Isotopes 

Atoms of an element may have different masses. An isotope is all the atoms of an element having the same mass.

All atoms have masses that are close to whole numbers. The whole number closest to the mass of the atoms of an isotope is the mass number of that isotope.

 At. #

  Element

 Mass numbers of stable isotopes

   1 Hydrogen 1, 2
   2 Helium 3, 4
   3 Lithium 6, 7
   4 Beryllium 9
   5 Boron 10, 11
   6 Carbon 12, 13
   7 Nitrogen 14, 15
   8 Oxygen 16, 17, 18
   9 Fluorine 19
  10 Neon 20, 21, 22
  11 Sodium 23
  12 Magnesium 24, 25, 26