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, Atomic structure and isotopes
Mass number- number of protons+neutrons
Atomic number- number of protons and electrons
Isotope- atoms of the same element with different numbers of neutrons and
mass number. Isotopes can’t be identified by experiment as the arrangements &
number of electrons are the same & these are what cause the reaction.
Relative isotopic mass- is the mass of an atom of an isotope compared with
1/12th of the mass of an atom of carbon-12. (same as mass number)
All the atoms in a single isotope are identical and so they all have the same
mass measured against carbon-12.
Relative atomic mass, Ar – the weighted mean mass of an atom of an element
compared with 1/12th of the mass of an atom of carbon-12.
Most elements contain as mixture of isotopes, each in a different amount &
with a different mass. The term ‘weighted mean mass’ is used to account for the
contribution made by each isotope to the overall mass of the element.
Ar= the sum of(relative isotopic mass x abundance) for each isotope
100
Relative formula mass/molecular mass, Mr- the weighted mean mass of the
formula unit of a compound compared with 1/12th of the mass of an atom of
carbon-12.
, Determining masses using mass spectrometry
A mass spectrometer is a piece of apparatus used to:
Identify an unknown compound
Find the relative abundance of each isotope of an element
Determine structural information about molecules
1. A sample is placed in a mass spectrometer
2. The sample is vaporised & then ionised to form positive
ions
3. The ions are accelerated through the apparatus and
separated according to their mass and charge
4. A computer produces a mass spectrum
On a mass spectrum each peak is a different isotope. The x
axis shows the mass to charge ratio (m/z). As the charge on
an ion is usually 1 this is the isotopic mass of each isotope.
The y axis shows the percentage abundance of each isotope.
, Ionic formulae
Although an ionic compound is made up of oppositely
charged ions, its overall charge is 0.
Writing ionic equations
1. Write equations in full using state symbols
2. Write equations as ions (only if ionic and dissolved
in water (aq)- use state symbols)
3. Cancel spectator ions
4. Write the final ionic equations
e.g. write the ionic equation for the reaction of silver
nitrate and potassium chloride
1. AgNO₃(aq) + KCl(aq)KNO₃(aq)+AgCl(s)
2. Ag+(aq)+NO₃-(aq) + K+(aq) + Cl-(aq)K+(aq)+NO₃-(aq)+AgCl(s)
3. Ag+(aq)+NO₃-(aq) + K+(aq) + Cl-(aq)K+(aq)+NO₃-(aq)+AgCl(s)
4. Ag+(aq) + Cl-(aq)AgCl(s)
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