Brief History of the Discoveries of the Atom

Man has questioned his world and reality for as long as he has been around. One of the oldest questions ever asked was, “What, in it’s most basic form, is the nature of matter?”. We must travel back to ancient Greece for the first recorded answer. Philosophers debated on whether or not matter was indivisible or infinitely divisible. Around 460 BC, philosophers such as Democritus and Leucippus believed that matter was made up of tiny, indivisible parts, which is where we get the term ‘atom’ from, Greek for indivisible. Still, other thinkers such as Plato and Aristotle held that matter could be broken down an infinite number of times and no such indivisible atom existed. Back then there was really no experimental way of proving who was right so the winner was usually the best debater which in this case was Aristotle. Twenty-two thousand years of humanity would pass before anymore light was shed on this important question. In 1803, James Dalton, an English chemist, physicist and meteorologist proposed his famous Atomic Theory. Dalton had spent a lot of time studying various gases under different temperatures and pressures, eventually concluding that the vapor pressure for all liquids is equivalent under the same variation of temperature. As for his Atomic theory, a careful inspection of his lab notes tells us that the idea arose in his mind as a purely physical concept. The first mention of the atom was in a paper he wrote in 1803 (just 27 years after America became a country!): Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases. Dalton went on to weigh the atoms of various elements including hydrogen, oxygen, nitrogen and carbon. As for his atomic theory, Dalton proposed the following: 1) Elements are made of extremely small particles called atoms, 2) Atoms of a given element are identical in size, mass, and other properties, 3) Atoms cannot be subdivided, created, or destroyed, 4) Atoms of different elements combine in simple whole-number ratios to form chemical compounds, 5) In chemical reactions, atoms are combined, separated, or rearranged but never destroyed.

All of Daltons points have been proven correct except for the third one. But before we can sub-divide the atom we need to look at how it’s fundamental particles can be divided. Another hundred years would pass before the next big atomic discovery would happen. In 1897 J.J. Thomson sent ionized gas particles through a magnetic field (which he knew would interact with the particles from Maxwell’s work) in order to observe how they behaved. He noticed that the particles were deflected towards the positive side of the glass tube, meaning that they must have a negative charge. Thus he discovered the electron. Thomson went on to develop his plum-pudding model of the atom which said that atoms are a positively charged sphere spiked with electrons buzzing around inside. Then, about 10 years later, a New Zealand physicist by the name of Ernest Rutherford conducted a ground breaking experiment. He shot alpha particles, which are two protons and two neutrons stuck together (an atom of Helium without the electrons), at a very thin sheet of gold foil and set up a 360 degree wall around the experiment to track where the particles landed. Because gold is so ductile it can be hammered into sheets no more than 0.00004 cm thick. As Rutherford expected the large majority of alpha particles went right through the thin gold sheet (even though they had to pass through roughly 2,000 gold atoms) being deflected minimally or not at all. A very small amount of the alpha particles were deflected and sent flying at an angle towards the wall. However, an even smaller amount of alpha particles, perhaps 1/20,000 bounced off the gold foil and flew back towards Rutherford. From these data he declared that atoms were almost completely empty space and that there was a very small dense center with a positive charge. Rutherford had discovered the nucleus. He concluded that the radius of the nucleus is at least 10,000 times smaller than that of the atom. Realizing how much empty space there is in the atom, it is rumored that Rutherford reluctantly stepped out of bed the next morning half expecting his foot to pass right through his floorboards-such is the case when you are the first person and perhaps the only person alive to understand a physical law so intimately. It turns out Rutherford had good reason to be afraid-we now know that an atom is 99.999999999999% empty space. For some perspective, if a BB was the nucleus of an atom, sitting on a dugout in a baseball stadium, then the actual atom would be the size of the entire stadium. So, why do things not constantly fall through each other if matter is almost completely empty space? The answer, interestingly enough, is that matter actually levitates on an electrostatic field. For instance, when you are sitting on a chair, you are not really touching the chair with your pants- at least on the atomic level. Every atom in your pants and on the chair is surrounded by an electron shell (or cloud). Hence, because both surfaces (and every surface) has a negative charge, the two cannot pass through each other, in fact they really cant touch each other at all. This is actually a very useful characteristic of nature, imagine if surfaces could pass through each other, what a mess our universe would be. In addition to the nucleus, Ernest Rutherford named the positively charged particles protons and knowing that there had to be more mass than was accounted for with only the electrons and protons, named the neutral particles in the atom neutrons. So, Rutherford was really a ground-breaking scientist, literally redefining how we look at the ground-and all surfaces for that matter! The next superstar physicist to make a major break through came from Denmark and went by the name Neils Bohr. He received his masters and doctorate in physics from the University of Copenhagen in 1911 before working at the Cavendish Laboratory in England under J.J. Thomson and later Rutherford. He worked out details of the atomic structure of the atom and described specifically the electron orbital’s and went on to win a Nobel prize for his work. He was a pioneer of quantum physics along with other physicists such as Edwin Schrodinger, Werner Heisenberg and of course the almighty Einstein. The first three decades of the 20th century proved to be a monumental time in physics with possibly the most significant discoveries made in the shortest amount of time that the world had (and has) seen.


About ben james

"The stars keep me up at night" -some song I heard one time Human. Studying the intersection of Neuroscience and Computation. Wanna be (astro) physicist.
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2 Responses to Brief History of the Discoveries of the Atom

  1. Pingback: Brief History of the Discoveries of the Atom | Bens Blog

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