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- Thread starter Shreyan
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alxm

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Now, the opposite charges attract. But this is not a problem if the electrons are moving - in which case they could form stable 'orbits', like a planet revolving around a sun. (if the Earth stopped it would fall in).

The real problem with this picture, is that the laws of electromagnetism (Maxwell's laws) predict that a charging particle that moves like that, would emit radiation (heat/light), losing kinetic energy and slowing down. So the electron would end up spiraling into the nucleus, which obviously doesn't happen.

Then Bohr (in 1913 IIRC) made his model of the hydrogen atom, where he postulated (assumed) that the electrons were only 'allowed' to move in certain orbits, corresponding to energy (angular momentum) levels proportional to 1/(n^2) (where n is an integer, 1,2,3..etc). From this, he was able to correctly predict the approximate radius of the hydrogen atom (now known as the 'Bohr radius'), and also (approximately) explain the hydrogen atom spectrum - the spectral lines corresponded to photons who's energy difference was equal to the difference between such levels. (So the spectrum is understood as coming about from moving between these 'allowed' levels)

There are several problems with Bohr's model - to begin with, it didn't really explain

At the same time, people were investigating the 'wave-particle duality' - that microscopic particles could act like waves, in some ways. Every particle has a wavelength known as the

The explanation for why the electron simply couldn't fall into the nucleus came about in 1925, with the Heisenberg Uncertainty Principle. Which states that a particle cannot simultaneously have a well-defined momentum

The 'final' solution to the problem of atoms came with the Schrödinger equation in 1927, when quantum physics was 'invented'. Using the Schrödinger equation, an equation similar to the wave equation, to describe the electron moving around the nucleus, you only get certain 'allowed' solutions. But the 'why' in this case is now purely mathematical - the equation only has a certain set of allowed solutions, analagous to the situation with the wave equation and standing waves.

The Heisenberg uncertainty principle is 'built in' to the Schrödinger equation, so it too predicted that the electron would not fall into the nucleus. But unlike Bohr's model, the electron did

The Schrödinger equation description of the hydrogen atom also explained quite a number of experimental results the Bohr model had not - and more importantly, it worked for

The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact solution of these laws leads to equations much too complicated to be soluble.

So we still need chemists and experiments. But as computers get faster, and the methods of approximating the solutions to the Schrödinger and Dirac equations for atoms and molecules get better, 'quantum chemistry' as its called, is playing an increasingly important role.

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