The word atom comes from the Greek Atomos,

This idea of atoms being the most small and indivisible building block of all nature date back to 450 BCE in Greece but before that to 600 BCE from India

Usual understanding depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus.

In 1661, Irish born Robert Boyle, in his work, 'The Sceptical Chemist' argued that all matter was made up of corpuscles, which we now call atoms.

1798 Antoine Lavoisier, realises that matter is made up of different elements that cannot be broken down to less complex particles.

John Dalton, from England, publishes his own Table of Relative Atomic Weights, this further adds to the idea that they truly believed there were just different types of atoms at this time. A great deal of the information he put in this table was little more than arranging other peoples work. Yet it is the first ordered table Chemistry has seen and the year is 1803.

Demitri Mendeleev produces his Table in 1869

Electrons are found by JJ Thompson

radioactive particles

Frederick Soddy & Alexander Fleck working in University of Glasgow realised that the atoms mutate from one element to another on the emission of an Alpha Particle.

Soddy was awarded the Nobel Prize in 1921

Rutherford discovers the atom is empty

In 1909, an undergraduate, Ernest Marsden, was being trained by Geiger. To quote Rutherford (a lecture he gave much later):

"I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?" I may tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle's being scattered backward was very small. Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

This led Rutherford to propose in 1911 the idea that the atom looked something like the solar system, with a small dense positively-charged nucleus with electrons swirling around it.

Each atom is 99.999999999 % empty. Now all matter, including you, me and whatever you happen to be sitting on, is made up of atoms. Which means you are almost completely empty space! In fact if we could somehow remove all this empty space in your space in your body and just keep the solid bit, it would be about the size of a grain of sand! So why do objects look and feel solid?

This was famously summed up by one scientist who claimed that “the world is not only queerer than we suppose, it is queerer than we can suppose”.

Chronographically it is after the Geiger & Marsden expt, that Bohr produces his model, however it will be more beneficial to your understanding if we look next at the center of the Nucleus.

Mosely measured X-rays from 38 elements. He started with aluminum, atomic mass 27, and ended with gold, atomic mass 197. As he moved across the periodic table, one element at a time, he found that the charge on the nucleus increased by one unit. This proved that the charge on the nucleus was equal to its atomic number.

In 1930, the German physicists Walther Bothe and Herbert Becker noticed something odd. When they shot alpha rays at beryllium (atomic number 4) the beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. In contrast, it takes less than one millimeter of lead to stop a proton. Bothe and Becker assumed the neutral radiation was high-energy gamma rays.

Rutherford predicted the existence of the neutron in 1920. Twelve years later, his assistant James Chadwick found it. Chadwick had been a student at Manchester University. After graduating in 1911, he stayed at the laboratory doing research for Rutherford.

Chadwick had another explanation for the beryllium rays. He thought they were neutrons. He set up an experiment to test his hypothesis.

Chadwick put a piece of beryllium in a vacuum chamber with some polonium. The polonium emitted alpha rays, which struck the beryllium. When struck, the beryllium emitted the mysterious neutral rays.

In the path of the rays, Chadwick put a target. When the rays hit the target, they knocked atoms out of it. The atoms, which became electrically charged in the collision, flew into a detector.

Chadwick's detector was a chamber filled with gas. When a charged particle passed through the chamber, it ionized the gas molecules. The ions drifted toward an electrode. Chadwick measured the current flowing through the electrode. Knowing the current, he could count the atoms and estimate their speed.

Chadwick used targets of different elements, measuring the energy needed to eject the atoms of each. Gamma rays could not explain the speed of the atoms. The only good explanation for his result was a neutral particle.

To prove that the particle was indeed the neutron, Chadwick measured its mass. He could not weigh it directly. Instead he measured everything else in the collision and used that information to calculate the mass. He found it WAS 1.026 the mass of a proton.

Bohr model, descriptive treatment only.

in 1913, physicist Niels Bohr suggested that the electrons were confined into clearly defined, quantized orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.

Energy levels.

A really fantastic page that explains the section very well

Actual energy levels for H atom ... good for calculation of Homework Problems (to be set)

Emission line spectra:

hf = E2 – E1.

Experiment may be simulated using a large-scale model or a computer or demonstrated on a video.

Demonstration of line spectra and continuous spectra.


Spectroscopy as a tool in science.