Bonding

• Ionic bonding involves electrostatic attraction between oppositely charged ions in a lattice.
• Ionic compounds are formed because of a transfer of electrons.
• An ionic lattice is a regular repeated three-dimensional arrangement of atoms, ions, or molecules in a metal or other crystalline solid.
• Properties of ionic compounds:

  • Ionic compounds have a high melting point or boiling point, or are solid at room temperature = strong electrostatic attraction between oppositely charged ions.
  • Ionic compounds are usually soluble in water.
  • Ionic compounds conduct electricity when molten or when in aq solution = in the liquid and aq state, the ions are free to move and carry charge.
• A covalent bond consists of one or more shared pairs of electrons between two atoms. Multiple bonds contain multiple pairs of electrons.
• Covalent bonds are found in:

  • Molecular elements and compounds
  • Macromolecular (giant) covalent elements and compounds
  • Molecular ions
• A single covalent bond is represented as a line between two atoms, e.g H-Cl.
• A double covalent bond is two pairs of shared electrons, represented as a double line between two atoms, e.g C=O.
• A triple covalent bond is three pairs of shared electrons, represented as a triple line between two atoms.
• A co-ordinate or dative covalent bond contains a shared pair of electrons with both electrons supplied by one atom, these are represented using an arrow.
• Metallic bonding involved attraction between delocalised electrons and positive ions arranged in a lattice.
• Properties of metals:

  • Metals conduct electricity = delocalised electrons free to move and carry charge.
  • Metals conduct heat.
  • Metals are ductile and malleable.
  • Metals have high densities.
  • Most metals have high melting points = large regular structure with strong forces of attraction.
• Types of crystal structure:

  • Ionic, e.g sodium chloride
  • Metallic, e.g magnesium
  • Macromolecular (giant) covalent, e.g diamond and graphite
  • Molecular (covalent crystals), e.g iodine and ice
• Molecular covalent crystals have low melting points due to them not having strong bonds to hold their shape. They do not conduct electricity as there are no charger particles to carry charge.
• Macromolecular (giant covalent) structures have high melting points as they contain strong covalent bonds which require a lot of energy to break. Some substances such as diamonds cannot conduct heat or electricity as there are no charged particles which can move. Others like graphite can conduct electricity due to delocalised electrons between layers that slide over each other and carry charge.
• Shapes of simple molecules and ions:

  A) Linear

  B) Trigonal planar

  C) V shaped

  D) Pyramidal

  E) Octahedral

  F) Trigonal pyramidal

  G) Tetrahedral

  
• Bonding pairs and lone pairs of electrons as charge clouds that repel each other.
• Lone-lone pairs repulsion is greater than lone-bond pair repulsion, which is greater than bond-bond pair repulsion.
• Electronegativity of an element is the power of an atom to attract the pair of electrons in a covalent bond.

  Electronegativity depends on:

  • The distance of the bonding electron from the attractive power of the nucleus.
  • The size of the nuclear charge.
  • The attractive power of the nucleus being shielded by inner electrons.
• Trend in electronegativity:

  • Electronegativity increases across a period. This is due to atomic radius decreasing, giving a progressively stronger attraction between the positive nucleus and two electrons in covalent bond. Nuclear charge also increases, causing greater attraction.
  • Electronegativity decreases down a group. This is due to the atomic radius increases, giving a progressively weaker attraction between the positive nucleus and two electrons in covalent bond. Shielding also increases so there are more electrons in inner energy levels.
• The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical. This produces a polar covalent bond, and may cause a molecule to have a permanent dipole.
• An induced dipole or van der Waals' forces occur when electrons orbiting electrons are distributed more on one side of the molecule than the other. Another molecule approaching this side of the molecule will have its electron repelled.
• In induced dipole forces, the electrons are in continuous movement so they keep shifting. This creates an attraction between all atoms in close proximity.

  The larger the molecule, the higher the Mr, the greater the number of electrons. The more electrons there are, the greater the induced dipole, so the greater the van der Waals' forces.
• Van der Waals' forces are much weaker than covalent bonds, permanent dipole forces or hydrogen bonding. Meaning, substances with only van der Waals' forces will have lower melting and boiling points.
• Hydrogen bonds occur between a slightly positive hydrogen atom, (which is covalently bonded to O, N or F) and the lone pair of an O, N or F atom of another molecule.
• The important features of drawing a hydrogen bonding diagram:

  • Show δ+ and δ- on all atoms.
  • Show the lone pair of electrons forming the hydrogen bond.
  • Draw a dashed line to show the hydrogen bond between the long pair and the δ+ H atom.
• The hydrogen bonds between water molecules explain many physical properties:

  • Water is fluid as the hydrogen bonds can break and reform allowing water molecules to move around each other.
  • Water has a higher boiling point than expected, the hydrogen bonds between water molecules mean that it requires substantially more energy to separate the water.
  • Ice has a lower density than water, the hydrogen bonds hold the water molecules in a more open 3D crystalline structure. Meaning the molecules are further apart from each other than in liquid form, giving it a lower density.
• Permanent dipole-dipole forces cause the molecules to be attracted to each other. The δ+ end of the dipole on one molecule is attracted to the δ- end of the dipole on another molecule.
• Permanent dipoles are formed when there is a large difference in electronegativity between two atoms bonded together in a covalent bond. This causes the shared pair of electrons to be shared unequally. They are pulled towards the more electronegative atom.
• Polar molecules have permanent dipole-dipole forces.They'll have weak electrostatic forces of attraction between δ+ and δ− charges on neighbouring molecules, meaning relatively low melting and boiling points.