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...Igcse chemistry study notes unit atoms elements compounds atomic structure and the periodic table state relative charges core c approximate masses of protons neutrons electrons define proton number as in nucleus an atom nucleon mass total use simple to explain basis see section with special reference isotopes same element which have but a different two types being radioactive non understand that supplement s properties because they their outer shell educatalyst one medical industrial describe build up shells significance noble gas electronic structures ideas distribution p orbitals d block are not required charge electron e negligible neutron n note for all practical calculations is assumed be zero z positive negative therefore electrically neutral no net modern arranged order increasing...