Bonding, structure and the properties of matter

• Types of chemical bonding

  ionic, covalent, metallic
• ionic bonding


  The strong electrostatic attraction between oppositely charged ions. Ionic bonding occurs in compounds formed from metals combined with non-metals. Atoms lose or gain electrons to get a full outer shell. An ionic bond is formed when electrons are lost and gained by two or more atoms.
• covalent bonding


  The particles are atoms which share pairs of electrons to give both atoms full outer shells. Covalent bonding occurs in most non-metallic elements and in compounds of non-metals.
• metallic bonding


  The particles are atoms which share delocalised electrons. Metallic bonding occurs in metallic elements and alloys. The electrons arefree to move around the structure which makes it a good conductor of electricity.
• ionic bonding process


  When a metal atom reacts with a non-metal atom electrons in the outer shell of the metal atom are transferred. Metal atoms lose electrons to become positively charged ions. Non-metal atoms gain electrons to become negatively charged ions. The ions produced by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 have the electronic structure of a noble gas (Group 0). The electron transfer during the formation of an ionic compound can be represented by a dot and cross diagram, eg for sodium chloride

  
• ionic compounds


  An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding.

  
• limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent a giant ionic structure


  Dot and cross: It doesn't show how the ions are arranged in space. A dot and cross diagram for sodium chloride suggests that it is made up of pairs of sodium and chloride ions but it is a giant ionic lattice structure with equal numbers of sodium and chloride ions.

  
  

  Ball and stick: The ions are not joined by sticks/physical bonds. The oppositely charged ions are attracted by strong electrostatic forces of attraction

  
  

  2 dimensional diagrams: giant ionic lattice are 3D, so the 2D diagram only shows 1 'layer' of it.

  
  

  3 dimensional diagrams: Only easily show ionic substances that exist in a 1:1 ratio such as NaCl. It is difficult to show ionic substances such as Na₂O.
• How to work out the empirical formula of an ionic compound


  The empirical formula is the simplest ratio of atoms or ions within a structure. To work out the empirical formulas from a diagram, count the number of each ion and finally work out the lower common ratio.

  
• Covalent bonds


  When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Covalently bonded substances may consist of small molecules. Students should be able to recognise common substances that consist of small molecules from their chemical formula. Some covalently bonded substances have very large molecules, such as polymers. Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide.

  
• limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures


  Dot and cross: Can easily be used to show the bonding in simple molecules but not giant covalent structures. The dot and cross arrangement implies the electrons are fixed but they move around a lot.

  Ball and stick: In giant covalent structures the atoms at the edge haven't got the correct number of bonds

  2 dimensional diagrams: Don't show the shape of the molecule

  3 dimensional diagrams: Often only show a small portion of the giant covalent structure.
• How to work out the molecular formula of a substance from a given model or diagram showing the atoms and bonds in the molecule


  - To deduce a molecular formular in an exam question means you already know the empirical formula

  - Add up the atomic masses of the atoms in the empirical formula.

  For example, the empirical formula of a hydrocarbon is CH2 and its Mr is 42.

  - the mass of the atoms in the empirical formula is 14

  42 ÷ 14 = 3

  - so you need to multiply the numbers in the empirical formula by 3

  - The molecular formula of the hydrocarbon is therefore C3H6.
• Metallic bonding


  Metals consist of giant structures of atoms arranged in a regular pattern. The electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure. The sharing of delocalised electrons gives rise to strong metallic bonds.

  
• Three states of matter

  The three states of matter are solid, liquid and gas. The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres.
• Limitations of the simple model for the 3 states of matter


  There are no forces

  All particles are represented as spheres

  The spheres are solid.

  
• Limitations of the particle theory in relation to changes of state when particles are represented by solid inelastic spheres which have no forces between them


  The particle model can be represented in different ways but these are limited as they do not accurately represent the scale, size, mass or behaviour of actual particles, they assume that particles are inelastic spheres, and they do not fully take into account the different interactions between particles e.g. the gaps between the particle in a gas must enable a diagram to fit on a page when they are much larger relative to the size of the particle.

  
• State symbols


  Solid (s)

  Liquid (l)

  Gas (g)

  Aqueous (aq) - dissolved in water
• Properties of ionic compounds


  They have regular structures (giant ionic lattices) and there are strong electrostatic forces of attraction in all directions between oppositely charged ions. They have high melting and boiling points because of the large amounts of energy needed to break the many strong bonds. When melted or dissolved in water, they conduct electricity because the ions are free to move and so charge can flow
• Properties of small molecules


  Usually gases or liquids that have relatively low melting and boiling points. These substances have only weak intermolecular forces. It is the intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.

  The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.

  These substances do not conduct electricity because the molecules do not have an overall electric charge.
• Polymers


  They have very large molecules. The atoms are linked to other atoms by strong covalent bonds. The intermolecular forces between polymer molecules are relatively strong and so these substances are solids at room temperature.

  
• Giant covalent structures


  They are solids with very high melting points. All the atoms are linked together by strong covalent bonds. These bonds must be overcome to melt or boil these substances. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures

  
• Properties of metals and alloys


  Metals have giant structures of atoms with strong metallic bonding. This means that most metals have high melting and boiling points. In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped. Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.

  
• Why are allows harder than pure metals?


  Alloys contain atoms of different sizes which DISTORT layers of the regular arrangement and structure of atoms in an metal so it is more difficult for the layers to slide over each other making them harder
• Metals as conductors


  Metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal. Metals are good conductors of thermal energy because energy is transferred by the delocalised electrons.
• Diamond structure


  In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure.

  
• Properties of diamond


  -tetrahedral shape, crystal lattice structure

  -high melting point

  -hard

  -good thermal conductor

  -can't conduct electricity

  -insoluble in any solvent
• Graphite structure


  In graphite, each carbon atom forms three covalent bonds with three other carbon atoms, forming layers of hexagonal rings which have no covalent bonds between the layers. In graphite, one electron from each carbon atom is delocalised

  
• Properties of graphite


  Soft

  Slippery

  Good conductor of heat and electricity
• Graphene


  Graphene is a single layer of graphite and has properties that make it useful in electronics and composites.

  
• Properties of graphene


  -Very strong

  -Very light material

  -Harder than a diamond

  -300 times stronger than steel

  -Lightest known material

  -Transparent

  -Conducts electricity and heat better than copper

  -Extremely flexible
• Fullerene


  Fullerenes are molecules of carbon atoms with hollow shapes. The structure of fullerenes is based on hexagonal rings of carbon atoms but they may also contain rings with five or seven carbon atoms. The first fullerene to be discovered was Buckminsterfullerene (C60) which has a spherical shape. Carbon nanotubes are cylindrical fullerenes with very high length to diameter ratios.
• Uses of fullerenes and carbon nanotubes


  Pharmaceutical delivery

  Lubricants

  Catalysts

  Nanotechnology

  Electronics

  Materials
• Nanoscience and nanoparticles


  Nanoscience are structures that are 1-100 nm in size, of the order of a few hundred atoms. Nanoparticles, are smaller than fine particles, which have diameters between 100 and 2500 nm (1 x 10-7 m and 2.5 x 10-6 m). Coarse particles have diameters between 1 x 10-5 m and 2.5 x 10-6 m. Coarse particles are often referred to as dust. As the side of cube decreases by a factor of 10 the surface area to volume ratio increases by a factor of 10. Nanoparticles may have properties different from those for the same materials in bulk because of their high surface area to volume ratio. It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes.
• Uses of nanoparticles


  Medicine

  Electronics

  Cosmetics

  Sun cream

  Deodorants

  Catalysts
• Advantages of nanoparticles


  - Large surface area to volume ratio over larger particles meaning they can make things stronger while being lighter
• Disadvantages of nanoparticles


  May have undiscovered side effects

  Often react quickly due to surface area to volume ratio. Might speed up reactions in living things.