CHEMICAL BONDING

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CHEMICAL BONDING

• Why bond?
• Atoms try to acquire particularly stable electron
  configuration (i.e the configuration maximizing
  binding energy) for their outer electrons ?
  form ions or molecules.
• Most stable configuration for outer electrons is
  noble gas configuration (octet structure)
  ns2np6
• strategies to achieve more stable configuration
• give away electrons
• accept electrons
• share electrons
• chemical bond is formed when it is energetically
  favorable, i.e., when the energy of the bonded
  atoms is less than the energies of the separated
  atoms.
• Quantities which play a role in chemical bonding
• ionization energy (the energy required to remove
  an electron from a neutral atom)
• electron affinity (the energy change when a
  neutral atom attracts an electron to become a
  negative ion)
• electronegativity (the ability of an atom in a
  molecule to draw bonding electrons to itself)
• types of bonds
• ionic bond
• covalent bond
• metallic bond

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IONIC BOND

• atoms achieve noble gas configuration by giving
  up or accepting electrons (usually electron
  transfer from metal to non-metal) ? formation of
  ion
• chemical bond is formed due to electrostatic
  attraction between two oppositely charged ions
• bond is strong, but becomes quickly weak when ion
  is displaced ? materials usually brittle (e.g.
  glass, rock, egg shells,..)
• compounds formed by ionic bond e.g. NaCl,
  CaCl2,..
• examples of ions with noble gas configuration

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METALLIC BOND

• metals have low ionization energy ? atoms give up
  outer (valence) electron, to be shared by all
  
• metallic lattice positive ions at fixed
  positions, in sea or gas of mobile electrons
  
• electron gas has pressure due to Pauli
  principle
• electrons mobile ? good thermal and electrical
  conductivity
• positive ions not free to move, but can vibrate
• electron not tied down in particular bond ? can
  absorb and re-emit light over wide frequency
  range ? good reflector
• bond is elastic since attraction due to mobile
  electrons bond holds even if ions displaced ?
  metals are malleable
• ability of the electrons to spread between the
  cations and neutralize their charges ? metal ions
  ions can pack closely together closeness of the
  packing of the atoms ? high densities of metals.
• In some sense, piece of metal is like an
  extremely large molecule

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Covalent Bond

• COVALENT BOND
• well-defined cluster of neighboring atoms share
  electrons - form molecule
• state with shared electrons has lower energy than
  individual atoms
• valence electrons outer electrons
• group number number of valence electrons
• valence of element number of electron pairs
  shared to complete octet of electrons
• examples H2 , O2 , N2
• in H2, H atoms share one electron pair
  
• n O2, O atoms share two electron pairs
  
• in N2, N atoms share three electron pairs
• single H, O, N (in statu nascendi) are much
  more reactive than pair
• Carbon
• Carbon's outer electron configuration is4s2 4p2
  ? it needs 4 electrons to complete octet ?
  shares 4 pairs of electrons
• e.g. methane (swamp gas)
• possibility of 4 covalent bonds ? large variety
  of possible compounds with C ? organic chemistry

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Polar bonds

• POLAR BONDS
• Electron pairs shared between two different atoms
  not necessarily shared equally - sharing ratio
  depends on electronegativity
• electronegativity ability of atom to attract
  an electron to itself
• electronegativity increases from left to right
  (i.e. grows with group number) in every row of
  the periodic table
• alkali metals (group 1) are least
  electronegative, halogens (group 7) are most
  electronegative
• non-polar bond bond in which electrons are
  shared equally
• polar covalent bond bond in which one of the
  atoms exerts greater attraction for the electrons
  than the other (has larger electronegativity)
• if difference in electronegativity is large
  enough, bond becomes ionic bond
• GEOMETRY OF BONDS
• electron-pair repulsion rule (EPRR)
• Electron pairs surrounding an atom (be they
  shared or un-shared with other atom) repel each
  other and are directed to be as far apart as
  possible. (example water molecule)

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INTER-MOLECULAR FORCES

• electric dipole forces polar molecules exert
  electric dipole forces on each other
• e.g. water molecule H() attracted to (-)
  partner of other molecule (oxygen of other water
  molecule, or solute constituent) ? hydrogen
  bond
• hydrogen bond is reason for water to be liquid at
  normal temperatures (note e.g. CO2 , CH4 are
  gases!)
• Van der Waals forces
• fluctuations, shifts of charges within covalent
  molecule ? temporary dipole moment ?
  electrostatic dipole forces
• charge unbalances small ? forces weak (Van der
  Waals binding energies are ? 10-2eV )
• most liquids held together by VdW forces,
• cohesion, surface tension

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Water

• water molecule
• oxygen in water has 4 electron pairs, 2 shared
  and 2 unshared
• EPRR e-pairs point to corners of tetrahedron,
  with O atom in center of tetrahedron,
• H atoms sit at two of the corners of the
  tetrahedron ?
  constituents of water form triangle angle
  between the two lines from O to H is 105o
  (109o predicted from this model)
• Mickey Mouse shape of water molecule
• O is more electronegative than H ? water is polar
  molecule, more negative near O atom, more
  positive near H atoms.
• polarity of water molecule ? good solvent

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• Water and Ice

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• Why ice is less dense than liquid water
• water is most dense as a liquid and least dense
  as a gas solid water (ice) is less dense than
  liquid water, but more dense than a gas
• water vapor (gaseous water) is made up of
  individual water molecules that are not hydrogen
  bonded to other water molecules
• cooling, pressure ? hydrogen bonds form ? liquid
  but in liquid, hydrogen bonds keep forming and
  breaking up on average every water molecule in
  3.4 H-bonds
• at 4oC, water molecules as tightly packed as
  they will go
• below 4oC , more and more stable hydrogen bonds
  form, eventually reaching 4 per molecule the
  hydrogen bonds push the water molecules further
  apart, making it less dense? ice floats on water

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