SIR JOHN JOSEPH THOMSON (1856-1940)

1897 – England

’Not only was matter composed of particles not visible even with the modern microscope, as scientists from DEMOCRITUS to DALTON had surmised, but those particles were themselves composed of even smaller components’

photo of JJ THOMSON at work in the laboratory ©

JJ THOMSON

By the end of the nineteenth century scientists had cleared up much of the confusion surrounding atomic theory. The discovery of the sub-atomic particle was made in April 1897. They believed that they now largely understood the properties and sizes of the atoms of elements; without question, hydrogen was the smallest of all.

When JJ Thomson announced the discovery of a particle one thousandth the mass of the hydrogen atom the particles were named ‘electrons’ and have been a fundamental part of the understanding of atomic science ever since.

Thomson was investigating the properties of cathode rays, now known to be a simple stream of electrons, but at the time the cause of widespread debate. The rays were known to be visible, like normal light, but they were quite clearly not normal light. He devised a series of experiments, which would apply measurements to the cathode rays and clarify their nature. The rays were created by passing an electric charge through an airless or gasless discharge tube.

By improving the vacuüm in the tube, it was demonstrated that the rays could be deflected by electric and magnetic fields. Thomson drilled a hole in the anode of the tube to allow the mysterious rays from the cathode to pass through. In the space after the anode, he arranged that a magnetic force field from a magnet would tug the cathode rays in one direction, and an electric force field between two electrically charged metal plates would tug them in the opposite direction. The rays would eventually strike the glass wall of the tube to create a familiar greenish spot of light on the phosphor-coated tube.

Thomson concluded that the rays were made up of particles, not waves. He saw that the properties of the particles were negative in charge and didn’t seem to be specific to any one element; they were the same regardless of the gas used to transport the electric discharge, or the metal used at the cathode. From his findings he concluded that cathode rays were made up of a jet of ‘corpuscles’ and, more importantly, that these corpuscles were present in all elements. Thomson devised a method of measuring the mass of the particles and found them to be a fraction of the weight of the hydrogen atom.

The position of the spot indicated how much the beam of cathode rays had been deflected. The deflection could be made zero by adjusting the magnetic and electric forces so that they perfectly balanced. In such a situation, Thomson could read off the strength of the electric force. He knew in theory how the magnetic force on a charged particle depends on its speed. By equating the two forces, he was able to deduce the speed of the cathode rays. The deflection was also influenced by the electric charge carried by the cathode ray particles, and their mass. The larger the charge, the greater the force the particles felt and the greater their deflection, the smaller the mass, the easier it was for any force to push the particles about and again, the greater their deflection.

Independent evidence from electrolysis (passing electricity through liquids) that electric charge came in discreet chunks, which he assumed to be carried by individual cathode ray particles, enabled Thomson to calculate their mass.
He arrived at a figure that was a thousand billion billion billionth of a kilogram – a 1000th of the mass of a hydrogen atom.

Knowing the deflection of the dot and the velocity of the particles (the slower the particles, the longer they were exposed to the electric force and the greater the deflection of the glowing dot), Thomson expected to be able to deduce their charge and mass. What he actually deduced was a combination of their charge and mass.

Atoms were made of smaller things, but the fundamental building-blocks were not hydrogen atoms, as had been maintained by PROUT.

Thomson’s particles were christened ‘electrons’ and were the first subatomic entities. Thomson visualized a multitude of tiny electrons embedded ‘like raisins in a plum pudding’ in a diffuse ball of positive charge.

‘The atom is a sphere of positively charged protons in which negatively charged electrons are embedded in just sufficient quantity to neutralise the positive charge’

This was the accepted picture of the atom at the start of the twentieth century until RUTHERFORD found a way to probe inside the atom in 1911.

picture of the Nobel medal - link to nobelprize.org

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DEMOCRITUS of ABDERA (c.460 – c.370 BCE)

Democritus of Abdera

DEMOCRITUS

Fifth century BCE – Greece

‘Matter is made up of empty-space and an infinite number of tiny invisible particles called atomos or atoms’

Democritus’ atomic theory was probably based on previous ideas of other Greek philosophers. It was the first scientific attempt to explain the nature of matter; however, many of Democritus’ assumptions have now been proved wrong.

Democritus left no written record of his work and little is known about his life, but we know about his atomic theory from the second century CE Greek cynic and biographer Diogenes Laertius’ book ‘Lives of Eminent Philosophers’.

Democritus reasoned that, if he were to attempt to cut an object in half over and over, he would eventually reach a tiny grain of matter that could not be cut in half. Democritus called these hypothetical building blocks of matter “atoms”, after the Greek atomos, ‘uncuttable’ – suggesting that atoms could not be divided indefinitely into smaller parts and that it is impossible to create new matter. All that you can see is a product of packing together a myriad of miniscule atoms.
He said that atoms were always in motion and as they moved about they collided with other atoms; sometimes they interlocked and held together, sometimes they rebounded from collisions. The Roman poet Lucretius (c.94 – c.55 BCE) imagined Democritus’ atoms with hooks that fastened them together.

His remains one of the earliest attempts to explain the universe with a few simple physical and mathematical laws. For Democritus, there were only two things, space and atoms. Both had always existed and always would exist because de nihilo nihil ‘nothing could come from nothing’. Balancing the idea that the atom is the only unit of being, the Void is the single kind of not being. The higher the atom-to-void ratio, the denser the material. Atoms simply combined with other atoms in the Void; solid, impenetrable, indivisible blocks which never change; they combine to form different things, from rocks to plants to animals. When these things died or fell apart the structure disintegrated and the atoms were free to form new things by combining again in a different shape with other atoms.

Democritus reasoned that the method by which the atoms could combine was through their different shapes. While all the atoms were the same in substance, liquids were thought to have smooth round edges so they could fall over each other, while those that made up solids had toothed rough edges, which could hook on to each other.
Atoms of fire had tetragonal shape; atoms of earth, cubic; atoms of air, octahedral; and those of water were icosahedral.

 

Democritus’ thesis completely rejects the notion of the spiritual or the religious. The soul was explainable through a fast-moving group of atoms brought together by encasement in the body. The motion produced sensations that interacted with the mind (itself a collection of atoms) to produce thoughts and feelings. Once dead, the object that held the fast-moving atoms disintegrated and thus released they could separate and interact with other atoms to form new things. Leaving no place for abstract notions of the supernatural or an afterlife, this marked the arrival of materialism.

THE PLATONIC SOLIDS

As with physical form, Democritus argued that other perceived differences in things, such as their taste, could be explained by the edges of the atoms. Likewise the colour of things was explained by the position of the atoms within a compound, which would result in darker or lighter shades.

Democritus had envisaged free atoms as flying about ceaselessly through empty space. What was needed therefore, was a precise picture of how atoms moved through space. Such a picture required knowledge of the laws that governed all motion.

Picture of the moon crater named after Democritus

DEMOCRITUS CRATER

The Greek philosopher ARISTOTLE rejected Democritus’ idea of the atom and said that matter was completely uniform and continuous. The influence of Aristotle was extraordinary. His concept of matter was at variance with modern thinking, but it was accepted for around 20 centuries until it was replaced by DALTON‘s atomic theory in 1808.

Link to WIKIPEDIA

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