Probably the most startling revelation of Quantum Theory was the postulate that all particles have a waviness to them. Initially, when this was proposed for photons, physicist were more open to accepting it – since light was already clearly a wavy phenomenon to begin with. However, when De-Broglie applied it to all matter – including cherished solid notions of atoms, the result was a bit harder to digest.
Firmly established since then, we no longer worry about how it is possible for matter to be wavy – we just accept that it is.
This is the first cool application of the wave like nature of particles.
The velocity obviously depends on the charges that cause the electron to move where Z is the number of protons (the atomic number). That part is classical. If we want to accommodate the quantum nature of the electron, planck’s constant, , needs to figure in the equation somewhere. The only combination of and that has the dimensions of velocity is :
Another way to relate the velocity to the atomic constants is to realize that the Potential Energy () is of the order of the kinetic energy .
This also helps us get an estimate of the atomic radius a:
So – here we have the first appearance of Quantum Theory () in an otherwise mundane calculation – that of the size on an atomic orbit (orbital radius).
If electrons are really waves, let us think about the electron bound in a Hydrogen atom. It is subject to a potential energy, V (we say the particle is in a potential well). The size of the box (well), a represents the range of the potential that binds the electron. So, we have the following assumption:
Assumption 1 (completely based on the wave like nature of the particle): Since the particle must ‘fit’ in the box, the ‘wave’ must fit in the box. The dimension a of the box that contains the particle, must contain an integral number of wavelengths (actually ‘half-wave-lengths). So – we have the equation below – the energy level L(n) is proportional to the wavelength of the ‘wave’ aspect of the particle.
The formal proof of the above assumption comes from solving the SE:
For V < E (classical region), the equation has an oscillatory solution. Back to our assumption – energy levels, L(n), are proportional to the wavelength of the particle.
where n is the integral number of wavelengths – and is the wavelength of the particle (this is where our particle’s waviness enters the picture).
De-Broglie didn’t just postulate that all matter behaves like waves, he was kind enough to provide a mathematical relationship between the matter-like nature (momentum of the particle) and the wave-like nature (wavelength) of particles.
So – energy levels are proportional to the wavelength, the wavelength is inversely proportional to the momentum. All we need is way to relate the momentum to the energy of the particle (E-V) – and we should have our estimate. From , we have .
With the equation for , we are almost there. Now – if we just choose V=0 as our origin (physically, this means taking the top of the well as our origin), we have an equation for p.
Hence, our estimate for the wavelength becomes:
And our estimate for our energy levels, L(n), which are just proportional to the wavelength above:
L(n) should just be the width of the well = L
So – we have our final equation Or
One does not need to solve the Schrodinger Wave Equation simply to get estimates of atomic values such as the electron velocity or atomic radius. We calculated the velocity of an electron in an atom as well as the atomic radius – with just dimensional analysis. The energy levels for a bound electron could also be calculated without solving the Schrodinger equation. These energy levels (for a particle in a box) are proportional to the quantum number n as well as inversely proportional to the width L of the well. This is in agreement with Bohr’s correspondence principle in the limit of large n. We used a combination of dimensional arguments (in the case of the electron velocity and atomic radius) and proportionality arguments (in the case of the stationary states).