Structure and bonding

• Ionic bonding occurs between a metal and non-metal and is where the metal transfers its' electrons to the non-metal to form a positive metal ion and negative non-metal ion.
• Oppositely charged ions attract through electrostatic forces to form a giant ionic lattice.
• A sulfate ion (SO4) has a 2 - charge.
• A hydroxide ion (OH) has a 1 - charge.
• A nitrate ion (NO3) has a 2 - charge
• A carbonate ion (CO3) has a 2 - charge.
• An ammonium ion (NH4) has a 1 + charge.
• Covalent bonding occurs between two non-metals. Electron pairs are shared between the two outer shells so both atoms have a full outer shell.
• Dative covalent bonds, also known as coordinate bonds occur when both of the electrons in the shared pair in covalent bonding come from one of the atoms.
• Dative covalent bonds react in the same way as standard covalent bonds.
• Metallic bonding occurs between metals and consists of a lattice of positively charged ions surrounded by delocalised electrons, producing a very strong electrostatic force of attraction between the oppositely charged particles.
• In metallic bonding, the greater the charge on the positive ion, the stronger the attractive force as more electrons are delocalised.
• In metallic bonding, the larger the size of the ion, the weaker the attractive force as the ions have a larger atomic radius.
• There are four main types of crystal structure: ionic, metallic, simple molecular, and macromolecular.
• Substances with an ionic crystal structure have a high melting and boiling point due to the strong ionic lattice, can conduct electricity when molten or in solution as the ions are free to act as charge carriers, and are often brittle as when the layers are distorted like charges repel causing the lattice to break apart into fragments.
• Substances with a metallic crystal structure have a high melting and boiling point due to strong electrostatic forces of attraction between the positive ions and delocalised electrons, can conduct electricity as the delocalised electrons are free to act as charge carriers and are malleable as the layers of ions can slide over each other with the delocalised electrons preventing fragmentation as they can move around.
• Substances with a simple molecular crystal structure have a low melting and boiling point as only weak van der waals forces need to be broken, and cannot conduct electricity as there is nothing free to act as a charge carrier.
• Substances with a macromolecular crystal structure have a high melting and boiling point as all atoms are bonded by strong covalent bonds, usually cannot conduct electricity as there is nothing free to act as a charge carrier, and are rigid due to the strength of the covalent lattice.
• Diamond is a macromolecular substance made of carbon atoms each bonded to four other carbon atoms, making it one of the strongest, hardest materials known.
• Graphite is a macromolecular substance made of carbon atoms each bonded to three other carbon atoms arranged in a hexagonal shape in flat sheets. The electrons not used in bonding are delocalised, meaning graphite can conduct electricity.
• The shape of a molecule is determined by the number of bonding pairs and lone pairs. Each electron pair repels each other so that the largest angle possible exists between the bonds.
• Any lone pairs provide additional repulsive forces to bonding pairs, and decreases the bond angle between bonding pairs by 2.5 degrees.
• An atom with two bonding pairs exhibits a linear shape with a bond angle of 180 degrees.
• An atom with three bonding pairs exhibits a trigonal planar shape, with a bond angle of 120 degrees.
• An atom with four bonding pairs exhibits a tetrahedral shape, with a bond angle of 109.5 degrees.
• An atom with five bonding pairs exhibits a trigonal bipyramidal shape, with bond angles of 90 and 120 degrees.
• An atom with six bonding pairs exhibits an octahedral shape, with a bond angle of 90 degrees.
• If one bonding pair out of three is replaced with a lone pair or two bonding pairs out of four, the shape produced is called bent or v-shaped.
• If one bonding pair out of four is replaces with a lone pair, the shape produced is called triangular pyramidal.
• Electronegativity is the power of an atom to attract a lone pair of electrons.
• Electronegativity increases along a period as atomic radius decreases and decreases down a group as shielding increases.
• If two atoms that are bonded have different electronegativities, a polar bond forms as the more electronegative atom becomes partially negative as it pulls electrons away from the less electronegative atom, and so the less electronegative atom becomes partially positive. This is called a permanent dipole.
• Polar molecules with a permanent dipole can align to form a lattice of molecules similar to an ionic lattice.
• Electron density in a particular region is constantly fluctuating causing temporary charges on each side of an atom, and so an induced dipole can be formed in a nearby atom due to the influence of the charge on it.
• Van der Waals forces is the weakest type of intermolecular force and acts between all molecules as an induced dipole between molecules. The strength increases with molecule size and with straight chain molecules as they can get closer together.
• Permanent dipole forces are a type of intermolecular force that acts between molecules with a polar bond as the partially charged regions hold the molecules together in a lattice - like structure.
• Hydrogen bonding is a special case of permanent dipole forces between hydrogen and nitrogen, oxygen or fluorine, and is the strongest type of intermolecular force. Molecules with hydrogen bonding have much higher melting and boiling points compared to similarly sized molecules.