Topic 4: Structure and Bonding

• Ionic bonding


  Electrostatic attraction between the electric charges of a cation (positive ion) and an anion (negative ion)
• Formation of ions


  1. Atoms lose or gain electrons to form cations and anions

  2. Driven by the formation of a noble gas electron configuration
• Formation of sodium and chloride ions

  • Sodium loses 1 electron to form Na+
  • Chlorine gains 1 electron to form Cl-
• Formation of magnesium and oxide ions

  • Magnesium loses 2 electrons to form Mg2+
  • Oxygen gains 2 electrons to form O2-
• Ionic compounds


  • Usually solids with lattice structures consisting of repeating units of positive and negative ions
• The octet rule states that elements tend to lose, gain or share electrons to acquire a noble gas electron configuration
• Physical properties of ionic compounds

  • High melting and boiling points due to strong electrostatic forces
  • Low volatility due to strong electrostatic forces
  • Solid ionic compounds do not conduct electricity, but molten ionic compounds do
  • Soluble in polar solvents like water, insoluble in non-polar solvents
• Ionic liquids are efficient solvents and electrolytes, used in electric power sources and green industrial processes
• Covalent bond


  Electrostatic attraction between a shared pair of electrons and the positively charged nuclei
• Types of covalent bonds

  • Single bond - 1 shared pair of electrons

  Double bond - 2 shared pairs of electrons

  Triple bond - 3 shared pairs of electrons

  Bond length decreases and bond strength increases as number of shared electrons increases
• Bond polarity


  Results from the difference in electronegativities of the bonded atoms
• Single bond


  Shared pair can be represented by a line
• Lewis structure of F2

  • Total of six non-bonding pairs of electrons (lone pairs)
  • One bonding pair of electrons
• Oxygen, O2


  In group 16, has six valence electrons<|>Acquiring two more electrons attains a noble gas electron configuration with a complete octet
• Formation of covalent bond between two oxygen atoms

  1. Each oxygen atom shares two electrons

  2. Results in a double bond
• Lewis structure of O2

  • Total of four non-bonding pairs of electrons (lone pairs)
  • Two bonding pairs of electrons
• Nitrogen, N2


  In group 15, has five valence electrons<|>Acquiring three more electrons attains a noble gas electron configuration with a complete octet
• Formation of covalent bond between two nitrogen atoms


  1. Each nitrogen atom shares three electrons

  2. Results in a triple bond
• Lewis structure of N2

  • Total of two non-bonding pairs of electrons (lone pairs)
  • Three bonding pairs of electrons
• Hydrogen fluoride, HF


  Fluorine in group 17, has seven valence electrons<|>Acquiring one more electron attains a noble gas electron configuration<|>Hydrogen in group 1, has one valence electron<|>Acquiring one more electron attains the noble gas configuration of helium
• Formation of covalent bond between hydrogen and fluorine

  1. Fluorine and hydrogen each share one electron

  2. Results in a single bond
• Lewis structure of HF


  • Total of three non-bonding pairs of electrons (lone pairs)
  • One bonding pair of electrons
• Bond strength


  Triple bond > Double bond > Single bond
• Bond length


  Single bond > Double bond > Triple bond
• Electronegativity


  Relative attraction that an atom has for the shared pair of electrons in a covalent bond
• Fluorine is the most electronegative element with a Pauling electronegativity value of 4.0
• Electronegativity increases from left to right across a period due to decreasing atomic radii and increasing nuclear charge
• Electronegativity decreases down a group due to increasing atomic radii and primary screening (shielding) effect of inner electrons
• Ionic bond


  Formed between a cation (usually metal) and an anion (usually non-metal)
• Covalent bond


  Formed from atoms sharing electrons with each other to attain a noble gas electron configuration
• Ionic compounds have lattice structures, covalent compounds consist of molecules
• Ionic compounds have higher melting and boiling points, covalent compounds have lower melting and boiling points
• Ionic compounds tend to be soluble in water, covalent compounds are typically insoluble in water
• Ionic compounds conduct electricity in the molten state, covalent compounds do not conduct electricity
• Polar covalent bond


  Unequal sharing of the shared pair of electrons, resulting in partial charges δ+ and δ-
• Non-polar covalent bond


  Equal sharing of the shared pair of electrons, no partial charges
• ΔχP > 1.8 indicates ionic bonding, ΔχP = 0 indicates non-polar covalent bonding, 0 < ΔχP ≤ 1.8 indicates polar covalent bonding
• Microwaves interact with polar water molecules in food, causing them to flip and rotate, dissipating heat energy and increasing the temperature of the food
• representation of microwaves (showing their oscillating capacity) interacting with the water molecules present in food and causing them to flip
• Molecular


  Compounds that contain only non-metals and no ions