Thursday, June 14, 2012

Discovery of an electron

My previous post about the nuclear atom and its parent experiment occurs later than one I discuss in this post. Before the nuclear atom was  proposed in 1907, the electron was discovered.

This experiment rests on a fundamental understanding of attraction and repulsion between charges. Naturally it is logical to assume the positive and negative charges attract one another while the like charges (positive/positive and negative/negative) repel one another. Here is a diagram of that basic concept:


Millikan used an apparatus called a cathode ray tube. I've always found that apparatus very conceptually confusing. Although I understand it now I'd rather gloss over the actual appartus and focus on the concept of what he did with it.

Millikan basically used the concept of electricity to prove that electrons were negatively charged. He pointed a beam of electricity in a way that it could either bend toward a positively charged area or a negatively charged area. The beam was attracted to the positive plate. This proved that the electron had a negative charge.

Millikan was able to derive a charge/mass ratio from this experiment. Here is the apparatus.


 

Tuesday, June 12, 2012

Key Experiments in Chemistry: Part I- The Rutherford Gold Foil Experiment

Anyone who reads this blog knows that I stopped making entries this past month. I wasn't sure what direction I wanted to take the blog and wanted to think about it for awhile. Now that I'm preparing to teach again next semester I have been reviewing the first few chapters of my introductory chemistry textbook. The thought occurred to me that readers might find it helpful for me to describe the most important discoveries and experiments that have led up to our current understanding of atomic structure and basic chemistry. So for the next few entries (at least) I will present a key experiment in chemistry and discuss its significance.


I thought I'd start with the most important one from an introductory chemistry perspective. In my opinion, Rutherford's gold foil experiment is the most important experiment that has influenced the way we envision the modern atom.  Here is a picture of the apparatus:


Prior to this experiment, scientists thought that JJ Thompson's model of the atom was correct. In his model, electrons float around in a sea of positive charge. (See image to the left). Scientists believed that if they projected a stream of alpha particles (like helium atoms without the electrons) at an atom the alpha particles would stream straight through the "sea" of positive charge.

Instead, when Rutherford set up the apparatus above, the stream of alpha particles only partially penetrated the atoms (gold foil). Some were deflected backwards. This meant that there was a hard mass preventing the stream of particles from passing through the fold foil.

And this is where the idea of the nucleus was born.

We now know that the nucleus of the atom is a small, hard, dense center of the atom surrounded by electrons circling in orbitals. A comparison of scale is to envision a flea in the center of a domed stadium. This model of the atom dictates how we think of quantum science (behavior of electrons), nuclear science (protons/neutrons in nucleus), and even explains how the periodic table is structured (by atomic number or number of protons in nucleus). It also explains how we determine the mass of an atom (protons + neutrons).

The modern model of the atom changed the course of science.