Chemistry & Physics: History of the Atom (Dalton, Thomson, Rutherford, and Bohr Models)

We might say The Atomic Age started with the
work of John Dalton. Dalton proposed the Atomic Theory in 1803. This picture of the world
where everything is composed of atoms helped explain what had been observed in chemical
reactions. For instance, different elements always combined to form chemical compounds
in amounts that were simple whole-number ratios. Dalton proposed that each element had its
own unique type of atom, with a certain characteristic weight. These atoms were very small, solid
particles that were indivisible. That was the model of the atom for almost a hundred
years. Since Dalton’s first conception, the model
of the atom has evolved over time. Each time new experimental observations were gathered
that couldn’t be explained by the atomic model of the day, the model had to be revised
and refined. For instance, the discovery of subatomic particles meant Dalton’s model,
that said atoms were indivisible…needed some work. In 1897, J.J. Thomson was the first to discover
a subatomic particle, the electron, through his experiments with cathode rays. At this
time, people weren’t sure if cathode rays were waves, or particles. Thomson was using
magnets and electrically charged plates to deflect cathode rays (and thereby estimate
their mass). He showed that cathode rays must be made up of negatively charged particles
that were over a thousand times lighter than the smallest atom (that’s hydrogen). Before
this experimental result, it was thought that THE smallest particle WAS the hydrogen atom.
To account for his observations, Thomson proposed the “plum pudding” model of the atom.
If you’ve never had plum pudding, that name may not make a lot of sense to you. So imagine
you have a dense chewy cake with little raisins all through it – maybe like a raisin bagel,
or a chocolate chip muffin, if you prefer. The cake part is the bulk of the atom, and
it’s positively charged, while the little sweet bits of raisin, or chocolate would be
the negatively charged electrons. The Thomson model stood, for about a decade,
until the experiments of Ernest Rutherford. Rutherford and colleagues, in an experiment
performed in 1909, found that a beam of alpha particles (that’s a kind of positively-charged
radiation), shot at a target of very thin gold foil, mostly passed straight through
to a detector behind the foil. But occasionally the particle would be violently deflected
back, as if it had hit something massive. As Rutherford said, “It was quite the most
incredible event that has ever happened to me in my life. It was almost as incredible
as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.
On consideration, I realized that this scattering backward must be the result of a single collision,
and when I made calculations I saw that it was impossible to get anything of that order
of magnitude unless you took a system in which the greater part of the mass of the atom was
concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute
massive centre, carrying a charge.” – Ernest Rutherford The results implied that an atom must be made
mostly of empty space, but with a small core of positively charged material that contained
most of the mass of the atom. This was the nucleus. Again, the atomic model had to be
updated to take this new information into account. There was a problem almost immediately
seen with Rutherford’s model, however – if the electrons are out in this empty space
around the positively charged nucleus, what is keeping them from just giving in to the
electrostatic attraction towards the nucleus? Why doesn’t the atom just collapse on itself,
from all that attraction? Niels Bohr, who did postdoctoral work in Rutherford’s
lab, came up with an elegant solution to the collapsing atom problem. In a publication
in 1913, Bohr suggested that the negatively charged electrons are found in concentric
circular orbits around the positively charged nucleus, much like how the planets orbit the
sun. We sometimes refer to the Bohr model as the “planetary model.” According to this model, the electrons are
found at fixed energy levels, orbiting at fixed distances from the nucleus. The path
closest to the nucleus has the lowest energy level, and the energy is generally higher
the farther the orbits are from the nucleus. The farther a negatively charged electron
is from the positively charged nucleus, the less attraction the electron feels. In the Bohr model, electrons are able to jump
from one energy level to another, but they are not found between levels. Much like how
you have to take one entire step up or down a ladder, but you’re never found standing
between rungs. Thus an electron gains or loses a discrete package of energy each time it
changes energy levels. We call this a “quantum” of energy, and that’s where we get the term
“quantum leap.” The Bohr model explained a lot of observed
chemical behavior, including why each element has a certain number of electrons available
for reactions, the electrons found in the outermost orbits. These “valence electrons”
thus determine the chemical properties of an atom. The Bohr model is indeed, elegant
– it’s clear and pleasing, and yet – it, too, has been supplanted by another model,
the quantum mechanical model. We will talk all about the quantum mechanical model of
the atom in another video.

Comments 55

  • @Socratica You call yourself Socratica Studios (clearly in homage to Socrates), yet you fail to mention the first person recorded to have described the atom – another ancient Greek Philosopher named Democritus. Tsk tsk. Other than that glaring omission, good video 🙂

  • I've watched this video because of a confusion but this contains nothing about that the confusion is that although since the dalton's proposal chemist are using this concept & developing their theories more & more accurately but on the Other side in the Physics community it hadn't been accepted unless thomson results & finally the debate was closed by Einstein from his paper on brownian motion. But Ludwig Boltzmann had commited suicide in 1906 because this atomic controversy why on one side chemists are happy & on the other side everyone was accusing one man for that what is wrong there in the story can you guide me?

  • U guys have forgotten Democritus.he is the first not Dalton

  • 良く出来てる。

  • that is cool

  • @Socratica can we use your videos for our capstone project? We are going to create a mobile app for the periodic table of elements and we are in need of video tutorials then we came across your channel. This will be a lot of help, I wish it would be ok? But the final project will not be on google play because it will just serve as our thesis 🙂

  • Nice and clear, thanks.

  • this is the first vid that makes me get about history of the atom. I need more. you've done a great help, thanks

  • but where is James Chadwick?

  • nala waren apita chemistry ugannana enawa pol buruwa

  • is Rutherford home? ??

  • chemistry & physic are cute couple that have a different tool like microscope & telescope, different responsibilities, but in the same purpose. chemist peek smallest life object, physicist peek largest life object 🙂

  • if the last energy level gain quantum what will happen to the electrons there ? will they go outside the atom!

  • so a good video

  • great video, this helped a lot

  • Nice one for science students. ..

  • What about Thames eh?

  • helped me a lot in my hw! tnx!

  • Where can I find the video about the quantum mechanical model?

  • This is a fantastic video and exactly the sort of thing I would use with my class in cases where I wasn't making my own vids (mine look like this due to time mid-term. I would like there to be notes for students to take down but not having them means that the students are having to think for themselves (just means they need to be a bit more capable).

  • these videos would be perfect If it's Spanish. there's not many videos which explains correctly and sometimes it's difficult to find videos like this

  • Energy is the ability to do work . But the closer to the nucleus the electron moves faster … then the ability to do work increases … then the electron close to the atom should have higher energy??? is it ???

  • what about democrjfkspabskcjod ?

  • Thank You, Maam!
    Helps a lot. I hope your channel grows further.

    Greetings from India!

  • thank you soooooo soooo much for this amazing work. i hate chemistry because i ll never get it in lecture or somewhere else but you explanation is awesome and very understandable it helps me so much with revision for the final exams,


  • а particles are helium ions. day is positive charged due to no electrons.

  • I like your bohr model of helium Eye only got a glimpse of it butt it was cute.

  • I see that 'Socratica' text on the top right… Is it russian!? It looks russian, that 'I' though is what makes me question if it isn't or not, but I may be totally wrong.

  • Please add QM video. Thanks.

  • Ah dammit, really? just when you got to the one i was interested about. Where is the video of the quantum mechanical model anyway

  • Does that Greek word at the top right spell out Socratica?

  • why is your picture an owl?

  • Electrons are able to jump!! 😮

  • nice interesting for everyone

  • video espetacular….parabéns!!!

  • 🙂☺😏🙄

  • Thank you so much.

  • FYI, Atom first appeared in the koran.

  • Democritus(the Greek philosopher) was the first one who gave atomic theory of universe. however, good video though…

  • Thanks……. Man. It helps me a lot

  • Hi. I love her, I love this professor:-) I feel like I'm approaching heaven. I love all Socratica narrators and profs of course. God bless, Proverbs 31

  • Maharishi Kanad named atom as paramanu (param means ultimate and anu means particle).

  • My 6 year old watched this and wanted comment and say " it was really interesting. I liked the little owl minion that was hiding." She liked the bohr model electrons spinning. It kept her interested the whole way through. She knows that this was about atoms. When I asked what are atoms she replied " particles that we're made out of". Great job!!

  • Like nr 1000

  • what software did you use to make video It looks so nice!

  • am really educated

  • Paradoxes of atomism

    If it were possible to continue the division of matter indefinitely, I would have thought it more probable that this process could be carried out to infinity (thesis of infinite divisibility, contrary to atomistic antithesis).

    The problem is that we can not and do not have the colossal force to do this, because we are physically limited, we can only at most split up to a few fractions of sand, because we can not get the pieces too small to be divided again and so on , only the cosmic forces of the universe could make or a God out of infinite power.

    It results in unsustainable paradoxes and absurdities to defend the thesis of the existence of indivisible material entities / elements, the atoms, as they considered Democritus and Leucippus, a thesis that Aristotle correctly rejected.

    Why do we have to accept the existence of atoms if experience shows us that all compound bodies are divisible indefinitely to our last tactile-sensitive limits?

    If all the material elements are breakable into smaller parts, from the softest to the hardest, an iron bar when we hit it kneads and looses small pieces of metal and sparks of fire – energy, revealing its divisibility to us, why then do we have to to accept that atoms (indivisible fragments of matter) exist?

    This atomistic thesis leads us to the paradox well demonstrated by Anaxagoras and Aristotle, that the parts are greater than the sum of the whole, for the components of the self are indivisible and not eternal. The results are the only and most perfect to be realized in all corpus of corruptible and mortal, which results in the refutation of atomism by reduction to the absurd.

    Another paradox reveals itself is not a fact of existing atoms but is not necessarily indivisible, it is not necessary to prepare an atom for its existence, for who can be indivisible, the ultimate of existence, eternal, indestructible, immune to all sorts of shocks and destructions. Existing and eternal exist, to probe and to separate the various clusters in concentrated points without space, resulting in an inexistence of cohesion / physical concretion and consequent non-existence of visualizing the bodies and material bodies! In what results in a further logical – qualitative refutation of atomism, by a new reduction to the absurd.

    And finally, indivisible and eternal atoms unite with other equally indivisible and eternal atoms, through connections made of finite and divisible matter as is our physical – corporeal composition and that of all the animate and inanimate bodies of the world, is an absurd total in this thesis, for where would arise a divisible and finite matter that binds atoms, if these same atoms are all indivisible, eternal and indestructible particles? Of the very primordial atoms that gave birth to the whole universe? But would a finite and divisible matter arising from indivisible and eternal atoms not be an unacceptable corruption of the eternal and indivisible essence of atoms? An indivisible atom that gives rise to a divisible matter would not have to possess the germ of divisibility in its essence, revealing in the truth that it is no atom, but a corruptible and perfectly divisible matter, which would refute the very Democritean thesis of existence of atoms?

    Do you perceive so much of metaphysical absurdities, paradoxes, and idiosyncrasies that the theory of atoms has borne since over 2600 years ago?

    In the antithesis to the atomist theory, we can not observe and test the process of division ad infinitum, because obviously we have spatial and physical – temporal limits, but at least it is indirectly based on ordinary experience, being a much more rational and scientific hypothesis than considering the hypothesis of finite divisibility in final and eternal atoms, for we have no example of phenomenon or object observable in experience that is indivisible, indestructible, incorruptible, and eternal, whereas for the philosophical hypothesis of indefinite or infinite divisibility we have the support of a sensory experience that all objects, bodies and physical phenomena are divisible or decomposable into smaller, corruptible and destructible parts!

  • Awesome video

  • It's very easy to understand
    Thank you so much

  • Thank you.

  • how does the Bohr model solve the electrostatic attraction problem?

  • The sound…I could barely hear the narrator of the video, even with headphones on!

  • What happened with Mendeleev? The dude predicted the whole chemical properties of all the atoms.

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