So what are we made of? Well if you were to ask any person from the age of 10 and above, then the vast majority would answer ‘Atoms’, and they would of course be correct. But then the question arises of what an atom is made up of, and once again most people would be able to answer correctly – electrons, protons and neutrons. But then what are these particles made of? At this point most would suggest that these particles were the smallest things that exist in the universe. However that is not the case.
I will start at the beginning. The concept that matter is composed of discrete units and cannot be arbitrarily divided into smaller units has been around for thousands of years; however this was only maintained through philosophical reasoning rather than experimentation. It was not until the late 18th and early 19th century that scientific theory came to the fore, as many famous scientists such as Antione Lavosier and John Daton would introduce the concept of atoms into their calculations. This culminated in Dmitri Mendeleev’s Period table of elements, first published in 1869, which ranked the know elements by their atomic number. (Mendeleev’s original and modern equivalent below)
The major breakthrough came in 1897 when J.J Thomson, whilst working on cathode rays, discovered the electron. At this point in time we had our first proposed model of the atom.
Thomson suggested that the atom was similar to that of a round plum pudding, with electrons scattered throughout the atom (the plums), and with their charge balanced by the presence of a uniform sea of positive charge (the pudding). This theory held true for just over 10 years, until 1909 when Hans Geiger and Ernest Marsden performed their gold foil experiment, under direction of famous physicist Ernest Rutherford.
The experiment consisted of firing alpha particles, then known to be positively charged helium atoms, at a piece of gold foil and using a detector to locate the particles on the other side. If the ‘plum pudding’ model was true then the particles would safely pass through the foil in a straight line with very few minor deflections. However, what they found was although a lot of the particles did follow the right path, a few particles were deflected at small angles whilst some were even reflected back to the alpha source.
Rutherford concluded that the atom must consist of a central positive charge, which had mostly open space around it, which was the reason why most atoms passed through the foil directly. The reasons for the slight deflections he put down to the central charge of the alpha particles passing close to the central charge of a gold particle, and thus pushing it of course. For a full reflection the central charge of the alpha particle must have directly hit the central charge of a gold particle and rebounded. This central charge later became known as the nucleus.
Around the same time a young Danish physicist named Niels Bohr had been studying on a year’s grant in England, notably under both Thomson and Rutherford. He was able to take on Rutherford’s idea of a small nucleus surrounded by electrons; however he found that this was still not mathematically possible using current theories of physics at the time. The problem was that in a similar way to an object being pulled to the ground by the Earth’s gravity, current theory would only suggest that as electrons orbited the nucleus they would gradually be pulled towards the middle of the atom, which would make atoms impossible. Fortunately, the beginning of the 20th century was also the introduction of quantum theory, which revolutionised physics. Using this he was able to work out that electrons could have specific energy levels and could exist in something he called stationary states, in which an electron could orbit a nucleus without being pulled towards that nucleus. Bohr published his modified model of the atom in 1913, and it is still the accepted model 100 years later.
The final piece of the puzzle, the Neutron, was proposed by Rutherford in 1920, and finally confirmed in 1932 after James Chadwick’s experiments revealed uncharged particles which had the same mass as a proton.
For the next 30 years everyone assumed that the atom could be divided no further, however during the 1960’s, 70’s and 80’s the standard model was devised. Gradually, it was discovered that protons, neutrons and electrons were in fact made up of smaller particles known as Quarks, Leptons and Bosons.
Quarks can be split into 6 types: up (u), down (d), strange (s), charm (c), bottom (b) and top (t). Each of these has a charge +2/3 or -1/3,
Leptons can be split into another 6 types: electron neutrino (νe), muon neutrino (νµ), tau neutrino (νƮ), electron (e), muon (µ) and tau (Ʈ). All the neutrinos have 0 charge, with the other particles -1 charge.
Bosons can be split into 4: photon (Ƴ), gluon (g), Z boson (Z) and W boson (W±). The first three have zero charge, whereas the W boson is unique in that it can change other particles in the standard model.
Finally there is the one that everyone has heard of, The Higgs Boson. Only just confirmed in March of this year, the higgs boson has been the one missing piece of the Standard model puzzle, which makes it all work by giving everything mass.
And to even make it more confusing each particle has an antiparticle. For example, the antiparticle of an electron is a positron. In total there are 61 particles in the standard model all produced by a variety of combinations of the 12 particles in the table above.
So next time if you’re ever asked what particles make up an atom, don’t give the standard three particles. There is some much more than that!