.5 size and mass of atoms

• Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m)
• The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m)
• Almost all of the mass of an atom is in the nucleus
• the relative masses of protons, neutrons and electrons are:

  
• The sum of the protons and neutrons in an atom is its mass number.
• Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.