Atomic Structure by yurtgc548


									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
• Much of an atom is
  empty space
              The protons
• Each proton has a +1charge.
• Each proton has a mass of 1 atomic mass
• 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
• 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

• 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%
• 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
• To work out the relative atomic mass of an
• 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.

                                  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
• 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
The successive ionisation energy
          graph of Be
• The diagram indicates     Log IE/kJ mol-1
  two electronic energy                   x
• 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
• 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-
• 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
• 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
       x                  x
                    x x           • The first shell can
                                    have one sub-shell, s
   x                                subshell.
                                  • 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
• 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
• A p sub-shell can have a maximum of six
• A d sub-shell can have a maximum of ten
          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
 Shape of p-orbitals - dumb-bell
• The three p orbitals,   py
  px,, py and pz.              P=x

       Electron configuration of
•   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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