Atomic and mass numbers

• Atomic number


  The number of protons in an atom
• Mass number


  The total number of protons and neutrons in an atom
• Calculating the number of neutrons


  Subtract the atomic number from the mass number
• Isotope


  Atoms of the same element with different numbers of neutrons
• Ion


  An atom that has lost or gained one or more electrons, giving it an overall positive or negative charge
• When an atom loses one electron

  The ion has a +1 charge
• When an atom loses two electrons

  The ion has a +2 charge
• Atoms can also gain electrons to form negative ions
• Plenty of questions on atomic and mass numbers, isotopes and ions in the physics specification