Ionic and covalent bonding: SAQ 2

Chemical Bonding

• Chemical bonding occurs when 2 or more elements combine in a chemical reaction, forming a compound.
• A compound is a substance made up of two or more different elements chemically combined.
• Examples of chemical formulas: H₂O, SO₂, HCl

Water of Crystallisation

• Hydrated substances contain molecules of water in definite proportions.
• Water bound in this way is called water of crystallisation.
• Heating removes this water, and compounds without this water attached are called anhydrous.

The Octet Rule

• Noble gases are extremely stable due to their electronic configuration, which includes 8 electrons in their outer energy level.
• The Octet Rule states that when bonding occurs, atoms tend to reach an electronic arrangement with 8 electrons in their outermost energy level.
• Exceptions to the Octet Rule:
  • Elements close to He tend to lose electrons to have 2 electrons in their outermost energy level.
  • Transition elements (d-block metals) do not obey the Octet Rule.
  • The rule sometimes works for sulphur and phosphorous, but not always.

Valency

• The valency of an element is the number of bonds an atom of the element forms when it reacts.
• Valency can be found by identifying the number of electrons an atom must lose, gain, or share to get a stable electronic configuration (i.e. Nobel Gas configuration).
• Examples of valency:
  • Sodium (Na): 1 bond
  • Oxygen (O): 2 bonds
  • Carbon (C): 4 bonds

Ionic and Covalent Bonding

• Ionic bonds are created by the transfer of electrons.
• Ionic bonds are formed between the electrostatic attraction (unlike charges attract) between the cation and anion.
• Examples of ionic bonds:
  • Lithium fluoride (LiF)
  • Calcium and flourine
• Covalent bonds involve the sharing of electrons.
• Covalent bonds are formed when two non-metal atoms share a pair of electrons.
• Examples of covalent bonds:
  • Hydrogen (H₂)
  • Hydrogen chloride (HCl)

Crystalline Structure of Ionic Compounds

• Ionic bonding does not create individual small molecules.
• Instead, crystal structures in the shape of a lattice are formed.
• The anions and cations attract each other and form a rigid 3-D shape.

Characteristics of Ionic Compounds

• High melting and boiling points.
• Solids at room temperature.
• Dissolve in water.
• Conduct electricity.
• Examples of ionic compounds:
  • Sodium chloride (NaCl)
  • Calcium carbonate (CaCO₃)

Covalent Bonding

• Covalent bonds can be formed between:
  • Atoms of the same element (e.g. H₂)
  • Atoms of different elements (e.g. HCl)
• Electrons in a covalent bond:
  • Bond pairs (shared electrons)
  • Lone pairs (unshared electrons)
• Examples of covalent bonds:
  • Water (H₂O)
  • Ammonia (NH₃)

Sigma and Pi Bonding

• Sigma (σ) bonds:
  • Formed by the end-on overlap of orbitals.
  • Single pair of electrons shared.
• Pi (π) bonds:
  • Formed by the sideways overlap of p-orbitals.
  • Multiple pairs of electrons shared.
• Examples of sigma and pi bonds:
  • Oxygen (O₂): 1 σ bond, 1 π bond
  • Nitrogen (N₂): 1 σ bond, 2 π bonds

Polar and Non-Polar Covalent Bonding

• Non-polar bonds:
  • Electrons shared equally between atoms.
  • Example: Hydrogen (H₂)
• Polar bonds:
  • Electrons shared unequally between atoms.
  • Example: Hydrogen chloride (HCl)
• Electronegativity:
  • The ability of an atom in a covalent bond to attract the shared electrons.
  • Examples of electronegativity values:
    • Hydrogen (H): 2.20
    • Chlorine (Cl): 3.16

Predicting Bonds

• Electronegativity values can be used to predict what types of bonds will be formed between atoms.
• The greater the difference in electronegativity values, the greater the degree of polarity.
• Examples of predicting bonds:
  • CS₂: non-polar covalent bond
  • HBr: polar covalent bond
  • NaCl: ionic bond