Troubles with the planetary model of the atom:

Even as he articulated his planetary model of the atom, Rutherford was aware that there were serious problems with it. For example, since like charges repel and unlike charges attract, why don’t the multiple protons in the nuclei of elements heavier than hydrogen repel each other, and cause the nucleus to fragment? What keeps them so close to each other? On the other hand, if electrons were orbiting the nucleus like planets round the sun, why didn't they repel each other, leading to quite complex and presumably unstable "orbits"?

Why weren’t they ejected spontaneously and why didn’t the electrostatic attraction between the positively-charged nucleus and the negatively-charged electrons force the negatively-charged electrons to “fall into” the positively charged nucleus.

Assuming that the electrons are moving, they should speed up (increasing their kinetic energy) as they were attracted to the positively charged nucleus – that energy should be released (as light) when they collided with the nucleus. But as we know atoms are, generally, quite stable. So many questions – so few answers! Clearly, Rutherford's model was missing something important (and assuming something that cannot be true) with regard to forces within the nucleus, the orbital properties of electrons, and the attractions between electrons and protons. To understand what these are leads us into the weird world of quantum mechanics.

Light and getting quantum mechanical

While work on the nature of atoms was progressing there was also significant progress in our understanding of the nature of light. Historically, there had been a long controversy about the nature of light; there were two apparently mutually exclusive viewpoints, namely that light is a type of wave phenomena (like sound or water waves) traveling through a medium (like air or the surface of water) versus the position that light is composed of particles (Isaac Newton called them corpuscles). There was compelling evidence to support both, seemingly mutually exclusive points of view, and the attempt to reconcile these observations into a single model proved difficult.

By the end of the 1800s, most scientists had come to accept a wave model for light because it better explained behaviors such as interference and diffraction (the phenomenon that gives rise to patterns when waves pass through or around objects that are of similar size to the wave itself). James Clerk Maxwell (1831 – 1879) developed the electromagnetic theory of light in which visible light, and other forms of radiation (such as microwaves, radiowaves, x-rays, and gamma rays), were viewed in terms of mutually perpendicular electric and magnetic fields. A light wave can be described by defining its frequency (ν) and its wavelength (λ). For all waves, the wave's frequency times its wavelength equals the wave's velocity. In the case of electromagnetic waves λν = c, where c is the velocity of light.

In these studies, an object is heated to a particular temperature; such heated objects emit radiation. Consider your own body, which typically has a temperature of approximately 98.6ºF or 36ºC. Your body emits infrared radiation and there are cameras that are sensitive to infrared radiation. Some animals, like snakes, have infrared detectors that enable them to locate their prey – typically small (warm-blooded, infrared light emitting) mammals. Because mammals tend to be warmer than their surroundings, infrared vision can be used to find them in the dark or when they are camouflaged.

Planck had been commissioned by an electric power company to produce a light bulb that emitted the maximum amount of light using the minimum amount of energy. In the course of this project he studied how the color of the light emitted (a function of its wavelength) changed as a function of an object's (such as a light bulb filament) temperature. We can write this relationship as λ (wavelength) = f(t) where t = temperature and "f" indicates "function of". He found that current theories of light did not explain his observations, in particular as the frequency of the light ν increased – his measurements diverged more and more from the predictions of then current, wave-based theory. His result was, believe it or not, known as the "ultraviolet catastrophe". It was a catastrophe for the conventional theory because there was no obvious way to modify the theory to explain the observation (and this was important because these observations were quite reproducible.) Once again, we see an example of the rules of science, a reproducible discrepancy, even if it seems minor, must be addressed or the theory must be considered incomplete or just plain wrong. To fit the data, Planck had to invoke a rather strange and non-intuitive idea, namely that matter absorbs and emits energy only in discrete chunks, which he called quanta. These quanta occurred in multiples of E (energy) = hν, where h is a constant, now known as Planck's constant. Planck’s constant is considered one of the fundamental numbers that describes our universe.

The idea that atoms emit and absorb energy only in discrete packets is one of the most profound and revolutionary discoveries in all of science, and set the stage for a radical rethinking about the behavior of energy and matter on the atomic and subatomic scales.

Question to answer:

• What is a constant? what is a function?
• What happens to the energy of a photon of light as the frequency increases? What about as the wavelength increases?
• Why is it difficult to detect "cold blooded" animals using infrared detectors?
• The wavelengths of X-rays are about 10^-10 m while the wavelengths of radiowaves are about 10 m.
• What other factors can you calculate about these two types of radiation? What factors are the same, what are different.
• Does this help explain why we are not bothered by radiowaves but X-rays are harmful?
• Why are the phenomena of diffraction and interference used as evidence that light is a wave?
  

Questions to ponder:

• Can an atom of one element change into an atom of another element?
• What is temperature?

• Why do we not worry about being constantly bombarded by radiowaves (we are), but yet we guard out exposure to X rays??