Bonding and intermolecular forces

• Polarizability (anion) is when more electrons are further from the nucleus

  1. Nucleus has less pull due to more energy shells and so more polarizability
  2. More negative charge due to a higher electron to proton ratio and so more polarizability
• The greater the difference between the size of an atom or ion, the stronger the dipole-dipole interaction
• Polarizing power is when their are less electrons and they are closer to the nucleus.

  1. The Nucleus has a larger effect on electrons as they are closer. Their are less electron shells and so more polarizing power.
  2. More positive charge because of a higher proton to electron ratio and so more polarizing power
• high charge and small size of cations result in a large polarizing power
• High charge and large size of anions results in more polarizability
• Polarizing power is the ability of a cation to distort the electron density a neighboring anion
• polarizing power is calculated by finding charge density
• charge density is roughly equal to charge/r2
• polarization is the distortion of the electron density of an anion
• covalent bonding is when two atoms atomic orbital containing a single electron from one atom overlaps with a similar atomic orbital
• covalent bonding occurs due to electrostatic attraction
• sigma bond forms by overlapping at one area
• a pi bond is formed by a second overlap after a sigma bond
• a pi bond forms due to the high electron density both above and below the molecule
• bond length is the distance between the nuclei of two atoms and that are covalently bonded together
• bond strength is measured by the amount of energy required to break one mole of the bond in gaseous state.
• the shorter the bond, the greater the bond strength due to an increase in electrostatic attraction between two nuclei and the electron in the overlapping atomic orbitals
• The electron pair repulsion theory is the shape adopted by a simple molecule or ion is that which keeps repulsive forces to a minimum
• covalent bonds are a pair of electrons and the bonds repel each other and push each other apart to reduce the repulsive forces until the repulsion’s are equal and the bonds are equally spaced.
• when their are only bonded pairs the repulsions are equal and they are standard shape
• 2 bonded pairs is a linear shape (180 degrees)

  3 bonded pairs is a trigonal planar shape (120 degrees)

  4 bonded pairs is a tetrahedral shape (109.5 degrees)

  5 bonded pairs is a Trigonal planar shape (90 and 120 degrees)

  6 bonded pairs is an octahedral shape (90 degrees)
• Irregular shapes is when a molecule or ion has lone pairs on the central atom. This makes the shapes slightly more distorted due to the extra repulsion by the lone pairs.
• 3 bonded pairs and 1 lone pairs is a pyramidal shape with a bond angle of 107 degrees
• 2 bonded pairs and 2 lone pairs is an angular shape with a bond angle of 104.5 degrees
• 4 bonded pairs and 2 lone pairs is a square planar shape with a bond angle of 90 degrees
• Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons
• It increases across a period and decreases down a group in the periodic table
• Fluorine has the highest electronegativity value of 4.0, while Francium has one of the lowest values at 0.7
• It increases across to the right as the number of protons increase as a result the nuclear charge also increases.

  There is a greater charge and so a greater attraction to the pair of electrons.

  Ions closest to the right has a stronger attraction
• increases up the group because the bonding pair of electrons are closer to the nucleus.
• slightly negative = delta negative
• slightly positive = delta positive
• < 0.5 = non polar covalent bond

  0.15 to 1.7 = polar covalent bond

  > 2.0 = ionic bond
• shapes can cause them to cancel out polarity of the molecule

  e.g Boron trifluoride has a Trigonal planer shape which causes the molecule to be non-polar
• Covalent bonds is the sharing of electrons
• simple molecular substances have strong covalent bonds holding elements together but weak intermolecular forces between each molecule
• larger structures are polymers and have giant covalent structures that are very strong
• polymers are long chains of monomers
• simple molecular substances have low temperatures to melt or boil and it increases as you go down the group
• in simple molecular substances you break the intermolecular forces instead of splitting the atoms. The more a molecule has intermolecular forces the stronger the attraction.