The History of the Development of the Atom Mr. Tsigaridis

Empedocles – 450 B.C. A Greek scholar named Empedocles proposed that matter was made of four elements For the greater part of the history of man, we believed that everything around us from the stars in the sky to the rocks beneath our feet was made of those four elements Earth Fire Air Water

Empedocles– 450 B.C. It was believed that different combinations of elements formed different substances Ex. For Lava, the ancients believed that it was 2 parts fire and one part Earth For many millennia this model ruled the world around us and was the dominant model for explaining the existence of everything

Democritus – 400 B.C. In 400 BC there was a Greek philosopher known as Democritus He was the first person to postulate that the world was not made up of the four element model but rather was made of tiny little particles called atoms. The word atom comes from the Greek word Atomos which means singular or indivisible

Democritus – 400 B.C. He came to the conclusion that the world around him must have been made of these tiny indivisible particles because he took a mortar and pestle and pounded materials until they were small indivisible particles Unfortunately he was not the most famous of the philosophers of the time and because names like Socrates believed in the four element model, his theory was tossed aside

Alchemists – 500 – 1600 A.D. Do metals grow like plants and ripen into gold? Many alchemists believed that they did For many of them, the idea of finding the philosopher’s stone, which was a mythical stone that could turn any metal into gold, was of the upmost importance For centuries they performed numerous experiments attempting to make gold from cheaper metals such as iron and lead

Alchemists – 500 – 1600 A.D. The devised chemical symbols for substances and are now recognized as elements and compounds They also invented the laboratory equipment that we use still today like beakers, filters, stirring rods and distillers They still however accepted the four element model More importantly, no one ever turned lead into gold!!

Robery Boyle – 1650 A.D. An english scientist took what some would consider the second step in no believing in the four element model He devised a new definition for the word element “I mean by element, simple unmitigated bodies.” This became the modern definition of an element: “a pure substance that cannot be chemically broken down into simpler substances”

Robert Boyle – 1650 A.D. He also believed that air was not an element, but rather a mixture It was not proven until later in the 1700’s

Joseph Priestly – 1700’s He was the first person to isolate oxygen scientifically, but he did not know that oxygen was an element It was not until later that it was recognised as an element by Antoine Lavoisier This played havoc on the first element of the four elements model, Air This proved that air was a mixture and not an element

Antoine Lavoisier – 1700’s He was experimenting with Priestly’s oxygen and he concluded that air must be a mixture of at least two gases, one of which was oxygen At the same time that Lavoisier was working with oxygen, another scientist named Henry Cavendish experimented with mixing metals with acids

Henry Cavendish – Late 1700’s He determined that a gas was being released from the reaction of the metal with the acid He found that this acid was lighter than air and that it was flammable He called the gas phlogisten We know that this gas is hydrogen but he discovered that when he burned this gas with Priestley’s oxygen they formed water

Henry Cavendish – Late 1700’s Infact, that is where hydrogen gets its name, hydro = water gen = birth This last point had implications for the second of the four fundamental elements, water Until that point in time, everyone believed that water was an element (could not be made from other more simple compounds)

John Dalton - 1809 In 1809 he pictured the atom as a tiny solid indestructible sphere The word atom itself comes from a Greek word meaning singular or indivisible This brought the world back into scope of Democritus’s theory that was made nearly 2500 years earlier He attempted to describe matter in chemical reactions with this model

John Dalton - 1809 He concluded the following All Matter is made of atoms, which are particles too small to see Each element has its own kind of atom, with its own mass Compounds are created when atoms of different elements link to form molecules Atoms cannot be created or destroyed or subdivided in chemical changes

Michael Faraday – 1800’s Dalton’s atomic model was excellent for that day and age however it could not explain how you could get a spark when you touch a metal doorknob Faraday postulated that atoms could gain and lose charges and so came into understanding the idea of ions He postulated the following Matter must contain negative and positive charges Opposite charges attract and like charges repel Atoms combine to form molecules because of electrical attractions between atoms

John Dalton’s Problem Dalton’s model was lacking because it could predict the formation of very simple compounds like CO2, and Li2O More complex compounds like, NH3 and CH4, started causing problems The problem was that it could not predict the formation of compounds with these particular molar ratios C:H = 1:4 N:H = 1:3

J.J. Thomson - 1904 In 1897 Dalton’s atomic theory was shattererd by a chemist named J.J. Thomson He had discovered the existance of negatively charged particles with a mass of less than 1/1000 that of the nucleus of a hydrogen atom This particle was called the electron (actual mass is 1/1836 of hydrogen atom)

J.J. Thomson - 1904 He discovered this by doing a discharge tube experiment The tube was evacuated and then a specific gas was placed there with extremely low pressure The high voltage passed through the tube caused the ionization of the gas The particles, first called cathode rays, were later renamed electrons

J.J. Thomson - 1904 J.J. Thomson came up with the plum pudding model which had a solid positive core with tiny electrons embedded into it The electrons, if given enough energy, could leap out of the atoms (ionization energy) This was what the CRT experiment found

Ernest Rutherford In 1909, Ernest Rutherford created an experiment to test J.J. Thomson’s model He placed a radioactive material sample in a box and surrounded it around the room with a film that was sensitive to the α-particles being emited by that radioactive material α-particles are just helium nuclei (2p, 2n)

Gold Foil Experiment

Gold Foil Experiment (cont’d) The gold foil experiment proved three things about the atom The majority of the atom is open space There is a very positive charge located at the center of every atom The majority of the atoms mass is located in the very dense nucleus of the atom

Ernest Ruthrerford’s Atomic Theory Using the information that he had gotten from the gold foil experiment, along with the information that had been present from Dalton’s and Thomson’s models, Rutherford came up with the following This is known as the planetary model of the atom

Neils Bohr’s Experiment One of Rutherford’s students, Neils Bohr, furthered his atomic model by studying the emission spectrum of atoms He found that when a hydrogen atom had its electrons jump up into an excided state (by adding electricity) the release certain lines on a emission spectrum The important thing is that they only released specific amounts of light

Neils Bohr’s Experiment (cont’d) He found that when the electrons jump they jump up to only specific levels and release a specific amount of energy (which appears in the spectrum) called quanta (little packets of light) He added to Rutherford’s experiment that not only do electrons orbit the nucleus, but they orbit on specific levels or quanta

Emission Sepctra Ephoton=hν There is a formula which is used to determine the amount of energy difference between the two states (ground vs. excited state) Ephoton=hν E – is the energy of the photon ν – is the frequency of the photon h – is Planck’s Constant (6.626068 × 10-34 m2 kg / s) This concludes that photons that leave an atom only have certain energies (energy levels)