Photo by Xaotherion

 

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An atom is the smallest piece of an element that can exist and still be recognized as that element. Nearly all of these elements, such as the oxygen that you breathe and the carbon in your skin, were made in stars about 5 billion years ago, at around the time that the Earth was first forming. Hydrogen and helium are even older, most hydrogen having been made soon after the Big Bang, later to provide the fuel of the stars within which the other elements would be created. 

Think again of that breath of oxygen and its million billion billion atoms within your lungs. That gives some idea of how small each atom is. Another way is to look at the dot at the end of this sentence. Its ink contains some 100 billion atoms of carbon. To see one of these with the naked eye, you would need to magnify the dot to be 100 metres across. 

 

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Look at the full stop once more. Earlier I said that toe see an atom with the naked eye would require enlargement of the dot to 100 metres. While huge, this is still imaginable. But to see the atomic nucleus you would need that dot to be enlarged to 10,000 kilometres: as big as the Earth from pole to pole. 

Between the compact central nucleus and the remote whirling electrons, atoms are mostly empty space. That is what many books assert, and it is true as concerns the particles that make up an atom, magnetic force fields, so powerful that they would stop you in an instant if you tried to enter the atom. It is these forces that give solidity to matter, even while its atoms are supposedly ’empty’. As you read this, you are suspended an atom’s breadth above the atoms in your chair due to these forces. 

Powerful though these electric and magnetic forces are, they are trifling compared to yet stronger forces at work within the atomic nucleus. Disrupt the effects of these strong forces and you can release nuclear power; disrupt the electric and magnetic forces and you get the more ambient effects of chemistry and the biochemistry of life. These day to day familiar effects are due to the electrons in the outer reaches of atoms, far from the nucleus. Such electrons in neighbouring atoms may swap places, thereby helping to link the atoms together, making a molecule. It is the wanderings of these electrons that lead to chemistry, biology, and life. This book is not about those subjects, which deal with the collective behaviour of many atoms. By contrast, we want to journey into the atom and understand what is there. 

 

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It is that although intense electrical forces are at work deep within the atoms of our body, we are not much aware of them, nor are we ourselves electrically charged. The atom of the simplest element, hydrogen, consists of a single proton and a single electron. The number of protons in the nucleus is what differentiates one element from another. A cluster of 6 protons forms the nucleus of the carbon atom, iron has 26, and uranium 92. 

 

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However you cannot put too many protons in close quarters; eventually the electrical disruption is too much. This is one reason why there is a heaviest naturally occurring element, uranium, with 92 protons in each nucleus. Pack more protons than this together and the nucleus cannot survive. Beyond uranium are highly radioactive elements such as plutonium whose instability is infamous.

Atomic nuclei of all elements beyond hydrogen contain protons and also neutrons. The neutron is in effect an electrically neutral version of the proton. It has the same size and, to within a fraction of a percentage, the same mass as a proton. Neutrons grip one another with the same strength that protons do. Having no electrical charge, they feel no electrical disruption, unlike protons. As a result, neutrons add to the mass of a nucleus, and to the overall strong attractive force, and thereby help to stabilize the nucleus.

When neutrons are in this environment, such as when part of the nucleus of an iron atom, they may survive unchanged for billions of years. However, away from such a compact clustering, an isolated neutron is unstable. There is a feeble force at work, known as the weak force, one of whose effects is to destroy the neutron, converting it into a proton. This can even happen when too many neutrons are packed with protons in a nucleus. The effect of such a conversion here is to change the nucleus of one element into another. This transmutation of the elements is the seed of radioactivity and nuclear power. 

 

Inside the atom. Atoms consist of electrons remotely encircling a massive central nucleus. A nucleus consists of protons and neutrons. Protons are positively charged; neutrons have no charge. Protons and neutrons in turn are made of yet smaller particles called quarks. To our best experiments, electrons and quarks appear to be basic particles with no deeper constituents.

 

Particle Physics by Frank Close