IGCSE›Cambridge IGCSE Chemistry›Mind mapsAtomic Structure and the Periodic TableCambridge IGCSE Chemistry: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsInside an atomNucleus holds protons and neutronsElectrons orbit in shellsNucleus is tiny but holds most of the massParticlesProton: charge +1, mass 1Neutron: charge 0, mass 1Electron: charge −1, mass 11836\frac{1}{1836}18361Neutral atom: protons = electronsKey numbersProton number Z defines the elementMass number A = protons + neutronsA=Z+NA = Z + NA=Z+NSodium-23: 11 protons, 12 neutronsAtomic structureand the periodic tablep⁺n⁰e⁻ConfigurationShells fill 2, 8, 8 up to Z = 20Na is 2,8,1; Cl is 2,8,7Cations lose electrons from the outer shellAnions gain electrons: ClX−\ce{Cl-}ClX− is 2,8,8Table positionPeriod number = occupied shellsGroup number = outer electronsNoble gases: full outer shell, unreactiveHelium has a full shell of 2Exam tipsIgnore electron mass in atomic massConfiguration total must equal electron count