module 2

• isotopes = atoms of the same element with different numbers of neutrons
• isotopes have the same chemical properties because the have the same number of electrons in the outer shell and the same electron configuration
• relative isotopic mass = mass of the isotope compared to 1/12th of the mass of an atom of carbon 12
• graphite is in a giant covalent lattice in layers and it is a good conductor because it has delocalized electrons that are free to move and It has a high boiling point because strong covalent bonds have to be broken, also it is soft because there are weak londons forces between the layers which slide over each other
• covalent bond = shared pair of electrons
• ammonia has a pyramidal shape and a bond angle of 107 because there are 3 bonded pairs and 1 lone pair and lone pairs repel more than bonded pairs
• dot and cross diagram of NH4+

  
• NH4+ has a tetrahedral shape and an angle of 109.5
• compounds can conduct electricity when dissolved in water or in liquid state because ions and electrons cannot move in a solid but are mobile in solution and can therefore carry electrical charge
• salt = when the H+ in an acid is replaced by a metal ion
• adding aqueous silver nitrate to halogens:

  1. chlorine = white ppt
  2. bromine = cream ppt
  3. iodine = yellow ppt
• equation for halogen + aqueous silver nitrate:

  Ag+ (aq) + X- (aq) -> AgX (s)
• benefits of adding chlorine to water:

  1. removes or kills bacteria
  2. kills germs / microorganisms
  3. makes it safe to drink
  4. sterilises water
• disadvantages of adding chlorine to water:

  1. it is a naturally toxic element
  2. poisonous
  3. could form chlorinated hydrocarbons
  4. forms carcinogens or toxins
• disproportionation reaction = when an element has been both oxidised and reduced in the same reaction and its oxidation state has increased and decreased
• the number in brackets in roman numerals e.g chloric (V) acid stands for the oxidation number/state of chlorine
• across a period, e.g from Na to Mg in period 3, the charge increases so the more to the right the element is, the more outer electrons it has and it has a greater attraction between ions and electrons with stronger metallic bonds
• relative atomic mass = weighted mean mass of an atom compared to 1/12th of the mass of an atom of carbon 12
• when magnesium reacts with sulfuric acid the mg dissolves and it fizzes/effervesces and gas is produced
• metallic bonding is the electrostatic attraction between the electrons and the positive ions

  
• ionic bond = electrostatic attraction between oppositely charged ions
• calcium oxide reacting with water = Ca(OH)2
• calcium oxdie reacts with nitric acid = Ca(NO3)2
• atomic number = number of protons in the nucleus
• hydrogen bonding diagram:

  
• .xH2O part of a formula shows the number of waters of crystallisation
• hydrogen bonding diagram of ammonia: NH3

  
• electronegativity = the attraction of an atom for the shared pair of electrons in a covalent bond
• ionic compounds are formed when metals react with non-metals, they have high melting points and boiling points due to strong electrostatic attractions between oppositely charged ions.
• covalent bonds form between non-metal atoms, they can be polar or nonpolar depending on whether there is unequal sharing of electrons
• the larger the difference in electronegativities, the greater the polarity of the bond
• molecules that contain polar bonds but cancel out their effects so that there is no overall charge separation are called nonpolar
• a molecule that has no net dipole moment is called nonpolar
• the shape of a molecule depends on its polarity - linear if all bonds are polar, bent if one bond is polar, trigonal planer if two bonds are polar, tetrahedral if all four bonds are polar
• intermolecular forces are weak attractive forces between molecules
• dipole dipole interactions occur between two polar molecules where the positive end of one molecule attracts the negative end of another molecule
• hydrogen bonding occurs when hydrogen is attached to highly electronegative elements such as fluorine, oxygen or nitrogen
• in hydrogen bonding, the positive end of one molecule interacts with the negative end of another molecule
• hydrogen bonding causes substances to have higher boiling points than would otherwise be expected because it requires energy to break these weak interactions
• strong acids release all of its hydrogen atoms as H+ ions and completely dissociate into aqueous solutions