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1.

When it was an era of classical mechanics we used to believe in the Bohr's atomic model. It interpreted electrons as particles (although I couldn't understand how come Bohr who interpreted electron as a particle, formulated an equation for electron's angular momentum which shows its mathematical proof to be a wave.) and are revolving around nucleus with their own particular energy for each orbit. This solved drawbacks of Rutherford's model which lacked to explain electron's stability. Bohr suggested that electrons could only have certain classical motions:

  1. Electrons in atoms orbit the nucleus.

  2. The electrons can only orbit stably, without radiating, in certain orbits (called by Bohr the "stationary orbits") at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels. In these orbits, the electron's acceleration does not result in radiation and energy loss as required by classical electromagnetics. The Bohr model of an atom was based upon Planck's quantum theory of radiation.

When Bohr restricts an electron's orbiting distance from nucleus by providing only a certain set of distances, isn't it called quantization? And why this set of laws?

Why does one restrict a particle's motion to some discrete set of distances? Is it to provide a theory on the particle's stability?

2.

When one says the electron behaves like a standing wave, which can be compared with waves on string, then from where those nodes arise in the case of electrons revolving around a nucleus?

enter image description here

Simply when one compares first harmonic of wave on a string with electrons moving around nucleus, from where do the nodes shown in the figure arise in case of orbiting electron?**

enter image description here

Let me say, electron, a wave, which has certain wavelength and frequency owing to its distance from nucleus (energy of different electrons at different distances from nucleus differ) executes a harmonic based on its energy. So as we move towards some nth harmonic, the trajectory becomes complicated. Is it that case? Am I wrong? But still I can't understand the arising of nodes..

3.

How do s,p,d and f orbitals (are they the way in which the electron, a wave, moves around nucleus as a function of time?) exist without interfering each other? I mean, an atom is so small. Don't they mix up, i.e. superpose? Is it that the n+1 nodes in an arbitrary orbital are there because of its preceding orbital?

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4.

Also, when I look at the 3D shape of d orbital esp. the one with ring (d-z^2), I feel it's so complicated. How do they arise?! What are those +/- written over the orbitals?

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    $\begingroup$ No offence, they are good questions, but, imo, too long for one answer. Would you consider splitting them up, and also they may already have answers on this site. Regards $\endgroup$ – user81619 Jul 26 '15 at 9:40
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    $\begingroup$ I am very sorry, I was not clear in my earlier comment. I should have specified splitting them into separate individual questions, asking them one at a time, getting an answer to each section, and then build on that answer for the next days question when you follow the answer to the previous one. That's what I do, again, apologies for not being clearer. $\endgroup$ – user81619 Jul 26 '15 at 10:04
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    $\begingroup$ each of them is a good question but you really need to ask them separately $\endgroup$ – sintetico Jul 26 '15 at 10:10
  • $\begingroup$ Thank you for your feedback..And please don't apologize. I love to improve myself.But I feel there is a sequence in my question and as I am a beginner I will fall short of technical words to enhance the meaning of my question if splitted.. I will try. Thanks. $\endgroup$ – Preeti Jul 26 '15 at 10:11
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    $\begingroup$ Yes, quantum mechanics developed because of the OBSERVED stability of the atoms/molecules and the OBSERVED spectra of transitions between energy levels. The Bohr theory had to postulate the stability. Schrodinger's equation for the hydrogen atom contains the stability from more general postulates. hyperphysics.phy-astr.gsu.edu/hbase/quantum/qm.html that could explain atomic spectra generally. An please note that the wave nature displayed at the particle level is a PROBABILITY WAVE. hyperphysics.phy-astr.gsu.edu/hbase/quantum/wvfun.html , NOT an energy or mass wave. $\endgroup$ – anna v Jul 26 '15 at 14:46
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Electron as a standing wave

Yes, the electron is a standing wave. See atomic orbitals on Wikipedia: "The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves".

I couldn't understand how come Bohr who interpreted electron as a particle, formulated an equation for electron's angular momentum which shows its mathematical proof to be a wave.

Maybe you need to check out De Broglie and matter waves: "All matter can exhibit wave-like behaviour. For example a beam of electrons can be diffracted just like a beam of light or a water wave". See this picture by artist Kenneth Snelson:

enter image description here

It isn't a totally accurate depiction. Electrons aren't actually thin coloured strips, but you should get the idea of these standing waves.

Simply when one compares first harmonic of wave on a string with electron moving around nucleus, from where the nodes shown in the figure arise in case of orbiting electron?

It's bit like a wave in a closed string. But it isn't a wave on a string, it's an electromagnetic wave that's configured as a standing wave. A field variation that's configured as a standing field. It has a Compton wavelength of 2.426 x 10⁻¹² m.

The electrons can only orbit stably, without radiating

Don't think of the electron as some little billiard-ball thing. Think of it as something more like a hula hoop.

So as we move towards some nth harmonic, the trajectory becomes complicated. Is it that case?

Yes. Check out spherical harmonics.

Also that how s,p,d, and f orbitals (May be, the way in which electron, a wave, moves around nucleus as a function of time taken?) do exist without interfering each other? I mean, an atom is so small and intact.. So, don't they mix up i.e., superpose?

Superposition is a wave thing. Two ocean waves can ride right over one another and then keep going. But for electrons in orbitals, like Acid Jazz said, the Pauli Exclusion principle applies. The simplest analogy I can think of for that is two whirlpools can't overlap.

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    $\begingroup$ Didn't think it could be answered so compactly, but you proved me wrong. $\endgroup$ – user81619 Jul 27 '15 at 13:00
  • $\begingroup$ My pleasure Preeti. The above wasn't a perfect answer, but it hopefully gets the concept across. Remember it's quantum field theory. Everything is fields and waves. The electron is not some billiard-ball speck that has a field, it is field. @Acid Jazz : I quite like copying and pasting the question then answering it bit by bit. It kind of keeps the answer focused and relevant to the question, and stops me rabbiting on too much. $\endgroup$ – John Duffield Jul 27 '15 at 16:12
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I now come to my point, why one restricts a particle's motion to some discrete set of distances? Is it to provide a theory on the particle's stability?

An attempt at an answer to your first question anyway.

The electrons surrounding an atom need to obey energy level (and other) rules. As you mention distance, if you imagine that the further away the electrons are from the nucleus, the more energy they have, whilst staying electrically attracted to the nucleus. It's not just a theory, it has been demonstrated experimentally that that that's how they are arranged.

They are discrete distances apart because the energy levels are discrete and the energy levels are based on distances. The electrons nearest the atom are called the ground state electrons, and have the minimum energy of all the orbiting electrons, as you probably already know.

Search through Google for the "Pauli Exclusion Principle" and you will find the other rule the electrons in an atom need to follow, in relation to discrete energies and therefore discrete distances.

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  • $\begingroup$ Thank you so much.. I was to split the questions but my internet connection failed.. Just got it corrected.. Sorry for that. And thanks for answering and clearing my doubts. This is a right place for igniting the mind and developing scientific way of thinking. $\endgroup$ – Preeti Jul 27 '15 at 12:58
  • $\begingroup$ John's answer is an object lesson for me. It's sometimes difficult to judge at what level you should answer, no offence, but I think he got it just right. Regards $\endgroup$ – user81619 Jul 27 '15 at 13:04
  • $\begingroup$ @Preeti Just one other thing about the p,d,f orbitals that might save you a headache. If / when you do study them, although as John says, they are described by spherical harmonic math functions, these functions don't actually describe spheres, despite their name. They are symmetrical orbitals, but they come in all sorts of odd shapes, I am pretty sure you know this already, but just in case. $\endgroup$ – user81619 Jul 27 '15 at 13:28
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  1. The discreteness of the allowed orbitals is a consequence of quantum mechanics, which was conceived precisely to explain this observation, among other things. However, the orbitals are not orbits - there is no "motion" in the classical sense going on, and an electron in an orbital does not have a fixed distance to the nucleus (it may even have non-zero probability to be found inside the nucleus). The discreteness of the orbitals has nothing to do with "stability of the particle" (however, with stability in time, see below), they are simply the only states that appear as solutions to the time-independent Schrödinger equation.

  2. Quantum objects are not waves. Quantum obejcts are not classical point-like particles. They are quantum objects, which may show wave-like and particle-like properties. You may represent a quantum state by its "probability wave" or wavefunction, whose square gives the probability density to find the object "as a particle" at certain locations. It is not a wave in the classical sense that anything physical would be oscillating here, and the Schrödinger equation does not always look like a wave equation.

  3. Electrons may be in more than one orbital at once, due to the general possibility of superposition of quantum states. But since the orbital are the solutions to the time-independent Schrödinger equation, being in one - and only one - orbital is the only stable state for an electron, while all other states will be changed by time evolution. The orbitals don't "interfere" because, well, they aren't actual waves.

  4. The shape of the orbitals is mainly governed by the spherical harmonics, which are the solutions to the angular part of the time-independent Schrödinger equation, at least for the Hydrogen atom.

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