Sunday, February 12, 2012

Why do electrons move in shells rather than in fixed orbits?

im doing a school science project and i need a relivitively basic explanation. thanks

Why do electrons move in shells rather than in fixed orbits?
Quantum mechanical objects (ie, very small, like the size of electrons in atoms) don't have fixed speeds and positions. So you can't talk about their orbits as you would in classical mechanics. Their properties (such as position) are described by a wave function, a function which obeys a differential equation (Schrodinger's equation). For an electron in an atom, there are only certain, specific, solutions to that equation that are mathematically possible. So the electron is limited to being in one of these wave functions (or a combination of them--that's allowed, too), which give you a probability distribution for its position, the shell. The electron isn't exactly moving in the shell (it may or may not have orbital angular momentum). It's just hanging out there. It doesn't need to decide exactly where it is unless you go looking for it.
Reply:Electrons do not move in the way big objects like e.g. cars move. The motion of objects in the macroscopic world we live in is described by classical physics, the 3 Laws of Newton. But in the microscopic world of atoms, these laws are not valid. Classical physics completely break down at this level.



The laws of physics as we understand them today are not based on classical mechanics but instead on quantum mechanics. These laws are valid also for atoms. When objects become large the quantum laws become more like the classical laws.



Now, the very concept of an object in motion, i.e. an object being at one place at some time and simultaneously having a certain velocity breaks down at the quantum level. This concept of motion can be thought of as an emergent thing that gradually arises when objects become very large.



According to quantum physics, the electron in an atom cannot have arbitrary values for the energy. The state of an electron can be completely specified by specifying the energy, and the so-called angular momentum. These numbers can be given in terms of integers, so called quantum numbers.



According to quantum physics when you have specified these numbers the state of the electron is completely fixed, i.e. there is nothing more to know about the electron. Now, if you were to measure the position of the electron you can still find it in many different places. There is a probability distribution for the position that depends on the quantum numbers.



For an electron in the lowest possible energy the electron can be found with very high probability close to the nucleus. For the higher energies the probability of finding it further away from the nuvcleus becomes larger.



Now if you consider that an electron in some fixed energy state can be found in many different places then that suggest that one should picture the electron being in all the possible places at once in some sense. One can do certain experiments in which an electron can move in two ways from one position to another position. One can then demostrate that it cannot be the case that an electron choses only one path when we don't observe how the electron is moving.



So, to summarize, it is wrong to think of electrons as ordinary objects. Quantum mechancs correctly describes atoms. This theory predicts that the possible states the electrons in atoms are cannot be changed continuously but they are labeled by integers.
Reply:Well the electron isn't a 'thing' as such - there's no way of telling exactly where it is or where it is going to be (well, you can find out one but not both). So instead of giving it a defined orbit we simply give an area in which it can be found at any moment - this area is the 'shell'.
Reply:multiple shells are needed to accommodate the many electrons in heavier elements. this does not affect the way electrons move, they buzz around in the shells as opposed to moving in orbits, their positions are unpredictable.


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