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Atomic Model

The History of the Atom

The idea that matter is made up of tiny particles is very old. Democritus was a Greek who lived in the 5th century BCE, and is known as the 'Father of the Atom' for his idea that every substance had a limit to how much it could be divided and remain that substance. The Greeks also thought that the atoms of different types of materials had different geometric shapes, which is not really true.

However, following the ancient Greeks, the idea of a fundamental basic building block of all matter was lost for two thousand years. People in this time thought matter was composed of four elements: air, water, earth, and fire. These express characteristics of matter much like the modern concepts of gas, liquid, solid and energy. However, this idea did not explain the great variety of differences between substances.

In the 17th century the microscope was invented, and the modern sciences of chemistry and biology began in earnest. In the 18th century, scientists began to ask what the air was made of. Experiments showed that it was not a single substance, but consisted of different gases. With this, and many similar discoveries, people began to wonder if the traditional 4-element model was correct.

Brownian Motion gets Boltzman and Einstein thinking

In 1827, a biologist called Robert Brown observed tiny particles of pollen moving around on their own in still water. He observed it, but could not explain it.

It was not until the late 19th century that a German physicist, Ludwig Boltzman, reproposed the atomic structure model. It seems surprising today to hear that his fellow scientists thought this idea to be preposterous!

In 1905, Albert Einstein demonstrated how this atomic model explained Brownian motion in one of his 4 famous Annus Mirabilis papers.

Thomson Atom

Thomson atomic structure model
J.J. Thomson envisaged an atom as a homogenous sphere of positive charge speckled with negative electrons like 'raisins' in a fruit cake

Sometimes called the 'plum pudding' model, Joseph (J.J.) Thomson proposed in 1904 that the atom consisted of a more or less homogeneous sphere of positive charge, within which were 'raisins' of negatively charged electrons.

The electron was discovered by Thomson in 1897, and since there was very little knowledge about atoms, and many scientists even disputed their existence, this seemed a reasonable proposal. The proton was not discovered for another 22 years, 1919, and it took a further 13 years before the neutron was discovered in 1932. Without the right equipment, the theory was slow to find the solution. The first clue came in 1908-13, when two New Zealanders and a German tried a very unusual experiment, with a surprising result which sank Thomson's short-lived model forever.

Instead of plum pudding, these New Zealanders preferred gold....

Rutherford-Geiger-Marsden Experiment

Gold Foil Experiment

Gold foil Experiment
Gold Foil Experiment: demonstrating that the nucleus of an atom exists

Between 1908 and 1913, Ernest Rutherford and two students, Hans Geiger and Ernest Marsden, conducted an experiment which established the model of the atom we still use today. In this experiment, the scientists fired alpha particles (nuclei of helium atoms, produced from the decay of radon) at a very thin sheet of gold foil (only a few atoms thick), to measure the diffraction pattern. To their surprise, they saw that a very small percentage (one in ten thousand) reflected (bounced back towards the source), while a few others deflected at unexplainable angles. This was unexpected and not the objective of the experiment.

Rutherford's brilliant insight was to see that this demonstrated that atoms were mainly empty space, with a tiny nucleus at the centre, and a 'cloud' of electrons at the periphery.

Before this accidental discovery, the prevailing model of the atom was a solid 'plum pudding' (Thomson's Model), with electrons evenly distributed through a 'sea of positive charge'.

The discovery led to a complete rethink of the atomic model, quantum mechanics, and the discovery of the proton (1917) and neutron (1932).

The Rutherford Atom

Ernst Rutherford realised that the Gold Foil experiment had only one possible meaning: most of an atom was empty space!

As the small positively-charged alpha particle were fired at the very thin gold foil, only a few atoms thick, most passed through with no discernible deflection. However, some did deflect, at very varying angles, and a very few (one in every ten thousand) actually reflected back towards the source. What could this mean?

To cause the alpha particle to bounce back would require a relatively large force. This could only be supplied by a very positive nucleus - but since only very few did reflect the alpha particles, the nucleus had to be very small in diameter compared to the diameter of the atom. Doing the maths, Rutherford calculated that the diameter was of the order of $10^{-15}$ m, compared to the diameter of the atom (of the order of $10^{-10}$ m).

It was not until 1919 that the source of the positive charge, the proton, was identified - by Rutherford again.

Rutherford proposed a model to replace Thomson's plum pudding model. The Rutherford model of the atom proposes a heavy positive nucleus, and electrons going around it in a circle (like planets in orbit around the Sun). This was a brilliant idea, and took physics a big step towards a more accurate picture of the atomic structure.

The Bohr Model

However, Rutherford's model ran into difficulties when Maxwell's laws of electromagnetic radiation were applied to it. If the electrons are small, solid objects in circular orbit around the nucleus, then they would be losing energy due to their centripetal acceleration. In fact, they would lose all their energy within a few nanoseconds. But atoms, like the matter it constitutes, are stable and permanent. A modification to the model was needed, if atoms were to be allowed to exist.

A student of J.J. Thomson, Niels Bohr, had arrived in England, and quickly made it his business to work out how electrons could be 'in orbit' and yet never lose energy. The results were the Bohr Postulates, published in 1911.

Bohr applied Max Planck's discovery - that of quantum energy levels observed in a black body radiating thermal energy - to the electrons in an atom. Electrons could absorb the energy of photons, and re-radiate photons at one or more frequencies (known as the spectra). Bohr conjectured that this indicated that electrons did not absorb and release energy of any amounts (continuum) but could only do so at distinct energy levels.

A metaphor is a ball rolling down steps. The ball can come down, or go up, only distinct whole numbers of steps. It cannot stop halfway between steps. At at each step it falls down, it emits a packet of sound energy.

In a similar way, an influx of energy of a specific frequency can cause an electron to 'jump up' an energy level. Not enough energy, and the electron does not change (it cannot store small amounts of energy and accumulate them). This is the 'quantum' nature of the electron, and indeed all sub-atomic particles: they can exist only at distinct energy levels, and nothing in between.

Spectra

An unbelievably useful tool derives from this quantum nature of electron energy absorption and emission.

Hydrogen Absorption Spectrum

If white light is shone on a gas of a single element, the energy of some of the photons will be absorbed by the electrons. Since these electrons can take only specific energy levels, they can absorb only photons of specific wavelengths, in accordance with Planck's Law. The light will therefore go 'dark' at those frequencies.

By observing the dark bands in a spectrum reveals which element, or elements are present. This is the technique used to determine the elements in astronomical bodies, such as distant stars.

Sub-Atomic Particles

All the matter that makes up the physical world around us and the universe consists of atoms. These come in 92 varieties, called elements. The Periodic Table is a representation of these elements, ordered by the number of particles they consist of.

These particles are the neutron, proton and electron.

Particle Relative Mass Charge
neutron heavy no charge
proton heavy positive
electron very light negative

The protons and neutrons are in a very tiny nucleus. The electrons are in 'shells' around the nucleus. In a neutral (uncharged) atom, there are exactly the same number of electrons as protons. The number of neutrons can vary, but, except for hydrogen, there are always at least as many, or usually more, neutrons than protons.

Some Examples of Atomic Structure

Hydrogen
Hydrogen
Hydrogen is the only element with more protons than neutrons - only one!

Hydrogen is the first and smallest element. In fact, most hydrogen atoms are just a single proton and a single electron. There are some rarer forms (isotopes) of hydrogen with one and two neutrons. These forms, called deuterium and triterium, are very important for nuclear energy.

Helium
Helium
Helium has two protons, two electrons, and two neutrons

The next largest atom is helium. This atom therefore has two protons, and two electrons. An atom with more than one proton must have neutrons. This is because protons are positive, and if they are next to each other in the nucleus, they would repel each other and the nucleus would fall apart. Neutrons, which have no charge, act like a glue, holding the nucleus together, preventing it from being blown apart by the positive charges of the protons.

Carbon
Carbon
Carbon has an atomic number of 6, and a mass number of 12 (6 protons + 6 neutrons = 12 nucleons)

Carbon is the element of 'life'. It is the backbone of all organic compounds. Most carbon is carbon-12 (6 protons and 6 neutrons), but there is an isotope, carbon-14 (6 protons and 8 neutrons), which is radioactive and is used to date organic residue in archeology.

Content © Andrew Bone. All rights reserved. Created : March 27, 2014 Last updated :September 6, 2015

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