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THE STRUCTURE OF THE ATOM
a) Protons, neutrons and electrons
Atoms are made up of three fundamental particles: protons, neutrons and electrons.
Protons and neutrons are found in the nucleus and are collectively called nucleons.
Electrons orbit the nucleus in a similar way to that in which planets orbit a sun. In between
the electrons and nucleus there is nothing (empty space).…read more

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Not all atoms of the same element have equal numbers of neutrons this may vary slightly.
The sum of the number of protons and neutrons in the nucleus of an atom is called its mass
number. It is represented by the symbol A.
The mass number is the sum of the number of protons and neutrons in the nucleus of an
atom
The nucleus of an atom can thus be completely described by its mass number and its atomic
number.…read more

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Relative atomic mass
The mass of an atom is measured in atomic mass units, where one unit is 12th of the mass of
one atom of carbon12.
The relative isotopic mass of an isotope is the ratio of the mass of one atom of that
isotope to 1/12th of the mass of one atom of carbon12.
It is usually very close to a whole number ratio:
Isotope Mass number Relative isotopic mass
1
1 H 1 1.006
4
2 He 4 4.…read more

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Eg The relative molecular mass of CO2 is 12.0 + 16.0 + 16.0 = 44.…read more

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MASS SPECTROMETRY
The mass spectrometer is an instrument used for measuring the masses of atoms and
molecules. It can also be used to measure the relative abundance of different isotopes and to
predict the structure of more complex molecules.
1.…read more

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The degree of deflection depends on the mass and the charge the greater the mass, the less
the deflection, and the greater the charge, the greater the deflection. It can be shown that the
deflection is inversely proportional to the m/e ratio.
In most cases, however, the charge is +1, so the deflection depends essentially on the
relative mass of the species in the mass spectrometer. If the spectrometer is calibrated, the
masses of all the species can be directly measured.…read more

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Calculating relative atomic masses
The relative atomic mass can be calculated by the formula:
(perentage abundance of each isotope x mass of each isotope)
100
eg Using the mass spectrum of neon above:
ram = (90 x 20 + 10 x 22)/100 = 20.2
All relative atomic masses have been found in this way.
3. Deducing relative molecular masses
It is also possible to put molecules into the mass spectrometer.…read more

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ELECTRONIC STRUCTURE
i) Energy levels
Electrons do not orbit the nucleus randomly they occupy certain fixed energy levels. Each
atom has its own unique set of energy levels, which are difficult to calculate but which
depend on the number of protons and electrons in the atom.
Energy levels in an atom can be numbered 1,2,3,.... To infinity.…read more

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An sorbital in the second energy level is a 2s orbital, etc
porbitals: these are shaped like a 3D figure of eight. They exist in groups of three:
Every energy level except the first level contains three porbitals. Each porbital in the same
energy level has the same energy but different orientations: x, y and z.…read more

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