2.2 CHEM covalent bonding

• When atoms of two non-metals react together, each seeking to gain valence electrons in order to achieve the stable electron structure of a noble gas. By sharing an electron pair, they are able to achieve this. The shared pair of electrons is concentrated in the region between two nuclei and is attracted to them both. The electrostatic attraction between shared pairs of electrons and the positively charged nuclei holds the atoms together and is known as a covalent bond.
• The system containing these two hydrogen atoms will be stabilized when the forces of attraction between the nuclei and shared electrons are balanced by the forces of repulsion between two nuclei. This holds the atoms at a fixed distance apart.
• Octet rule: the tendency of atoms to gain a valence shell with a total of eight electrons.
• The octet rule provide an insight into the chemical properties of the noble gases.
• Few molecules which are an exception to the octet rule. Beryllium (Be) and Boron (B) form stable molecules in which the central atom has fewer than eight electrons. Known as an incomplete octet. BeCl2, BF3
• A double bond forms when two electron pairs, a total of four electrons, are shared; a triple bond forms when three electron pairs, a total six electrons, are shared. CO2 (has a double bonds), HCN (H-C single bond, C-N has a triple bond)
• Every covalent bond is characterized by two values: Bond length- measure of the distance between two bonded nuclei and Bond strength- usually described in terms of bond enthaply, is the measure of energy required to break the bond.
• Atomic radius increases as we go down the group, we would expect the atoms to form molecules with longer bonds. As a result, the shared electron pairs is further from the pull of the nuclei in the larger molecules, and so the bond would be expected to be weaker.
• Multiple bonds have a greater number of shared electrons and so have a stronger force of electrostatic attraction to the bonded nuclei. There is a greater pulling power between the atoms, bringing them closer together, resulting in bonds that are shorter than single bonds.
• A coordinate bond is a covalent bond in which both shared electrons originate from the same atom. The bond forms by both the electrons in the pair originating from the same atom. This means that the other atom accepts and gains a share in a donated electron pair.
• Ammonium ion

  
• Hydronium ion

  
• Carbon Monoxide

  
• CO, the triple bond consists of two bonds that involve sharing an electron from each atom, and the third bond is a coordination bond where both electrons come from the oxygen. The three bonds are identical to each other. This illustrates that coordinate bond are formed, they are no different from other covalent bonds.
• Electron pair is an electron donmain
• Species with two electron domains - 180 degrees - linear shape. Ex: BeCl2, CO2, C2H2
• Electronegativity is a measure of the ability of an atom to attract electrons in a covalent bond.
• Bond polarity results from the difference in electronegativities of the bonded atoms
• VSEPR for 2 electron domains

  Linear, bond angle 180
• VSEPR 3 electron domains

  3 electron domains, 0 lone pairs, trigonal planar, bond angle 120
• VSEPR 3 electron domains 1 lone pair

  2 bonding pairs, 1 lone pair, domain geometry trigonal planar, molecular geometry bent linear, bond angle 118
• VSEPR 4 electron domains 


  4 bonding domains, tetrahedral, bond angle 109.5
• VSEPR 4 electron domains 


  3 bonding domains, 1 lone pair, domain geometry tetrahedral, molecular geometry trigonal pyramidal, bond angle 107
• VSEPR 4 electron domains 


  2 bonding domains, 2 lone pairs, electron domain geometry tetrahedral, molecular geometry bent linear, bond angle 104.5
• VSEPR 5 electron domains 


  5 bonding domains, 0 lone pairs, electron domain geometry trigonal bipyramidal, molecular geometry trigonal bipyramidal, bond angle 120, 90, 180
• VSEPR 5 electron domains


  5 bonding pairs, 1 lone pair, molecular geometry seesaw, electron domain geometry trigonal bipyramidal, bond angle 90, 117
• VSEPR 5 electron domains 


  3 bonding pairs, 2 lone pairs, molecular geometry t-shape, electron domain geometry trigonal bipyramidal, bond angle 90
• VSEPR 5 electron domains 


  2 bonding domains, 3 lone pairs, molecular geometry linear, electron domain geometry trigonal bipyramidal, bond angle 180
• VSEPR 6 electron domains

  6 bonding pairs, 0 lone pairs, molecular/electron domain geometry octahedral, bond angle 90
• VSEPR 6 electron domains

  5 bonding pairs, 1 lone pair, electron domain geometry octahedral, molecular geometry square pyramidal, bond angle 90
• VSEPR 6 electron domains

  4 bonding pairs, 2 lone pairs, electron domain geometry octahedral, molecular geometry square planar, bond angle 90
• Bond dipole


  often used to indicate the fact that this type of bond has two partially separated opposite charges. The more electronegative atom with the greater share of electrons has become partially - and the less electronegative atom has become partially +
• Graphite, diamond, buckminsterfullerene and graphene are allotropes of carbon
• • Diamond is a giant lattice of carbon atoms
  • Each carbon is covalently bonded to four others in a tetrahedral arrangement with a bond angle of 109.5o
  • The result is a giant lattice with strong bonds in all directions
  • Diamond is the hardest substance known
  • For this reason it is used in drills and glass-cutting tools
• Graphite

  • In graphite, each carbon atom is bonded to three others in a layered structure
  • The layers are made of hexagons with a bond angle of 120o
  • The spare electron is delocalised and occupies the space in between the layers
  • All atoms in the same layer are held together by strong covalent bonds, and the different layers are held together by weak intermolecular forces
• Buckminsterfullerene

  • Buckminsterfullerene is one type of fullerene, named after Buckminster Fuller, the American architect who designed domes like the Epcot Centre in Florida
  • It contains 60 carbon atoms, each of which is bonded to three others by single covalent bonds
  • The fourth electron is delocalised so the electrons can migrate throughout the structure making the buckyball a semi-conductor
  • It has exactly the same shape as a soccer ball, hence the nickname the football molecule
• Graphene

  • Some substances contain an infinite lattice of covalently bonded atoms in two dimensions only to form layers. Graphene is an example
  • Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons
  • Graphene is one million times thinner than paper; so thin that it is actually considered two dimensional
• • The silicon atoms in silicon have a tetrahedral arrangement, just like that of the carbon atoms in diamond
  • Each silicon atom is covalently bonded to four other silicon atoms 
  • Silicon has a giant lattice structure 
• Silicon(IV) oxide

  • Silicon(IV) oxide is also known as silicon dioxide, but you will be more familiar with it as the white stuff on beaches!
  • Silicon(IV) oxide adopts the same structure as diamond -  a giant structure made of tetrahedral units all bonded by strong covalent bonds
  • Each silicon is shared by four oxygens and each oxygen is shared by two silicon atoms
  • This gives an empirical formula of SiO2
• Properties of Giant Covalent Structures

  • Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility