Topic 4: Chemical Bonding and Structure

4.1 Ionic Bonding and Structure

Octet rule

  • Electrons tend to lose, gain or share electrons in order to acquire a noble gas core electron configuration.

Ions

  • Cation: an atom that lost electrons and is positively charged.

  • Anion: an atom that gained electrons and is negatively charged.

Definition: The electrostatic attraction between oppositely charged ions.

Conditions:

  • Formed when one or more electrons are transferred from one atom to another. Atoms are either oxidized (loses electrons) or reduced (gains electrons).

  • Under normal conditions ionic compounds are typically solids and have a lattice type structure, as shown bellow.

Physical proprieties 

  • High melting and boiling points

  • Low volatility

  • Only conduct electricity when molten or in aqueous solution

  • Dissolve in polar solvents 

Common molecules

  • Ammonium:

  • Hydroxyl:

  • Nitrate:

  • Hydrogen carbonate:

  • Carbonate:

  • Sulphate:

  • Phosphate:

4.2 Covalent Bonding

Bonds

  • Definition: Electrostatic attraction between a shared pair of electrons and the positively charged nuclei.

  • Single bond: a bond that has only one pair of shared electrons.

  • Double bond: a bond that has two pairs of shared electrons.

  • Triple bond: a bond that has three pairs of shared electrons.

  • Bond length: Triple<Double<Single

  • Bond strength Single<Double<Triple

  • Electronegativity: The relative attraction that an atom of an element has for the shared pair of electrons.

  • Bond Polarity: Due to the difference in the electronegativity of the atoms in a bond.

  • Important: Bond polarity should not be mixed with molecular polarity!!!

  • Microwave: Water molecules absorb microwave radiation and constantly change their orientation to align with the waves, thus heating the food.

Bond polarity/character in relation to electronegativity

4.3 Covalent Structures

Lewis (electron dot) structure

  • Representation of molecules

Valence shell electron pair repulsion (VSEPR theory)

  • Used to deduce the shape of covalent molecules

Molecular geometry

  • Linear

    • number of domains:​ 2

    • Bond angle: 

    • Shape:

    • examples:

Valence shell electron pair repulsion (VSEPR theory)

  • Used to deduce the shape of covalent molecules

Molecular geometry

  • Linear

    • number of domains:​ 2

    • Bond angle: 

    • Shape:

    • examples:

  • Trigonal planar

    • number of domains:​ 3

    • Bond angle:

    • Shape:

    • examples:

  • V-shaped (bent)

    • number of domains:​ 3

    • Bond angle:

    • Shape:

    • examples:

  • Tetrahedral

    • number of domains: 4​

    • Bond angle:

    • Shape:

    • examples:

  • Trigonal pyramidal

    • number of domains:​ 4

    • Bond angle:

    • Shape:

    • examples:

  • V-shaped (bent)

    • number of domains:​ 4

    • Bond angle:

    • Shape:

    • examples:

Resonance structures

  • A molecule that has the same atom arrangement but different electron arrangement.

Note: Molecules may have the same V-shaped (bent) shape however may have different number of domains!

Molecular polarity

Using vectors and the atom structure we have:

Allotropes

  • Molecules that have the same molecular formula but different molecular structures

  • Examples: Oxygen gas (      ) and ozone (      )

  • Carbon allotropes:
    • Graphite​

      • Covalent bonds​

      • layered structure

      • London Forces between layers

    • Diamond

      • Covalent bonds

      • tetrahedral structure

    • Graphene

      • Covalent bonds

      • 2 dimensional crystal

      • One single layer

    • C 60 Fulerene

      • Molecular structure

      • Spherical (buckyballs, soccer ball shape) structure

Note: You should be able to compare and contrast the carbon allotropes.

Coordinate covalent bond

  • When a shared pair of electrons comes from a single atom

  • Example: 

Silicon dioxide, sio2

  • Giant lattice structure of SiO4 arrays

  • High melting and boiling point

  • As shown bellow each Si atom (Black) bonds with 4 O atoms (Red)

Exceptions to the octet rule

  • There are few exceptions to the octet rule

  • Examples:

4.4 Covalent Structures

van der Waals forces= London forces +Dipole-Dipole forces

Order of strength

London Forces< Dipole-Dipole Forces<Hydrogen Bonds

London Forces

  • Dispersion forces of instantaneous induced dipole-induced dipole forces

  • Even though the molecules are non-polar, one part of the molecule at a given instant of time has a slightly bigger electron density than the other part

  • Factors that affect London Forces:

    • Number of electrons:​ more electrons lead to more polarization

    • Shape of the molecule: the bigger the carbon chain, the bigger the boiling point ( this is further explained in Topic 10)

    • size (volume) of the electron cloud: the bigger the size, the smaller the attraction, therefore the bigger the polarization.

Dipole-Dipole forces

  • ​Permanent dipole forces

Hydrogen Bonds

  • The attraction between hydrogen and a pair of non-bonding electrons of F, O or N

4.5 Metallic Bonding

Lattice that consists of positive ions and surrounded by a "sea" of electrons

 

Proprieties:

  • High electric conductivity

  • High thermic conductivity

  • High melting and boiling points

  • Solid at Standard conditions (Exception: Hg)

 

Factors that affect the strength:

  • Number of Valence electrons

  • Charge of metal ions

  • ionic radius of ions

 

Alloys

  • 2 or more metals in the bond (there may be non-metals like carbon)

    • Brass = copper + zinc​

    • Steel = iron + carbon + tungsten and others

    • Dental amalgaman: mercury + zinc + tin

      • Harmful ​

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