Models of the Atom
We are going to be looking at how the modern understanding of atom has evolved over the time It is important to realise that a lot of what we know about the structure of atoms has been developed over a long period of time. This is often how scientiﬁc knowledge develops, with one person building on the ideas of someone else.
The idea of atoms was invented by two Greek philosophers, Democritus and Leucippus in the ﬁfth century BC. The Greek word ατoµoν (atom) means indivisible because they believed that atoms could not be broken into smaller pieces.
In the past, before the structure of the atom was properly understood, scientists came up with lots of diﬀerent models or pictures to describe what atoms look like.
The Plum Pudding Model/ J.J. Thomson model
After the electron was discovered by J.J. Thomson in 1897, people realised that atoms were made up of even smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet, and so the ’plum pudding model’ was put forward in 1904. In this model, the atom is made up of negative electrons that ﬂoat in a soup of positive charge. Thomson was awarded the Nobel Prize for his work in this ﬁeld. However, even with the Plum Pudding Model, there was still no understanding of how these electrons in the atom were arranged
Rutherford’s model of the atom
The discovery of radiation was the next step along the path to building an accurate picture of atomic structure. In the early twentieth century, Marie Curie and her husband discovered that some elements (the radioactive elements) emit particles, which are able to pass through matter in a similar way to X-rays .It was Ernest Rutherford who, in 1911, used this discovery to revise the model of the atom
Radioactive elements emit diﬀerent types of particles. Some of these are positively charged alpha (α) particles. Rutherford carried out a series of experiments where he bombarded sheets of gold foil with these particles, to try to get a better understanding of where the positive charge in the atom was.
Rutherford set up his experiment so that a beam of alpha particles was directed at the gold sheets. Behind the gold sheets, was a screen made of zinc sulﬁde. This screen allowed Rutherford to see where the alpha particles were landing. Rutherford knew that the electrons in the gold atoms would not really aﬀect the path of the alpha particles, because the mass of an electron is so much smaller than that of a proton. He reasoned that the positively charged protons would be the ones to repel the positively charged alpha particles and alter their path.
What he discovered was that most of the alpha particles passed through the foil undisturbed, and could be detected on the screen directly behind the foil (A). Some of the particles ended up being slightly deﬂected onto other parts of the screen (B). But what was even more interesting was that some of the particles were deﬂected straight back in the direction from where they had come (C)! These were the particles that had been repelled by the positive protons in the gold atoms. If the Plum Pudding model of the atom were true, then Rutherford would have expected much more repulsion since the positive charge, according to that model, is distributed throughout the atom. But this was not the case. The fact that most particles passed straight through suggested that the positive charge was concentrated in one part of the atom only.
Rutherford’s work led to a change in ideas around the atom. His new model described the atom as a tiny, dense, positively charged core called a nucleus, surrounded by lighter, negatively charged electrons. Another way of thinking about this model was that the atom was seen to be like a mini solar system where the electrons orbit the nucleus like planets orbiting around the sun.
The Bohr Model
There were, however, some problems with this model: for example it could not explain the very interesting observation that atoms only emit light at certain wavelengths or frequencies. Niels Bohr solved this problem by proposing that the electrons could only orbit the nucleus in certain special orbits at diﬀerent energy levels around the nucleus. The exact energies of the orbitals in each energy level depends on the type of atom. Helium for example, has diﬀerent energy levels to Carbon. If an electron jumps down from a higher energy level to a lower energy level, then light is emitted from the atom. The energy of the light emitted is the same as the gap in the energy between the two energy levels. The distance between the nucleus and the electron in the lowest energy level of a hydrogen atom is known as the Bohr radius.