Shapes of molecules

• What do metal atoms do to form positive ions?

  Metal atoms lose electrons to form +ve ions.
• What do non-metal atoms do to form negative ions?

  Non-metal atoms gain electrons to form -ve ions.
• How does magnesium change when it forms a Mg2+ ion?

  Mg goes from 1s$^2$ 2s$^2$ 2p$^6$ 3s$^2$ to Mg2+ 1s$^2$ 2s$^2$ 2p$^6$.
• How does oxygen change when it forms an O2- ion?

  O goes from 1s$^2$ 2s$^2$ 2p$^4$ to O2- 1s$^2$ 2s$^2$ 2p$^6$.
• What factors affect the strength of ionic bonding?

  Ionic bonding is stronger with smaller ions and/or higher charges.
• Why does MgO have a higher melting point than NaCl?

  MgO has smaller ions (Mg2+ & O2-) with higher charges than NaCl (Na+ & Cl-).
• What is the structure of ionic crystals?

  Ionic crystals have the structure of giant lattices of ions.
• What is the trend in ionic radii across a period and down a group?

  • Ionic radii decrease across a period due to increasing nuclear charge.
  • Ionic radii increase down a group due to additional electron shells.
• Why are positive ions smaller than their corresponding atoms?

  Positive ions are smaller because they have one less shell of electrons and a higher ratio of protons to electrons.
• Why are negative ions larger than their corresponding atoms?

  Negative ions are larger because they have more electrons than protons, reducing the attraction per electron.
• What is the definition of ionic bonding?

  Ionic bonding is the electrostatic force of attraction between oppositely charged ions formed by electron transfer.
• What is a covalent bond?

  A covalent bond is a shared pair of electrons.
• What is a dative covalent bond?

  A dative covalent bond forms when the shared pair of electrons comes from only one of the bonding atoms.
• What is metallic bonding?

  Metallic bonding is the electrostatic force of attraction between the positive metal ions and the delocalised electrons.
• What are the three main factors that affect the strength of metallic bonding?

  1. Number of protons/strength of nuclear attraction.
  2. Number of delocalised electrons per atom.
  3. Size of ion.
• Why does magnesium have stronger metallic bonding than sodium?

  Mg has more outer shell electrons and a smaller ion size, leading to stronger bonding.
• What are the properties of ionic compounds?

  • High melting and boiling points due to strong ionic bonds.
  • Generally good solubility in water.
  • Poor conductivity when solid.
  • Good conductivity when molten.
• What are the properties of molecular (simple) compounds?

  • Low melting and boiling points due to weak intermolecular forces.
  • Generally poor solubility in water.
  • Poor conductivity when solid and molten.
• What are the properties of macromolecular compounds?

  • High melting and boiling points due to strong covalent bonds.
  • Insoluble in water.
  • Poor conductivity when solid.
• What are the properties of metallic compounds?

  • High melting and boiling points due to strong metallic bonds.
  • Good conductivity when solid and molten.
  • Malleable due to identical positive ions in the lattice.
• What are the common molecular shapes and their bond angles?

  • Linear: 180° (e.g., CO2)
  • Trigonal planar: 120° (e.g., BF3)
  • Tetrahedral: 109.5° (e.g., SiCl4)
  • Trigonal pyramidal: 107° (e.g., NCl3)
  • Bent: 104.5° (e.g., H2O)
  • Trigonal bipyramidal: 90° and 120° (e.g., PCl5)
  • Octahedral: 90° (e.g., SF6)
• How does the presence of lone pairs affect bond angles?

  Lone pairs repel more than bonding pairs, reducing bond angles by about 2.5° per lone pair.
• What is electronegativity?

  • Electronegativity is the tendency of an atom to attract electrons in a covalent bond.
  • Increases across a period and decreases down a group.
• Which atoms are the most electronegative?

  F, O, N, and Cl are the most electronegative atoms.
• What happens to electronegativity across a period and down a group?

  Electronegativity increases across a period and decreases down a group.
• Why does electronegativity increase across a period?

  Electronegativity increases due to increasing protons and decreasing atomic radius.
• Why does electronegativity decrease down a group?

  Electronegativity decreases due to increased distance from the nucleus and increased shielding.
• What type of molecular shape is formed with 4 bond pairs and 1 lone pair?

  It is a variation of the trigonal bipyramidal shape.
• What is the bond angle in a square planar molecule?

  90°
• How does the presence of a lone pair affect bond angles?

  It reduces the bond angle from the ideal value.
• What is the bond angle in a linear molecule?

  180°

  
• What is electronegativity?

  It is the tendency of an atom to attract electrons in a covalent bond.
• Which atoms are the most electronegative?

  F, O, N, and Cl
• How does electronegativity change across a period?

  It increases as the number of protons increases and atomic radius decreases.
• How does electronegativity change down a group?

  It decreases due to increased distance and shielding of outer electrons.
• What type of bond forms when there is a small electronegativity difference?

  A purely covalent bond forms.
• What is a polar covalent bond?

  A bond formed when elements have different electronegativities.
• What does a polar covalent bond produce?

  It produces a charge separation, creating a dipole.
• What is the electronegativity difference range for polar covalent bonds?

  0.3 to 1.7
• What type of bond forms when there is a large electronegativity difference?

  An ionic bond forms.