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The History of the AtomTimeline: 400 BCScientist: Democritus (Greek Philosopher)Democritus was a Greek philosopher who wasthe first person to use the term atom (atomos:meaning indivisible).He thought that if you take a piece of matterand divide it and continue to divide it you willeventually come to a point where you could notdivide it any more. This fundamental or basic unit was what Democritus called anatom.He called this the theory of the universe:All matter consists of atoms, which are bits of matter too small to beseen.There is an empty space between atomsAtoms are completely solidAtoms have no internal structureEach atom (of a different substance) is different in size, weight andshape.Timeline: 1800’sScientist: John DaltonJohn Dalton was the first to adapt Democritus’ theory into the first modern atomicmodel.JOHN DALTON’S ATOMIC MODEL:1. All matter consists of tiny particles calledatoms2. Atoms are indestructible and unchangeable3. Elements are characterized by the weight of their atoms4. When elements react, it is their atoms that have combined to form newcompounds1

Timeline: 1890’sScientist: J.J ThomsonJ.J Thomson was a physicist who is credited for discovering theelectron. He used his research on cathode ray tube technology in thisdiscovery.CATHODE RAY TUBEHow does this Tube Work? -Modern Day Tube1. Nearly Empty tube (Air has been sucked out)2. An electric charge is passed through the tube. Travels from cathode toanode3. The charge is invisible, so to see where it traveled a fluorescent screen isplaced at back of tube. Where the beam hits, adot will appear on the screen. You could also use afluorescent gas and the whole tube will light up.4. This beam will always travel straight if notinterfered with.5. The deflection coils each have a specific charge.One is positive and the other is negative.6. Thomson showed (as in the diagram above) thatthe charge would deflect away from the negativecoil. He then stated that this charge was thus anegative charge.2

J.J Thomson was an excellent physicist and thus did not stop when he had found this negativecharge. Through a series of clever experiments he was able to predict the mass of this charge.SCIENTIFIC INTERPRETATION AT ITS BESTHe then found out that this charge was 1000 times lighter that a hydrogen atom. He made a boldstatement saying that this negative charge must be inside an atom. This negative charge (he calledcorpuscles) later became known as the electron.THOMSON’S ATOMIC MODELUsing what he had discovered, Thomson predicted what an atom should looklike. These are the key points to Thomson’s Atomic Model:1. Because of its design this model is known as the plumpudding model2. Each atom is a sphere filled with positively charged‘fluid’. This resembles the sticky jam part of a pudding.3. Corpuscles (later called electrons), are the negatively charged particlessuspended in this ‘fluid’. This resembles the plums in the pudding.4. He did not predict the movement of these electronsTimeline: 1910’sScientist: Ernest RutherfordErnest Rutherford was not convinced about the model of the atomproposed by Thomson. He thus set up his now famous Gold FoilExperiment.1. He fired alpha particles (positively charged) at a goldfoil.2. He measured the deflection as the particles came outthe other side.3. Most of the particles did not deflect at all.Every now and then a particle would deflectall the way back.4. He said that there must be a positive centre ofthe foil. He called this centre the nucleus.3

RUTHERFORD’S ATOMIC MODEL (AKATHE PLANETARY MODEL)1. The nucleus of the atom is a dense mass of positivelycharged particles.2. The electrons orbit the nucleus3. A problem raised was: Why are the negativelycharged particles not attracted by the positively chargednucleus4. Rutherford stated that the atom was like a mini solar system and that theelectrons orbited the nucleus in a wide orbit. That is why it is known asthe planetary model.Timeline: 1910’sScientist: Niels BohrNiels Bohr agreed with the planetary model ofthe atom, but also knew that it had a fewflaws. Using his knowledge of energy andquantum physics he was able to perfectRutherford’s model. He was able to answerwhy the electrons did not collapse into thenucleus.BOHR’S ATOMIC MODEL (AKATHE RUTHERFORD-BOHR MODEL)1.Electrons orbit the nucleus in orbits that have a setsize and energy.2. The lower the energy of the electron, the lower theorbit.3. This means that as electrons fill up the orbitals, theywill fill the lower energy level first.4. If that energy level is fill (or at capacity), a newenergy level will begin.5. Radiation is when an electron moves from one level to another.Problems with this theory:Electrons do not travel on a specific orbit or path.4

Timeline: 1920’sScientist: Erwin SchrödingerErwin Schrödinger was a revolutionary physicist who used Heisenberg’suncertainty principle to come up with the atomic model that we still use today.SCHRÖDINGER’S ATOMIC MODEL (AKA THE CLOUD MODEL)1. An electron does not travel in an exact orbit2. We can predict where it will probably be3. We cannot say for certain where it is, but only where it ought to be.4. The type of probability orbit is dependent on the energy level described by BohrSUMMARY OF ATOMThe smallest part of an element is called an atomEach atom (of an element) is different in structure fromother atoms (of other elements)An atom can be divided in smaller subatomic particles:Protons, Electrons and NeutronsThe nucleus is the centre of an atom. It contains protons and neutrons.Electrons orbit the nucleusAs we go up the periodic table, an electron andEnergy Levelsproton is added.Electrons occupy a certain energy level (of acertain size)Once the energy level is full, a new level beginsWithin each of these levels are specials typesof orbitals. These depend on the energy levelEach orbital can contain two electronsWithin these energy shellsare different shapes oforbitals5