2.2 Relative Mass
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The strong nuclear force holding together protons and neutrons comes at the expense of the loss of…
…a fraction of their mass (the mass defect) This was found out by Albert Einstein
What is the standard isotope to base all atomic masses on
Carbon-12 isotope One atom has a mass of approx. 1.99*10^-26kg
What is the unit of atomic mass
u
What is the mass of a carbon-12 isotope
12u Ie. Exactly 12 atomic mass units
1u is approximately the mass of…
…a proton or a neutron
What is relative isotopic mass
The mass of an isotope relative to 1/12th of the mass of an atom of carbon-12
What is relative atomic mass
The weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon- 12
What is the symbol for relative atomic mass
Ar
The weighted mean mass takes account of…
…the percentage abundance of each isotope and the relative isotopic mass of each isotope
How can we determine the relative atomic mass
Use a mass spectrometer
How do mass spectrometers work
1) A sample is placed in the mass spectrometer 2) The sample is vaporised and then ionised to form positive ions 3) The ions are accelerated. Heavier ions move more slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated. 4) The ions are detected on a mass spectrum as a mass-to-charge ratio m/z. Each ion reaching the detector adds to the signal so the greater the abundance, the larger the signal
How to find the mass to charge ratio m/z
Relative mass of ion/relative charge on ion
For an ion with one positive charge, the m/z ratio is equivalent to…
…the relative isotopic mass which is recorded on the x ais of the spectrum
How to find the relative atomic mass from the mass numbers of each isotope
Relative atomic mass=(%isotope number)+(%isotope number) / 100
How to determine the relative isotopic mass
Using a mass spectrometer to record the accurate m/z ratio