TOPIC 2

• Atoms combine

  Through the movement of electrons to achieve a greater level of stability by obtaining a full outer shell of electrons

  
• Ways atoms can combine

  • Ionic bonds
  • Covalent bonds
  • Metallic bonds
• Ionic bonds

  Take place when metals and non-metals react by transferring electrons, forming oppositely charged particles (ions) held together by electrostatic forces
• Covalent bonds


  Non-metal atoms share pairs of electrons between each other
• Metallic bonds

  Occur in metals and metal alloys, where atoms lose their valence electrons which become delocalised and form a "sea of electrons"
• Intermolecular forces are not chemical bonds, do not involve electron transfer or sharing, and are around one-tenth the strength of a chemical bond
• Ion

  An electrically charged atom or group of atoms formed by the loss or gain of electrons
• Anion

  Negative ion formed when atoms gain electrons

  
• Cation

  Positive ion formed when atoms lose electrons
• All metals lose electrons to become positively charged ions, all non-metals gain electrons to become negatively charged ions
• The number of electrons an atom gains or loses is the same as the charge
• Dot and cross diagram

  Diagrammatic representation of ionic bonds, showing the transfer of electrons

  
• Ionic lattice


  Regular arrangement of alternating positive and negative ions held together by strong electrostatic forces, resulting in high melting and boiling points

  
• Ionic compounds have giant ionic lattice structures containing huge numbers of ions
• Covalent bonds


  Form when non-metal atoms share pairs of electrons to obtain full outer shells, creating strong bonds and molecules

  
• Covalent bonds do not involve electron transfer like ionic bonds, the electrons are shared between atoms
• Simple covalent molecules

  Small molecular structures like Cl2, H2O, CO2 that can be represented by dot and cross diagrams
• Simple covalent molecules can be separated into individual units without breaking chemical bonds, unlike giant covalent structures
• Polymers and giant covalent structures

  Very large covalently bonded structures like graphite, diamond, and silicon dioxide
• Dot and cross diagrams show electron transfer but not 3D arrangement, ball and stick models show 3D arrangement but not electron movement, 2D representations are simpler but lack detail

  
• Metallic bonding


  Metal atoms lose their valence electrons which become delocalised, forming a "sea of electrons" that holds the positive metal ions together in a lattice structure

  
• States of matter

  • Solid
  • Liquid
  • Gas

  
• Solids

  • Strong forces of attraction between particles in a fixed regular pattern, particles vibrate in fixed positions, have fixed volume and shape, high density, low particle energy
• Liquids


  • Weaker attractive forces between particles in an irregular arrangement, particles can move and slide past each other, have fixed volume but not fixed shape, moderate to high density, medium particle energy
• Gases


  • Particles in random movement with no defined pattern, far apart and move quickly in all directions, no fixed volume or shape, low density, high particle energy
• Strength of forces between particles

  Determines the energy needed for state changes and the melting/boiling points
• Melting

  Solid changes to liquid, requires heat energy to overcome forces and allow increased particle movement
• Boiling


  Liquid changes to gas, requires heat energy to form gas bubbles and allow particles to escape the liquid surface
• Freezing


  Liquid changes to solid, reverse of melting, as temperature decreases forces increase and particles become fixed in place
• Interconversion of state


  A physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance
• Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state
• Melting

  1. Solid changes into a liquid

  2. Requires heat energy which transforms into kinetic energy, allowing the particles to move

  3. Occurs at a specific temperature known as the melting point which is unique to each pure solid
• Boiling

  1. Liquid changes into a gas

  2. Requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid

  3. Occurs at a specific temperature known as the boiling point which is unique to each pure liquid
• Freezing

  1. Liquid changes into a solid

  2. Reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same

  3. Water freezes and melts at 0 ºC

  4. Occurs at a specific temperature which is unique for each pure substance
• Evaporation

  1. Liquid changes into a gas

  2. Occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid

  3. The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate

  4. Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface
• Condensation

  1. Gas changes into a liquid, usually on cooling

  2. When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid

  
• Sublimation

  1. Solid changes directly into a gas

  2. Happens to only a few solids, such as iodine or solid carbon dioxide

  3. The reverse reaction is called desublimation or deposition

  
• Particle theory

  • Considers all particles, irrespective of their state or chemical identity, to be small, solid and inelastic
  • Doesn't consider the difference caused by different particles, such as atoms, ions or molecules or mixtures of all three
  • Fails to consider the intermolecular forces that exist between different particles in different substances
• The amount of energy needed to change from a solid to a liquid and from a liquid to a gas depends on the relative strength of the forces acting between the particles
• The stronger the forces between the particles, the higher the energy needed for melting and boiling to occur