1.4 - Isotopes and Relative Isotopic Masses

• isotopes


  atoms of the same element with the same number of protons but different number of neutrons in the nucleus
• relative atomic mass (RAM)

  average mass of an atom of an element, relative to 1/12th of the mass of an atom of carbon-12
• relative isotopic mass

  average mass of an atom of an isotope relative to 1/12th of the mass of an atom of carbon-12
• relative molecular mass

  average mass of a molecule relative to 1/12th of the mass of an atom of carbon-12

  the sum of relative atomic masses of all atoms in the molecular formula of an element or compound
• similarities in isotopes

  number of protons and electrons

  atomic number

  chemical properties
• differences in isotopes

  number of neutrons

  mass number

  physical properties
• relative atomic mass

  average mass of an atom of an element / (1/12 x mass of one atom of carbon-12)
• mass spectometer to study molecules

  bombarding electrons break molecules into fragments

  due to the high vacuum, we can study fragments and ions

  mass spectrum consists of a 'fragmentation pattern'

  peak of ion with highest mass = whole molecule ionised
• what can be deduced from data from mass spectometers

  isotopic composition of elements

  relative atomic masses of elements

  relative molecular masses of molecules
• difference between mass number and relative atomic mass

  mass number is an average and refers to the number of protons and neutrons where as RAM is more specific
• difference between relative formula mass and relative molecular mass

  relative formula mass is for ionic compounds and for other compounds with giant structures

  relative molecular mass is for an element or compound