In this essay, I will present the key scientists who studied the atomic theory and their contributions to science.

## Democritus

Democritus lived in 5th century B.C. in Greece. “Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then, at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter ‘atomos’, which translates to indivisible in English”. Democritus believe that atoms were different sizes and shapes, but were also in constant motion. Democritus’ theory of atoms followed many principles such as: atoms are indestructible, atoms always have been and always will be in constant motion, between atoms there lies empty space, and many more. Democritus was also the first philosopher to posit what we now know as ‘The Milky Way’.

## Joseph John Thomson

Thomson was born December 18, 1856 in Manchester England. In 1897, he discovered the electron by experimenting with a crooke, or tube. From this he realized that the accepted model of an atom did not account for negatively or positively charged particles. So, he opted to create a new model using plum pudding. The raisins represented the negative electrons and the dough represented the positive charged particles. Though years later, his own student had proven his plum pudding model incorrect.

## Marie Curie

Marie Curie was born November 7, 1867 in Warsaw, Poland. Many think the fact that her father was a physics teacher contributed to her interest in science. Curie discovered radioactivity. She was studying uranium rays, when she made the claim the rays were not dependent on the uranium’s form, but on its atomic structure. This formed a new field of study called atomic physics. In this she called her new discovery ‘radioactivity’. She also discovered Radium and Polonium through her work with pitchblende.

## Albert Einstein

Albert Einstein was born March 14, 1879 in Germany. Einstein became famous for his ‘Theory of Relativity’, which was the basis of ‘The Atomic Theory’. The third paper he published focused on the movements of tiny particles in a liquid or gas. This was renown as ‘Brownian motion’. This paper he wrote on Brownian motion confirmed the atomic theory. In 1921 he was awarded the Nobel Prize in Physics.

## Erwin Schrodinger

Erwin Schrödinger was born August 12, 1887 in Vienna, Austria. In 1926, Erwin and a friend Werner Heisenberg took the model Neils Bohr created a step further. He used mathematical equations to describe the chance of finding an electron in a certain position. Before the math came though, he created an ‘Electron Cloud Model’. This consisted of a dense nucleus surrounded by a cloud of electrons at different levels of orbit. He used the math to see if his theory would work and it did. Using the model and putting the math together he came up with what are now called lobes.

## Antoine Lavoisier

Antoine Lavoisier was born in 1743. He is most famous for ‘The Law of Conservation Of Matter’. Between 1772 and 1794 he discovered that mass is conserved in a chemical reaction. From this we know why matter is conserved, atoms cannot be destroyed nor created, only transferred. He discovered Hydrogen and Oxygen. By the end of the eighteenth century, Lavoisier had published 13 more elements to the list of 11 that were already known.

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## Ernest Rutherford

Ernest Rutherford was born August 30, 1871 in Spring Grove, New Zealand. He discovered that atoms have a small charged nucleus surrounded by empty space and are circled by electrons. In 1912, Bohr joined Rutherford at Manchester and adapted Rutherford’s nuclear structure with Planck’s quantum theory and came up with an atomic structure that is still valid to this day. In 1919, Rutherford was the first person to transmute one element into another. He is also credited with the discovery of the proton, when he noticed the signatures of hydrogen nuclei being emitted during this process.

## James Chadwick

James Chadwick was born October 20, 1891. In 1932 Chadwick discovered the neutron. He experimented by observing beryllium as it hit paraffin wax. This caused radiation to eject from it and Chadwick hypothesized that it was in fact a neutral particle. In 1935 he was awarded a Nobel Prize in Chemistry.

## Neils Bohr

Neils Bohr was born in Copenhagen in 1885. He was always very interested in physics. In 1913 Bohr published his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. Perhaps Bohr’s greatest contribution though was his atomic model that showed electrons orbit the nucleus in an atom. It also showed why they emit light in fixed wavelengths.

## Werner Heisenberg

Werner Heisenberg was born December 5, 1901 in Germany. Heisenberg created his uncertainty principle. This stated that an electrons velocity and location cannot possibly be known simultaneously. This is because an electrons mass is so tiny that trying to observe it with any kind of light would cause it to move. In 1932, Heisenberg won the Nobel Prize in Physics for the creation of Quantum Mechanics.

## John Dalton J.J

John Dalton was born September 6, 1766, in Eaglesfield, England. With his experiments and research into gases, Dalton discovered that certain gases could only be combined in certain proportions. The experiments further built two of the most used laws today. The law of conservation of mass, and the law of definite proportions. After studying these laws, Dalton came up with his own law: the law of multiple proportions. “This law states that if two elements can be combined to form a number of possible compounds, then the ratios of the masses of the second element, which combine with a fixed mass of the first element, will be ratios of small whole numbers”.

## Robert Millikan

Robert Millikan was born March 22, 1868 in Morrison, Illinois. In 1909 Millikan started a series of experiments to determine the electric charge carried by a single electron. He began by measuring the course of charge water droplets in one electric field. His results suggested that the charge on the droplets is a multiple of the elementary electric charge. Though the experiment wasn’t accurate enough to be used. So, in 1910 he got more precise results doing his oil-drop experiment.

## Max Planck

Max Planck was born April 23, 1858 in Germany. He best known for being the originator of the ‘Quantum Theory of Energy’. His work in thermodynamics led to his formulation of the quantum theory. While doing his research he came up with the same problems as other scientists, classic physics couldn’t explain his results. So, with this in mind he created an equation to explain his results. “E=Nhf, with E=energy, N=integer, h=constant, f=frequency. In determining this equation, Planck came up with the constant (h), which is now known as ‘Planck’s constant’”.

## Louis De Broglie

Louis De Broglie was born August 15, 1892 in France. Louis discovered the ‘wave nature’ of electrons. He called it the ‘Wave-Particle Theory’. It said that all matter has wave properties. “Putting together Planck’s equations (quantization of energy: E = hν) and Einstein’s (special relativity: E = mc2), de Broglie calculated what the length of these matter waves associated with each particle would be, depending on its velocity and mass”. In this our world is quantum not just light. If an electron were like wave, it would have to display properties of a wave. This hypothesis was demonstrated by the electron diffraction experiment of Davisson and Germer (1927), thus confirming the hypothesis of de Broglie. Later in 1929, Broglie was awarded the Nobel Prize for this finding.