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Atomic Structure

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					Atomic Structure
      Simple model of an atom
• An atom is made of a
  tiny nucleus with
  electrons orbiting
  around it.
• The nucleus is made
  up of protons and
  neutrons.
• Much of an atom is
  empty space
              The protons
• Each proton has a +1charge.
• Each proton has a mass of 1 atomic mass
  unit.
• The number of protons in an atom is called
  its atomic number(z).
             The neutrons
• The neutrons have no charge.
• Each neutron has a mass of 1 atomic mass
  unit.
• The total number of protons and neutrons in
  an atom is called its mass number (A).
             The Electrons
• Each electron has a charge of -1.
• Electrons have negligible mass of 1/1840
  that of a proton.
Convention for writing an atom


                A
                    zX
                Isotopes
• Isotopes are atoms of the same element
  which have the same number of protons but
  different number of neutrons.
       Relative Isotopic Mass
• Relative Isotopic Mass= Mass of one
  isotope of the element/(1/12) X Mass of one
  atom of 126C.
• An atom of 12C has a relative atomic mass
  of 12 exactly.
    Relative Atomic Mass(Ar)
• Ar = Weighted average of the isotopic
  masses/(1/12) X mass of one 126C atom
  Calculating the relative atomic
         mass of chlorine
• Chlorine has two isotopes 35Cl and 37Cl in
  relative proportions of 75% and 25%
  respectively.
• The weighted average mass of a chlorine
  atom is 35X(75/100) + 37 X (25/100) =
  26.25+9.25=35.50(no unit)
  Relative Molecular Mass(Mr)
• Mr = Mass of one molecule/(1/12)X Mass
  of one 126C atom.
• The relative molecular mass can be worked
  out by adding the relative atomic masses of
  all the atoms present in one molecule.
          Mass spectrometry
• A mass spectrometer separates the isotopes
  of an element according to their masses and
  shows the relative numbers of the different
  isotopes present.
• Before the isotopes can be separated, they
  must be converted to positive ions.
               The workings
•   Evacuation of the instrument
•   Vaporisation of liquid or solid samples
•   Production of positive ions
•   Acceleration of positive ion
•   Deflection of positive ions
•   Detection of positive ions according to
    mass(m)/charge(e). When charge =+1,
    mass/charge = mass
 The uses of a mass spectrometer
• To find the isotopic composition of an
  element.
• To work out the relative atomic mass of an
  element.
• To find the relative molecular mass and the
  fragmentation pattern of a molecule.
• In forensic science.
       The mass spectrum of Cl
• Chlorine has two          % Abundance
  isotopes: 35Cl and 37Cl
  with relative                  75
  proportions of 75%
  and 25% respectively.
                                      25




                                  35 37    m/ e
       First ionisation energy
• It is the energy required to remove one
  electron from each of one mole of gaseous
  atoms to form one mole of gaseous ions
  with single positive charge and one mole of
  electrons.
• The equation for first ionisation energy of
  element A is: A(g)      A+(g) + e
  Successive ionisation energies
• Successive ionisation energies provide
  evidence for the existence of quantum shells
  or electronic energy levels.
  Successive ionisation energies
• If an atom has two electrons, it will have
  two ionisation energies, first ionisation
  energy and second ionisation energy.
• If an atom has three electrons, it will have
  three separate ionisation energies.
• All these ionisation energies for each
  element are its successive ionisation
  energies.
The successive ionisation energy
          graph of Be
• The diagram indicates     Log IE/kJ mol-1
                                                 x
  two electronic energy                   x
  levels.
• Electrons 1 and 2 are
  at a higher energy                  x
  level                           x
• Electrons 3 and 4 at a
  lower energy level -           1 2 3 4
  nearest to the nucleus.        Ionisation no
    Electron configuration- key
              points
• Each element has a characteristic emission
  spectrum which can be used to identify it.
• The electrons in an element can exist only
  at certain energy levels - shells and sub-
  shells
• The region in which an electron moves for
  most of the time is called an orbital.
• An orbital can hold two electrons.
          The Line Spectrum
• An electron can absorb sufficient energy
  and move to a higher energy level.
• When such an electron drops to a lower
  energy level, the energy absorbed is given
  out.
• The amount of energy given out appears as
  a line in the line spectrum of the element.
    First ionisation energies of
   successive elements - H toNe
1st IE/kJ mol-1                   • These provide
                                    evidence of shells and
                                    subshells.
       x                  x
                    x x           • The first shell can
                                    have one sub-shell, s
                x
   x                                subshell.
           x
            x
       x
                                  • The second shell can
                                    have two subshells, s
   1 2 3 4 5 6 7 8 9 10       z     and p
         The aufbau principle
• Electrons always occupy the lowest
  available energy sub-level or subshell.
• Electrons pair up after a sub-level is half
  filled.
• Numbers 1, 2, 3 denote the shells. Letters s,
  p, d, f denote the subshells. A superscript
  indicates the number electrons.
• Sequence of energy levels: 1s2s2p3s3p4s3d
        Subshells and orbitals
• An s sub-shell has only one orbital.
• A p sub-shell has three orbitals.
• A d subshell has five orbitals.
      Sub-shells and electrons
• An s sub-shell can have a maximum of two
  electrons.
• A p sub-shell can have a maximum of six
  electrons.
• A d sub-shell can have a maximum of ten
  electrons.
          Shape of an s orbital
• An s orbital is
  spherical in shape
• The sphere is made up
  of a cloud of negative
  charge from the
  electrons
 Shape of p-orbitals - dumb-bell
            shaped
• The three p orbitals,   py
  px,, py and pz.              P=x


                               pz
                               px
       Electron configuration of
               elements
•   H 1s1             He 1s2
•   Li 1s2 2s1        Be 1s22s2
•   B 1s22s22p1       C 1s22s22p2
•   N 1s22s22p3       O 1s22s22p4
•   F 1s22s22p5       Ne 1s22s22p6
•   Na 1s22s22p63s1   Mg 1s22s22p63s2

				
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posted:3/18/2013
language:English
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