Chemistry - quantifying atoms and compounds

Relativity/scales - all things are relative

• advantage of relative scale → very large/small numbers can be compared easily

→ to generate the relative scale of atomic masses → carbon-12 is used → most abundant isotope of carbon

Relative isotopic mass

• the mass of an atom of an isotope relative to the mass of an atom of the carbon-12 isotope, which has a mass of exactly 12 unified atomic mass units (U)

→ unified atomic mass units (U) = standard unit of mass based on mass of C-12 isotope, where mass of an atom of C-12 - 12 U → approx = to one nucleon ← proton or neutron

• realative isotopic mass = mass of an atom of the isotope/ mass of an atom carbon-12 X12

Relative isotopic abundances

• many elements → have multiple isotopes

• → relative isotopic abundances = the amount of each isotope on earth

Relative atomic mass

• the average mass of atomic isotopes, taking into account their natural abundances

→ Ar of an element = RIM1 X % abundance + RIM2 X % abundance +… / 100

Mass spectrometry

• an instrument used to measure the relative masses of atoms with high precision

IT HELPS US OBTAIN ACCURATE VALUES FOR RIM’S :

• the no. Of isotopes in the element that was analysed

• Accurate values for each isotope relative irsotopic mass

• The relative proportion (% abundance ) of each isotope in the sample

HOW THE INSTRAMENT WORKS

→ vaporisation of sample - the sample is converted into gas

→ ionisation of sample - atoms are given a positive charge

→ acceleration of particles - the ions are accelerated by an electric field

→ deflection of particles - the ions ar passed through a magentic field which deflects them. The amount of deflection depends on the mass-to charge ration of the ion. Lighter ions or ions with a higher charge are elected more

> detection of particle → the deflected ions strike a detector, allowing their relative abundance to be measured