2.2 Flashcards
Mass defect description
Small amount mass lost, as strong nuclear force, hold protons and neutrons together, comes at expense of fraction of mass
Standard isotope:
Carbon-12
Working in kg too difficult so…
New unit, ‘atomic mass unit’ (u) used
- mass of carbon-12 exactly 12u
- standard mass for atomic mass 1u (mass of 1/12 atom C-12)
- 1u approx mass of 1 proton or neutron
Relative isotopic mass is…
Mass of isotope relative to 1/12 mass of carbon-12
Does relative isotopic mass have units
No, it’s a ratio of 2 masses
What can we assume about relative isotopic mass?
Most cases, same as Ar of isotope
Relative atomic mass is…
Weighted mean mass of an element compared to 1/12 mass of carbon-12 atom
Weighted mean mass takes into account…
%abundance of each isotope
Relative isotopic mass of each isotope
In periodic table, elements are show with relative atomic mass…
Ar
% abundance of isotopes in sample found how?
experimentally with mass spectrometer
How mass spectrometer work?
Sample placed inside
Sample vaporised, then ionised to form positive ions
Ions accelerated; heavier ions slower, more difficult to deflect than lighter, so different isotope ions separated
Ions detected on mass-to-charge ratio m/z; each ion reaching detector adds to signal, so higher abundance, larger signal
Mass to charge ratio =
Relative mass of ion/relative charge on ion
For ion with 1 positive charge this ratio is equal to relative isotopic mass, which is recorded on x-axis of spectrum
How to work out relative atomic mass:
chlorine-35: 75.87% abundance
chlorine-37: 24.22% abundance
Rel.at.mass = ((75.78x35)+(24.22x37))/100
Determination of rel.isotopic masses with mass spectrometer
Mass spectrometer can record mass to charge (m/z) ratio for each isotope, so accurate values of relative isotopic mass can be measured
