J1 THE STRUCTURE OF THE ATOMAtomsAll matter is made from atoms. It would take more than aAn atom has the same number of electrons as protons, so theMillion million atoms to cover this full stop.Amounts of negative and positive charge balanceAn atom has a tiny central nucleus made of protons andunlike charges (-and 4) attract each other This electric forceNeutrons (apart from the simplest atom, hydrogen whoseholds electronics in orbit around the nucleusNucleus is a single proton). Orbiting the nucleus are muchlike charges (-and also 4 and 4)repel eachLighter particles called electronshow ever the particles in the nucleus are held together by aLithhim stonstrong nuclear force. Which is strong enough to overcome theNeutron has no Charge repulsion between the portions.

NegativeAtoms can stick together in solids for example. The forces Electricthat bind them are attraction between opposite charges.Charge Moving electrons in metals some of the electrons are onlyNucleusloosely held to their atoms these free electrons can driftbetween the atoms. The electric current in a wire is a flowOf free electronsProton has aIf an atom gains or losses electrons it is left with an overallpositive electric -or + charge charged atoms are called ions.

Early models of the atomIn 1810 the mode of the atom was john Daltons Billiard ballHe measured a value for e/m. This led to the Plum PuddingModel. According to this model the atom was the smallestModel of the atom. According to this model. The atom was aPiece of an element that was possible it was imagined to be apositive material or pudding in which the electrons wereSmall solid spherenegative particles, or plums. Thomson imagined themCirculating inside the positive material. The bullet pointsAbove were still part of the new model.In this model atoms Of n element have identical masses Of different elements have different masses Combine only in small, whole number ratios (for example1:1,1:2,2:3) Cannot be created of destroyedIn 1987 Joseph John Thomson used electric and magneticFields to deflect electron beams (see H11) and showed that theElectron was a negative particle.

Evidence for nucleus This experiment was carried out by Hans Geiger and ErnestMarsden under the supervision of Ernest Rutherford.

A thin piece of gold foil was bombarded with alpha particles Nuclear model to take account of them According to thisWhich are positively charged (see 13) Most of the alpha model virtually all the mass of the atom is concentrated at Particles passed straight through the gold atoms. But a fewthe centre in a positively charged nucleus with much lighterWere repelled so strongly that they bounced back or werenegatively charged electrons in orbit around it. RutherfordDeflected through large angles, These results came as a bigcalculated the scattering this would cause and showed that theSurprise and in 1911 Rutherford proposed a new model, themodel was consistent with the experimental results. How Science WorksRecent models of the atomElements, nuclides and isotopesIn 1913 the Rutherford.-Bohr model took account of theEverything is made from about 100 substances calledWork of Henry Moseley who showed that the atomic number elements For most elements a sample contents a number ofCorresponded to the number of positive charges (protons) in different versions called isotopps . These have the sameThe nucleus and the work of Niels Bohr. Who suggested thatnumber of protions (and electrons). But different numbers ofThe elect ons have certain allowed circular orbits or sheltsneutrons This table shows some examples (italic numbers areSince then changes to the model have includedfor rarer isotopes).ElementElectronsProtonsNeutrons

HydrogenHelium Lithium Carbon uranium 123692 1236920 or1 or 2 1 or 23 or 46 or 7or 8142 or 143or 146

The wave mechanical model in which the electrons set Up stationary waves around the nucles (see12) The idea of neutrons in the nucleus, following theDiscovery of the neutron by james Chadwick in 1932 The idea that neutrons and protons are made of quarks The existence of other fundamental particles (seek2)

Our understanding of the atom is an example of the tentativeNature of science knowledge. The model we use is E= electron (-)P= proton (+)N= neutron

Continually being refined as we discover more about theAtom and its properties we can expect that this process mayContinue in the future

Atomic measurements Mass of proton 1.673 10 -27 kg Mass of neutron 1.675 10 -27kgMass of electron 9.110 10-31 kgCharge on proton +1.60 10-19CCharge of electron -1.60 10 -19 CDiameter of an atom -10 -10 mDiameter of a nucleus -10 -14 m

Nuclide; This is any particular version of anatom, above are Simple models of the two naturally occurring nuclides of Lithium along with the symbols used to represent them.

Note The proton and neutron have approximately the sameMass- about 1800 times that of the election. - means of the order of i.e within a factor ten of The diameter of an atom is -104 times that of its nucleus(Anton size varies from element to element) Confusingly the symbol e may be used to represented the Charge on an electron (-) or a proton (+) in this unit the Charge on an electron will called -e