Atoms and nuclei neutron proton a=Z+N Ato Mass CR 4 6 electron atom size≈10 Atomic‖ Neutron nucleus NumberNumber sie≈5x0-15m nucleus ['nju: klias Atomic Number:原子序数 Neutron number:中子数 Nuclei. pl Mass Number:质量数Atoms and Nuclei Atomic Number:原子序数 Neutron Number:中子数 Nuclei, pl. Mass Number:质量数