Module 2 chemical bonding

• Chemical bonding force that holds atoms

  together in an element or a

  compound.
• Metallic bond – formed when the

  valence electrons are not associated

  with a particular atom or ion, but exists as a cloud of electrons around the ion centers
• Covalent bond – formed when valence

  electrons from one atom are shared

  between two or more particular atoms
• Ionic bond – formed when valence

  electrons are transferred from one atom

  to another to complete the outer electron shell
• Metals have very high melting and

  boiling points.

  • Metallic bonds are strong and a lot of energy is needed to break them.
• Metals are good conductors of electricity and heat.

  • Metals contain electrons that are free to move in the metal structure, allowing the transfer of charge and thermal energy
• Metals have high densities.

  • Metals have a large number of atoms packed per unit volume.
• Metals are lustrous.

  • Light can be reflected back when light strikes a metal surface because the electrons oscillate at a collective frequency.
• Metals are malleable and ductile.

  • When stress is applied on a metal, atoms roll over each other onto new positions without breaking the metallic bonding.
• Covalent compound

  It is a compound whose structural units are called molecules.

  A molecule is a neutral collection of atoms held together by covalent bonds.
• Bond length

  • the equilibrium distance between the nuclei of two atoms that are bonded to each other
• Bond strength

  alternative name for bond dissociation energy

  • the amount of energy needed to break a bond homolytically and produce two radical fragments
• Bond strength

  • the amount of energy consumed when a bond is broken is equal to the amount of energy liberated when a bond is formed
• Bond angle

  • the angle formed between two adjacent bonds
• Bond polarity

  • a measure of the distribution of electrons between two bonded atoms
• Electronegativity

  • intrinsic ability of an atom to

  attract the shared electrons in a covalent bond
• Lewis dot symbol consists of the symbol of an element and one dot for each valence electron in an atom of an element.”
• Lewis structure is a representation of covalent bonding in which shared electron pairs are shown either as lines or as pairs of dots between two atoms, and lone pairs are shown as pairs of dots on individual atoms
• Formal Charge the electronic charge assigned to individual atoms in a Lewis structure
• Resonance structures 


  two or more structures of a molecule that differ in the placement of pi (π) bonds and nonbonded electrons the placement of atoms and sigma (σ) bonds stay the same
• Valence Shell Electron Pair Repulsion (VSEPR) model- a model that is useful in predicting the geometry (shape) of molecules formed from non metals
• main postulate of Valence Shell Electron Pair Repulsion (VSEPR) mode : the structure around a given atom is determined principally by electron minimizing pair repulsions
• for repulsive forces

  Since lone pairs (LP) occupy a larger volume than bonding pairs (BP), repulsion is greater between lone pairs than between bonding pairs, forming bond angles that deviate from normal by pushing the bonding pairs closer together
• Electron pair geometry is the name of the geometry of the electron pair/groups/domains on the central atom, whether they are bonding or non bonding.”
• “ Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule.” It does not consider the nonbonding pairs (lone pairs)
• “In a polar molecule, the presence of one or more polar covalent bonds leads to a separation of the positive and negative charge centers for the molecule as a whole.”
• A polar molecule has a resultant dipole moment.
• Valence Bond Theory (VBT)- a covalent bond results when two atoms approach each other closely so that a singly occupied orbital on one atom overlaps a singly occupied orbital on the other atom
• in a sigma bond (σ), the electron density is concentrated on the axis joining two nuclei
• in a pi bond (π), the electron density is not concentrated on the axis joining two nuclei
• Key idea of VBT

  1. covalent bonds are formed by overlap of two atomic orbitals (each contains one electron)
  2. bonded atoms retains its own atomic orbitals, but electron pair in the overlapping orbitals is shared by both atoms
  3. the greater the amount of orbital overlap, the stronger the bond
• Molecular Orbital Theory (MOT)- describes covalent bond formation as arising from a mathematical combination of atomic orbitals to form molecular orbitals
• Chemical Bonding Theories

  In subtractive combination, destructive interference occurs between the opposite phases leading to a node between the two nuclei.
• Chemical bonding teories

  In additive combination, the matching phases produce a constructive interference which enhances the electron probability between the two nuclei
• “ Bond order is one-half the difference between the numbers of electrons in bonding and in antibonding molecular orbitals in a covalent bond.
• A diamagnetic substance has all its electrons paired and is slightly repelled by a magnetic field
• A paramagnetic substance has one or more unpaired electrons and is attracted into a magnetic field.
• Key ideas of MOT

  1. molecular orbitals are to molecules what atomic orbitals are to atoms
  2. molecular orbitals are formed by combining atomic orbitals the number of molecular orbitals formed is equal to the number of atomic orbitals combined
  3. relative energies of molecular orbitals:
• Hybridization- combination of two or more atomic orbitals to form the same number of hybrid orbitals, each having the same shape and energy