GCSE›Edexcel GCSE Chemistry›Mind mapsAtomic StructureEdexcel GCSE Chemistry: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsChanging modelsDalton: atoms are solid, indivisible spheresThomson: plum pudding with embedded electronsRutherford: tiny dense nucleus, mostly empty spaceBohr: electrons in fixed shellsInside an atomNucleus holds protons and neutronsNearly all the mass is in the nucleusNucleus is tiny compared with the atomElectrons sit in shells around the nucleusParticle factsProton: charge +1, mass 1Neutron: charge 0, mass 1Electron: charge -1, mass 1/1836Atoms are neutral: protons = electronsAtomic structureinside the atomp+ne-NumbersAtomic number = number of protonsMass number = protons + neutronsNeutrons = mass number − atomic numberSodium (11, 23) has 12 neutronsIsotopesSame protons, different neutronsDifferent mass numbers, same elementAr is a weighted average of isotopesAr=∑(mass×%)100A_r = \frac{\sum (\text{mass} \times \%)}{100}Ar=100∑(mass×%)Cl: 75% Cl-35, 25% Cl-37 gives 35.5Exam tipsName Thomson, Rutherford, Bohr in orderCount electrons when explaining neutrality