|MadSci Network: Physics|
Hi Aaron, The question you ask about the quantum nature of atoms is very broad, and it would be impossible for me to give you any kind of definitive answer. Instead, I will try to give you a general overview of what quantum mechanics is and how it can be applied to atoms, followed by some references to where you can learn more about the subject. In quantum mechanics we describe the state of a system either in terms of a wave function, or in terms of a state vector. These descriptions are to different, but equivalent, ways of looking at the problem. In either case, we need what is called a basis of orthogonal states. What this basically means is that we need a set of states with the property that when we measure the system we are gauranteed that it will be in one of those states. One example of a basis for a single particle is position. If we try to measure the position of a particle we will only ever see it in one place, and will never measure it to be in 0,2,3... places at the same time. Where quantum mechanics differs from our everyday experience is that it is possible for a particle to have a wave function that is in what is called a superposition of basis states. So what does this mean? Well basically it means that a particle can be in a superposition of two or more positions at the same time. When we try to measure the particle we will only get one result, but we cannot know ahead of time which result. If a particle could only be in one of, say, 3 possible states, then we could write its state as: a|1> + b|2> + c|3>. Here I have used |1>, |2> and |3> to label the 3 possible basis states (this is known as Dirac notation, although there is no need to worry about it at the moment). a,b and c are known as amplitudes and are in general complex numbers (although you can think of them as real numbers between -1 and 1 for this example). The probability of measuring the particle to be in any state is given the square of the absolute value of the amplitude. If we keep a,b and c as real numbers, as above, then the corresponding probabilities are just a^2 (this means a squared), b^2 and c^2. Since we must measure the particle to be in one of the states, we must have a^2 + b^2 + c^2 = 1. This property of superposition is what separates quantum mechanics from classical mechanics, which we experience every day. At first most people don't believe that this incredible effect actually happens, but it has been verified many times (see for example http://en.wikipedia.org/wiki/Youngs_double-slit_experiment#Quantum_version_of_experiment). Atoms are made up of a very small positively charged nucleus containing protons and neutrons, around which are trapped electrons. The electrons are bound to the neucleus because they are negatively charged, and opposite charges attract. All of these particles are in a specific superpostion of states known as a shell. The nucleus is very tightly bound, so the neutrons and protons are spread over a very small area, while the much lighter electrons are spread over a much larger area. When energy is added to an atom (either through a collision with other particles, or when it absorbs a particle of light known as a photon), the electrons are rearraged into a different shell. The atom can only absorb light if it gives an electron just the right amount of energy to move from one shell to another. Because there is a gap between energy levels not all light can be absorbed. What determines the energy of light is it's frequency, which we percieve as it's colour. Different atoms and molecules have different arrangements of shells, and so absorb and reflect different coloured light. This is what gives materials their colour. If an electron is in an excited state, it can drop down into a lower shell emitting a photon. This is how light is 'made'. In a light bulb the electricity (a flow of electrons through the filament) excites the atoms, which then drop down into a lower energy level, while in the process emitting light. By studying the light emitted, reflected or absorbed by some material it is possible to determine what it is made of. This process is known as spectroscopy. For a general popular science introduction to quantum mechanics I would suggest "In search of Schrodingers Cat" by John Gribbin, but if you are interested in understanding the mathematics involved then I would suggest volume 3 of the "Feynman Lectures" by Richard Feynman (he won the Nobel Prize for physics in 1965 for his work on the interaction of particles with light, inventing what is called quantum electrodynamics). This second book is aimed at interested non-scientists, but introduces all the required mathematics and physics in a very conversational manner. There is also four videos online of Feynman giving introductory lectures at http://www.vega.org.uk/video/subseries/8). Hope this was helpful, Joe
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