Further developments to the atomic model
Bohr and energy levels
Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model.
In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.
By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. of that element.
Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
When atoms absorb energy, such as Energy travelling as waves in the form of changing electrical and magnetic fields., the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus).
After a short, but random time, they fall back down to a lower level releasing the energy they gained as electromagnetic radiation, such as light, of definite frequencies.
This 'solar system' model of the atom is the way that most people think about atoms today.
It is known as the Rutherford-Bohr model of atomic structure.
Chadwick and the neutron
The Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass.
However, there was still a difference between the atomic number (the number of protons) of the atom and the atomic mass.
Bohr and other scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass.
This was accepted, and scientists used models that included the neutron for a further 20 years before it was actually discovered.
The neutron was in fact difficult to discover because it has no charge.
It wasn't until 1932 that the English physicist, James Chadwick, was able to prove its existence.
Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil.
In doing so he discovered the proton-sized neutral particle - now known as the neutron.
Example
The Rutherford-Bohr model of the atom is based on evidence obtained from Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus). scattering experiments using the apparatus shown below.
Question
- Name the parts labelled A, B and C in the diagram above.
- Describe how the experiment was carried out.
- Briefly explain the finding from this experiment that 99.99% of particles were undeflected.
- Briefly explain the finding from this experiment that a very small number of particles bounce back.
- A - Alpha particle source; B - Gold foil; C - Scintillation screen (or flourescent screen or zinc sulphide screen)
- The scintillation screen is moved to different positions – rotating it 3600 about the alpha particle source. At each position the number of alpha particles detected (small flash of light observed) is counted and recorded.
- The atom is mostly empty space.
- This implies that the nucleus of the atom is positive and that most of the mass of the atom is concentrated in a small nucleus.