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Flashcards

2. Chemical bonding and structure

Georgina Peck A Level Chemistry A level English 51 cards

Covalent bonding

Cards in this deck (51)

  1. 1Properties of metalsMetals have the following properties:
    • High melting temperatures
    • Good electrical conductivity
    • Good thermal conductivity
    • Malleability
    • Ductility
  2. 2Metallic bondingThe strong electrostatic force of attraction between the nuclei of metal cations and delocalised electrons
  3. 3Melting temperatures of metalsMetals have a giant lattice structure where many forces of attraction between the nuclei of the cations and the delocalised electrons must be overcome. The energy required to do this is usually very large, so the melting temperatures are typically high
  4. 4Factors that effect the melting temperature of a metal• The number of delocalised electrons per cation
    • The size of the cation. The smaller the cation, the closer the delocalised electrons are to the nucleus of the cation. This results in an increase in the force of attraction between the nuclei and the delocalised electrons, and so an increase in melting temperature
  5. 5Electrical conductivity of metalsWhen a potential difference is applied across the ends of a metal, the delocalised electrons will be attracted to, and move towards, the positive terminal of the cell. This movement of electrical charge constitutes an electric current
  6. 6Thermal conductivity of metalsTwo factors contribute to the ability of metals to transfer heat energy:
    1. The free-moving delocalised electrons pass kinetic energy along the metal
    2. The cations are closely packed and pass kinetic energy from one cation to another
  7. 7Malleability and ductility of metalsMetals can be hammered or pressed into different shapes (malleability). They can also be drawn into a wire (ductility). Both of these properties depend on the ability of the delocalised electrons and the cations to move throughout the structure of the metal. When a stress is applied to a metal, the layers of cations may slide over one another. However, since the delocalised electrons are free moving, they move with the cations and prevent strong forces of repulsion forming between the cations in one layer and the cations in another layer
  8. 8Ionic bondingThe strong electrostatic attraction between oppositely charged ions
  9. 9Factors that affect the strength of ionic bonding• Size of the ion – the smaller the ion the more energy required to overcome the electrostatic interactions between the ions and to separate them
    • Charge on the ion
  10. 10Trends in ionic radiiThe ionic radius decreases as the number of protons increases. As the positive charge of the nucleus increases, the electrons are attracted more strongly and are therefore pulled closer to the nucleus
    Ionic radius increases down a group as the ions have more occupied shells
  11. 11Isoelectronic ionsIsoelectronic ions have the same electronic configuration and number of electrons
  12. 12Properties of ionic compoundsIonic compounds typically have the following physical properties:
    • High melting temperatures
    • Brittleness
    • Poor electrical conductivity when solid but good when molten
    • Often soluble in water
  13. 13High melting temperatures of ionic compoundsIonic solids consist of a giant lattice network of oppositely charged ions. There are many ions in the lattice and the combined electrostatic forces of attraction among all of the ions are large.
    So, a large amount of energy is required to overcome the forces of attraction sufficiently for the ions to break free from the lattice and be able to slide past one another
  14. 14Brittleness of ionic compoundsIf a stress is applied to a crystal of an ionic solid, then the layers of ions may slide over one another. Ions of the same charge are now side by side and repel one another. The crystals break apart
  15. 15Electrical conductivity of ionic compoundsSolid ionic compounds don’t conduct electricity. This is because there are no delocalised electrons and the ions are also not free to move under the influence of an applied potential difference.
    However, molten ionic compounds will conduct since the ions are now mobile and will migrate to the electrodes of opposite sign when a potential difference is applied. Aqueous solutions of ionic compounds also conduct electricity and undergo electrolysis, since the lattice breaks down into separate ions when the compound dissolves
  16. 16Evidence for the existence of ionsThe most convincing evidence for the existence of ions is the ability of an ionic compound to conduct electricity and undergo electrolysis when either molten or in aqueous solution
  17. 17Covalent bondingA covalent bond is formed by the overlap of two atomic orbitals each containing a single electron and is the strong electrostatic attraction between the nuclei of two atoms and the bonding pair of electrons
  18. 18Covalent bondingCovalent bonding
  19. 19Bond lengthBond length is the distance between the nuclei of two atoms that are covalently bonded together
  20. 20Relationship between bond length and bond strengthThe general relationship between bond length and bond strength is that the shorter the bond, the greater the bond strength. This is a result of an increase in electrostatic attraction between the two nuclei and the electrons in the overlapping atomic orbitalsRelationship between bond length and bond strength
  21. 21ElectronegativityElectronegativity is the ability of an atom to attract a bonding pair of electrons
  22. 22Trends in electronegativityThe electronegativity of elements, in general:
    • Decreases down a group of the periodic table
    • Increases from left to right across a period
  23. 23Polar covalent bondA polar covalent bond is a type of covalent bond between two atoms where the bonding electrons are unequally distributed. Because of this, one atom carries a slight negative charge and the other a slight positive charge
  24. 24Continuum of bonding typeCovalent and ionic bonding are the two extremes of a continuum of bonding types
    Polar covalent bonds can be described as covalent bonds with a degree of ionic character    Continuum of bonding type
  25. 25Discrete moleculeA discrete (simple) molecule is an electrically neutral group of two or more atoms held together by chemical bonds
  26. 26How is a dative covalent bond formed?A dative covalent bond is formed when an empty orbital of one atom overlaps with an orbital containing a lone pair of electrons of another atom
    It is represented by an arrow pointing from the atom that provides the electron pair towards the atom with the vacant orbital
  27. 27Electron pair repulsion theoryThe electron pair repulsion (EPR) theory states that:
    • The shape of a molecule or ion is caused by repulsion between the pairs of electrons, both bond pairs and lone (non-bonding) pairs, that surround the central atom
    • The electron pairs arrange themselves around the central atom so that the repulsion between them is at a minimum
    • Lone pair-lone pair repulsion > lone pair – bond pair repulsion > bond pair – bond pair repulsion
  28. 28Shapes of moleculesShapes of molecules
  29. 29Bond anglesBond angles
  30. 30Non-polar and polar moleculesA polar molecule must have:
    • One ore more polar bonds
    • Unbalanced dipoles because of their partial charges or the shape of the molecule
    Water molecules are polar. Carbon dioxide molecules aren't polar    Non-polar and polar molecules
  31. 31London forcesLondon forces are attractive forces between instantaneous dipoles and induced dipoles
    Electron density fluctuates over time. If at any instant the electron density becomes unsymmetrical in a molecule, an instataneous dipole will be generated. An induced dipole is caused when a molecule comes close to the molecule with the instantaneous dipoles    London forces
  32. 32Features of London forcesLondon forces have these features:
    • They exist between all molecules
    • Their strength depends upon the number of electrons in the molecule
    • Their strength depends upon the number of points of contact between molecules
  33. 33Permanent dipolesIf the molecules posess permenant dipoles, they will also interact with one another. If the dipoles are aligned correctly, then there will be a favourable interaction and the two molecules will attract one another. However, the random movement of the molecules is such that the dipoles aren’t always alligned to produce a favourable reaction. As a result, the interaction between permenant dipoles is usually much less than the interaction between instantaneous-induced dipoles
  34. 34Hydrogen bondHydrogen bonds are intermolecular forces. In general they can form if:
    • The compound contains a hydrogen atom covalently bonded to an atom of N, O or F
    • There is a lone pair of electrons on an atom of N, O or F    Hydrogen bond
  35. 35Boiling temperatures of unbranched alkanesThere are two reasons for the increase in boiling temperature with increasing molecular mass:
    • As molecular mass increases, the number of electrons per molecule increases and so the instantaneous and induced dipoles increase
    • As the length of the carbon chain increases, the number of points of contact between adjacent molecules increases. Instantaneous dipole-induced dipole forces exist at each point of contact between the molecules, so the more points of contact the greater the overall intermolecular force of attraction
  36. 36Boiling temperatures of branched alkanesBranched chain alkanes have lower boiling temperatures than their un-branched isomers as the more branching in the molecule, the fewer points of contact between adjacent molecules. This leads to a decrease in the overall intermolecular force of attraction between molecules and a decrease in boiling temperature
  37. 37Boiling temperatures of alcoholsThe boiling point of the alcohol is higher than that of the alkane. This increase can be attributed to the hydrogen bonding that exists between the alcohol molecules, which doesn’t exist between molecules of the alkane. The additional force of attraction increases the energy required to separate the molecules
  38. 38Boiling temperatures of the hydrogen halidesThe steady increase in boiling temperature from HCl to HI is the result of the increasing number of electrons per molecule, which in turn results in an increase in London forces
  39. 39Melting and boiling temperature of waterThe hydrogen bonds between water molecules are relatively strong. As a result, the overall intermolecular forces of attraction in water are greater than would be expected for the number of electrons in the molecule. Water, therefore, has an abnormally high melting and boiling temperature
  40. 40Density of iceThe density of ice is less than the density of liquid water at 0°C. The molecules in ice are arranged in rings of 6, held together by hydrogen bonds. The structure creates large areas of open space inside the rings. When ice melts, the ring structure is destroyed and the average distance between the molecules decreases, causing an increase in density
  41. 41Choosing a suitable solventIn order for a substance to dissolve, the following two conditions must be met:
    • The solute particles must be separated from each other and then become surrounded by solvent particles
    • The forces of attraction between the solute and solvent particles must be strong enough to overcome the solvent-solvent forces and the solute-solute forces
  42. 42Dissolving ionic solids in waterMany ionic solids dissolve in water. Water molecules surround individual ions and form strong electrostatic attractions with them. This is hydration. The energy required to separate the ions in the solid is either completely, or partially, supplied by the hydration of the ions
  43. 43Compounds that can form hydrogen bonds with waterAlcohols have a hydroxyl group –OH. This allows them to form hydrogen bonds
    As the number of carbon atoms in an alcohol molecule increases:
    • London forces become more important
    • The alcohol becomes less soluble
  44. 44Metallic latticesMetallic lattices are composed of a regular arrangement of positive metal ions surrounded by delocalised electrons
    Substances that have a giant metallic lattice typically have the following properties:
    • High melting and boiling temperatures
    • Good electrical conductivity
    • Good thermal conductivity
    • Malleability
    • Ductility
  45. 45Giant ionic latticesGiant ionic lattices are composed of a regular arrangement of positive and negative ions
    Substances that consist of giant ionic lattices typically have the following properties:
    • Fairly high melting temperatures
    • Brittleness
    • Poor electrical conductivity when solid but good when molten
    • Often soluble in water
  46. 46Giant covalent latticesGiant covalent lattices are sometimes called network covalent lattices. They consist of a giant network of atoms linked to each other by covalent bonds
    Four of the most common giant covalent substances are:
    • Diamond
    • Graphite
    • Graphene
    • Silicon oxide
  47. 47DiamondIn diamond each carbon atom forms four sigma bonds to four other carbon atoms, in a giant three-dimensional tetrahedral arrangement. All bond angles are 109.5°. Diamond is extremely hard because of the very strong C – C bonding throughout the structure. It also has a very high melting temperature because a great number of strong C-C bonds have to be broken in order to melt it. This requires a large amount of heat energy
  48. 48GraphiteEach carbon atom is bonded to three others by sigma bonds, forming interlocking hexagonal rings. The fourth electron on each carbon atom is in a p-orbital. The carbon atoms are close enough for the p-orbitals to overlap with one another to produce a cloud of delocalised electrons above and below the plane of the rings
    Graphite can be used as a solid lubricant since the layers slide over one another.
    Graphite is a fairly good conductor of electricity as the delocalised electrons between the layers are free to move under the influence of an applied potential difference. Graphite can only conduct electricity parallel to its layers, as the delocalised electrons are not free to move from one layer to the next.
  49. 49Molecular latticesTwo common solid molecular lattices are iodine and ice. Other molecular solids include sulphur, white phosphorous, Buckminster fullerene, dry ice, sucrose and solid alkanes
  50. 50Physical properties of molecular solidsMolecular solids will have low melting and boiling temperatures. In order to melt a molecular solid it isn’t necessary to break the covalent bonds within the molecule; it is only necessary to overcome the intermolecular forces of attraction.
    Since intermolecular forces of attraction tend to be much weaker than covalent bonds, little energy is required to either break down the lattice structure of the solid and cause it to melt, or to separate the molecules and cause the liquid to boil and vaapourise
  51. 51Structure and propertiesStructure and properties

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A level A Level Chemistry Flashcards on 2. Chemical bonding and structure, created by Georgina Peck on 28/02/2017.
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