2 Early Atomic Theory Democritus 440BC The smallest indivisible particle of matter is the atom.Aristotle 340BCcriticised an alternative model based on the four elements – fire, water, air and earth.This model fitted well with religious beliefs and held for 1600 years.Alchemy appeared 400 years after Aristotle.
3 End of the Dark Age Galileo (1564 – 1642) Returned to the Democritus model with a world made of void and atoms.Lavoisier (1743 – 1794)Law of Conservation of Mass during a chemical reaction.“the sum of the mass of the reactants equals the sum of the masses of the products.”
4 Dalton – a revolutionary theory In 1803 proposed that:Elements are made up of tiny particles called atoms.Elements contain only one type of atom.Atoms cannot be created or destroyed – a chemical reaction being there-arrangement of elements.Compounds contain more than one type of atom
5 J.J. Thomson ( )1897 measured charge to mass ratio of electrons e/m.Determined that cathode rays were electrons.
6 - Thomson’s plum-pudding model (1897) Rutherford’s model (1909) ++-Thomson’s plum-puddingmodel (1897)Rutherford’s model(1909)Bohr’s model(1913)Charge-cloud model(present)1803 John Daltonpictures atoms astiny, indestructibleparticles, with nointernal structure.1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding"model. He pictures electronsembedded in a sphere ofpositive electric charge.1911 New ZealanderErnest Rutherford statesthat an atom has a dense,positively charged nucleus.Electrons move randomly inthe space around the nucleus.1926 Erwin Schrodingerdevelops mathematicalequations to describe themotion of electrons inatoms. His work leads tothe electron cloud model.1913 In Niels Bohr'smodel, the electrons movein spherical orbits at fixeddistances from the nucleus.1924 Frenchman Louisde Broglie proposes thatmoving particles like electronshave some properties of waves.Within a few years evidence iscollected to support his idea.1932 JamesChadwick, a Britishphysicist, confirms theexistence of neutrons,which have no charge.Atomic nuclei containneutrons and positivelycharged protons.1904 Hantaro Nagaoka, aJapanese physicist, suggeststhat an atom has a centralnucleus. Electrons move inorbits like the rings around Saturn.
7 Thomson Model Plum-pudding model In the nineteenth century, Thomson described the atom as a ball of positive charge containing a number of electrons uniformly distributed.
8 meanwhile …….Spectra1886 Balmer - when an element is ionized in a discharge tube the light emitted produces a line spectrum in a spectroscope. picRadioactivityBequerel and the Curies (1898) identified elements that produced radioactive rays and particles.
9 Rutherford Scattering Experiment Tried to verify Thomson’s model of atom.Aim: to prove that atoms in the gold foil were spheres of positive fluid with negative charges embedded.Prediction: less than 1% of alpha particles would be deflected more than 3º.
10 The ExperimentTo test this he designed and experiment directing ‘alpha’ particles toward a thin metal foil.The screen was coated with a substance that produced flashes when it was hit by an alpha particle.
11 Experimental Observations most alpha particles passed through the gold foil undeflected.large numbers of α were deflected at angles >3º1/10,000 α were backscattered through 180ºpic
12 Rutherford’s proposed model most of the atom is spacea very small dense and positive nucleus in the centre of the atom.electrons were around nucleus but at a distance.
13 Problems of the model line spectra could not be explained electrons could not be at rest – they would be attracted to the nucleus and be neutralized.if electrons circled the nucleus, they would be undergoing centripetal acceleration – energy?accelerated charges emit electromagnetic radiation, so electrons should lose energy continuously and spiral into the nucleus.stability of atoms could not be explained
14 The AtomNucleusPositive protons and neutral neutrons each of atomic mass unit 1. Together they are the nucleons.held together by strong nuclear forces balanced by strong electric forcesAs the number of protons increases the nuclear forces decreases and the nucleus becomes more unstable
15 shorthandAtomic number Z = number of protons Mass number A = mass of nucleons
16 Radioactive Isotopes and radioactivity Radioisotopes - isotopes that have unstable nuclei and will spontaneously disintegrate and emit radiation.Becquerel 1896 mineral pitchblendeCurie 1898 radium and poloniumThree types or radiation:Alpha particlesBeta particles3. Gamma radiation
17 IsotopesIsotopes are atoms of the same element with different numbers of neutronsC C13 C14Atoms which give out radioactivity are called radioactive isotopes.The nucleus of a radioactive isotope is unstable.
19 Alpha particles Nuclei of helium atoms He =2p+2n Heavy particle – low speeds 2x10 7 ms-1Strong ionizers – attracting e from other moleculesLow penetration in airDeflected magnetic by a and electric fieldEmitted by elements>\
21 Half life of IsotopesThe rate at which a nucleus decays is characteristic of that nucleusRate is measure by half life t1/2 and is an exponential decay functionThe time it takes for half the given mass of an element to decay into a new element.It is constant but random processeg. at zero time if there is 10g of a radioactive element
22 eg. at zero time if there is 10g of a radioactive element with a half life of 6hr then: after 6 hrs there will be 5g leftgg