavy body present in an atom must be occupying a very small volume, from this experiment Rutherford made the following observations
- The vast majority of the molecule that is 99.9% gone through the thwart without experiencing any redirection.
- A few particles avoided through little points and a couple of diverted through a vast edge.
- Very few, which is 1 in 12000 were deflected back that is through an angle greater than 90 degrees.
From these observations Rutherford drew the following conclusion
- Since the vast majority of the alpha molecule went through the thwart without experiencing any redirection there must be adequate discharge space inside the atom
- The little overwhelming decidedly positive (+) charged molecule of the body introduce inside the particle is called nucleus.
Rutherford’s nuclear model of an atom
On the basis of the scattering experiments Rutherford put forward a model of atom known as the Rutherford’s nuclear model
- There is positively (+) charged center in an atom called the Nucleus in which all the mass of an atom resides
- The size of nucleus is very small as compared to the size of an atom
- The electrons (-) revolves around the nucleus in well-defined orbits
- As some of the alpha particle were deflected by small angles, and since alpha particles are positively charged, they could be deflected only by some other positively charged body present within the atom.
- The alpha particle which deflected through small angles were those which passed far from this positive body.
- The alpha particle that deflected through large angles were those which passed closed to the positive body.
- Since some alpha particles were deflected back, alpha particles are heavy particles they could be deflected back only when they strike concentrated positive charge inside the atom.
- Since number of alpha particle deflected back is very very small which shows that the positive charge is mass of the gold atom a very very small volume
Drawbacks of Rutherford’s model of an atom
- According to Rutherford’s model, an atom consists of a small heavy positively charged nucleus in the center and the electron are revolving around it.
- Whenever a charged particle like electron is revolving around a central force like that of nucleus it losses energy continuously in the form of radiation thus the orbit of the revolving electron will keep on becoming smaller and smaller following a spiral path and ultimately the electron should fall into the nucleus which means the atom should collapse
- But this actually doesn’thappen, and the atom is quite stable
Bohr atomic model
Neil Bohr was dealing with some problems that were arising with the structure of the atom. If an atom has a positively charged protons and negatively charged electrons, why are atom stable? Why doesn’t the electron just collide with the nucleus? What is these emission spectra that we can see? What is it about different elements that makes them emit light of different colors? Bohr answered these questions with his model of the hydrogen atom.
- He extended the idea of energy quantization and said that the potential energy of the electron in a hydrogen atom is also quantized. This means that an electron can’t have any imaginable energy but can only inhabit certain energy levels that are at fixed distances from the nucleus.
- Each types of atoms have its energy levels at different values due to its unique number of protons in the nucleus and an electron will transition from one energy level to another when a photon of a very specific energy is either absorbed or emitted by the electron
- The energy of the photon will correspond to the difference between the two energy levels so if the electron in a hydrogen atom goes from the n=3 to the n=2 energy level, a photon will be emitted that is equivalent to that specific energy gap.
- Another transition has a different energy gap associated with it and therefore generate a photon of that particular energy
- To go from lower to higher energy levels an electron must absorb a photon of that particular energy.
We group the transition according to the energy level they land on, all that end on n=1 are called the “Lyman series”. The one that ends at n=2 are the “Balmer series”. n=3 are the “Paschen series” and n=4 are the Brackett series.
If you notice the energy level gaps decreases as “n” increases, and n equals infinity is actually a finite distance from the nucleus. If an electron goes beyond that it is considered to have been ejected from the atom.
The Balmer series series happen to contain transitions that generates photons of visible lights, these are the ones found on the hydrogen emission spectrum.
Thomson’s Atomic Model
Are the charged particles arranged in any particular manner? Are the charged particles spread throughout the atom or are they concentrated in one place? Thomson’s atomic model answers all such question. Around the year 1900, J.J. Thomson conducted an experiment on the beams of particles inside the glass tube called a cathode ray tube.
- He found that the particles were attracted to the positive terminal of the tube.
- Thomson concluded that the particles must be negatively charged and called these particles electrons.
- An electron has a negligible mass and has a charge of -1.
These discoveries mean that scientists believe that an atom was divisible and made up of electrons and protons, what they didn’t know at the time was how this electrons and protons were arranged in an atom. They did various experiments to understand this arrangement.
J.J. Thomson was the first to put forward a model to explain the structure of an atom, in his model Thomson compared an atom to a Christmas pudding, where the electrons were like the raisins in the pudding and the pudding itself was like the positively charged particles. This can also be explained through the example of a watermelon. The positive charge in the atom is spread all over like the red fleshy part of the watermelon while the electrons are embedded into the atom like the seeds of the watermelons, the oppositely charged particles are held together by the electrical force of attraction. Thomson concluded that
- An atom consists of a positively charged sphere with electron set within the sphere
- An atom is electrically neutral as the positive and negative charges within it are equal
As per Tomson’s Conclusion
- The electrons are embedded in a sphere of positive charge this conclusion was however was incorrect
- Thomson asserted at the positive charge spread through the atom held the negatively charged electrons due to electrical forces
- The assertion by J.J. Thomson failed to explain many experimental observations
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