Structure of the Nucleus

The word Atom comes from the Greek language, átomos means indivisable, it cannot be broken down into more simple components. A Gold bar is made up of millions and millions of these atoms. The Iron bar is also made up of millions and millions of Iron atoms.

Each type of atom is different.In the 17th and 18th centuries chemists began to isolate these differrent elements.

They saw that each element had its own properties,

Oxygen (O) was required for fires to burn.

Copper (Cu) was a better conductor than other metals,

Sodium (Na) reacted easily with other substance and burned with a different colour (orange) to the other elements.

The next thing that happened the Atom was not for almost 100 years, when J.J. Thompson who was experimenting with Cathode Rays (more about that later) that had been discovered or first created by German scientist Johann Hittorf in 1869.

Crookes Tube

J.J. Thompson carried out 3 experiments

1 Was negative charge seperable from the cathode beam.

This experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays"

J.J. Thompson


Mr Kerr

2 We know they are deflected in a magnetic field, can they be deflected in an electric field

Yes they did, and with what was decided a Negative charge.

3 Lets try to measure the Mass to Charge ratio.

He compared the deflection of his cathode rays to hydrogen ions, he found that they were not as forcefu as the hydrogen ions. In fact they were over a 1000 times greater than the mass to charge ratio of the Hydrogen, this means the particle is over 1000 time more charged or less than 1/1000th the mass?

This last experiment was carried out in 1897, J.J. Thompson had discovered negatively charged particles that were very small parts of the atom. The idea of these particles existing had be forwarded by an Irish man by the name of George Johnstone Stoney, he had called these particles Electrons, and so the name stuck.

The new idea was that these electrons were stuck in the atom randomly like raisins in a christmas cake.

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.

Until Ernest Rutherford came along, he was born in New Zealand and educated there too. In 1895 in order to continue his studies he went to England to study in the well sponsored Cavendish Laboratory in the University of Cambridge.

The Apparatus

The Experiment

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.

The Nucleus

The findings of the experiment were unexpected. Most of the alpha particles when straight through the gold leaf, about 1% deviated slightly from the straight path. Geiger and Muller were good experimenters and continued with their work. They found that a few of the alpha particles were rebounded back along the path they came.

This suggested to the young students that the centre of the atoms were densly packed structures that obviously contained most of the mass of the atom. From this they proposed that the the majority of atoms space was empty. They also thought at this stage the electrons were randomly spread about the volume of the atom, 'like raisins in a plum pudding'

A 40 cm wide missile being fired at tissue paper and it bouncing back, something very strange going on here!!

The feeling at the time was that the atom had its mass shared evenly over space in what they called the plum pudding model. That the whole space taken up by the atom had the atoms mass spread evenly across the entire space. Those experiments carried out in the Cavendish Lab suggested things were not so simple. Some of the Alpha particles were reflected back along the path they came in ?? The majority passes straight trough with little or no deviation from the path it was sent along. 1 in every 8000 was reflected back at an angle that was unexpected.

It was as if all the mass must be concentrated at the center of the atom.

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.

Rutherford is known as the Father of Nuclear Physics because it was he that showed that very heavy elements change from one atom to another, in nuclear disintigration. He also demonstrated half-life and from this proved that the earth was far older than had previously been believed.

All of a sudden Atom could be interchanged?

While they were all different they were similiar ??

the element rutherfordium, Rf, Z=104

structure of the atom

So the Nucleus contains 2 different types of particles, the Proton and the Neutron, outside the Nucleus lie the electrons. To describe electrons as lying there is wrong, in fact the electrons go whizzing around the atoms but they do so in specific orbits often called shells.

location, relative charge, and relative atomic mass of the sub-atomic particles

atomic number Z

Each element is made up of different types of atoms,

the atoms in these elements differ because the number of protons for each element is different.

The Atomic number is equal to the number of protons in an atom. The number of electrons is usually the same as the number of protons, as there is no net charge on an atom, they are neutral. The number of neutrons varies from element to element obviously but also differs from atom to atom, the ne

Mass number A

There is another number that is always listed with the symbol for the element, this is the mass number. The mass number is defined as the number of protons + the number of neutrons together.

To find the number of Neutrons

# neutrons = Mass Number - Atomic Number

X = A - Z

The Mass number will be the bigger value in the atomic symbol (except for hydrogen which usually does not have a neutron attached).



An Isotope of an element are two atoms that have the same number of protons but have different numbers of neutrons.

for an interesting bit on isotopes

Where does the decimal comes from in the Mass No ?

The mass numbers of elements often has a decimal in it.

There can be no such thing as a fraction of a proton, or a neutron.

However due to their being different isotopes of elements, this gives us different mass numbers.

When we account for the relative abundence of each of the Isotopes we get average values for the mass number.


Chlorine has two Isotopes

Most of the Chlorine found on earth is the CL-35 variety, about 75% of it, the remaining 25% is made of Cl-37.

Do the math

(35 x 0.75) + (37 x 0.25)

▬▬▬▬▬▬▬▬▬▬▬ = the mass number for chlorine (on average)


So this decimal is found by finding the average of all the isotopes found in nature according to their abundence in nature.

Bohr Models

For the basics of Bohr Models