Size and Mass Of Atoms

• Atoms are very small, having a radius of about 0.1nm
• The radius of a nucleus is less than 1/10 000 than that of the atom
• Almost all of the mass of an atom is in the nucleus
• Relative masses

  • proton = 1
  • neutron = 1
  • electron = very small
• 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, which are called isotopes of that element
• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of that element
• Relative atomic mass is calculated by adding up the masses of all the isotopes of an element and then averaging them based on their relative abundance in nature