Mr. Strickland's Chemistry     pi-sigma

Topic 4: Chemical Bonding and Structure

 

4.1 Ionic bonding and structure

Essential idea: Ionic compounds consist of ions held together in lattice structures by ionic bonds.

Understandings:

  • Positive ions (cations) form by metals losing valence electrons.
  • Negative ions (anions) form by non-metals gaining electrons.
  • The number of electrons lost or gained is determined by the electron configuration of the atom.
  • The ionic bond is due to electrostatic attraction between oppositely charged ions.
  • Under normal conditions, ionic compounds are usually solid with lattice structures.

 

Applications & Skills:

  • Deduction of the formula and name of an ionic compound from its component ions, including polyatomic ions.
  • Explanation of the physical properties of ionic compounds (volatility, electrical conductivity, and solubility) in terms of their structure.

 

Guidance:

  • Students should be familiar with the names of these polyatomic ions: NH4+, OH-, NO3-, HCO3-, CO32-, SO42- and PO43-.

 

Theory of knowledge:

  • General rules in chemistry (like the octet rule) often have exceptions. How many exceptions have to exist for a rule to cease to be useful?
  • What evidence do you have for the existence of ions? What is the difference between direct and indirect evidence?

 

Utilization:

  • Ionic liquids are efficient solvents and electrolytes used in electric power sources and green industrial processes.

 

4.2 Covalent bonding

Essential idea: Covalent compounds form by the sharing of electrons.

Understandings:

  • A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.
  • Single, double, and triple covalent bonds involve one, two, and three shared pairs of electrons respectively.
  • Bond length decreases and bond strength increases as the number of shared electrons increases.
  • Bond polarity results from the difference in electronegativities of the bonded atoms.

 

Applications & Skills:

  • Deduction of the polar nature of a covalent bond from electronegativity values.

 

Guidance:

  • Bond polarity can be shown either with partial charges, dipoles or vectors.
  • Electronegativity values are given in the data booklet in section 8.

 

Utilization:

  • Microwaves—cooking with polar molecules.

 

4.3 Covalent structures

Essential idea: Lewis (electron dot) structures show the electron domains in the valence shell and are used to predict molecular shape.

Understandings:

  • Lewis (electron dot) structures show all the valence electrons in a covalently bonded species.
  • The 'octet rule' refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.
  • Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons.
  • Resonance structures occur when there is more than one possible position for a double bond in a molecule.
  • Shapes of species are determined by the repulsion of electron pairs according to VSEPR theory.
  • Carbon and silicon for giant covalent/network covalent/macromolecular structures.

 

Applications & Skills:

  • Deduction of Lewis (electron dot) structure of molecules and ions showing all valence electrons for up to four electron pairs on each atom.
  • The use of VSEPR theory to predict the electron domain geometry and the molecular geometry for species with two, three, and four electron domains.
  • Prediction of bond angles from molecular geometry and presence of non-bonding pairs of electrons.
  • Prediction of molecular polarity from bond polarity and molecular geometry.
  • Deduction of resonance structures, including C6H6, CO3-2, O3.
  • Explanation of the properties of giant covalent compounds in terms of their structures.

 

Guidance:

  • The term “electron domain” should be used in place of “negative charge center”.
  • Electron pairs in a Lewis (electron dot) structure can be shown as dots, crosses, a dash or any combination.
  • Allotropes of carbon (diamond, graphite, graphene, C60 buckminsterfullerene) and SiO2 should be covered.
  • Coordinate covalent bonds should be covered.

 

Theory of knowledge:

  • Does the need for resonance structures decrease the value or validity of Lewis (electron dot) theory? What criteria do we use in assessing the validity of a scientific theory?

 

4.4 Intermolecular forces

Essential idea: The physical properties of molecular substances result from different types of forces between their molecules.

Understandings:

  • Intermolecular forces include London (dispersion) forces, dipole-dipole forces, and hydrogen bonding.
  • The relative strength of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonding.
  • Bond length decreases and bond strength increases as the number of shared electrons increases.
  • Bond polarity results from the difference in electronegativities of the bonded atoms.

 

Applications & Skills:

  • Deduction of the types of intermolecular force present in substances, based on their structure and chemical formula.
  • Explanation of the physical properties of covalent compounds (volatility, electrical conductivity, and solubility) in terms of their structure and intermolecular forces.

 

Guidance:

  • The term “London (dispersion) forces” refers to instantaneous induced dipole- induced dipole forces that exist between any atoms or groups of atoms and should be used for non-polar entities. The term “van der Waals” is an inclusive term, which includes dipole–dipole, dipole-induced dipole and London (dispersion) forces.

 

Theory of knowledge:

  • The nature of the hydrogen bond is the topic of much discussion and the current definition from the IUPAC gives six criteria which should be used as evidence for the occurrence of hydrogen bonding. How does a specialized vocabulary help and hinder the growth of knowledge?

 

4.5 Metallic bonding

Essential idea: Metallic bonds involve a lattice of cations with delocalized electrons.

Understandings:

  • A metallic bond is the electrostatic attraction between a lattice of positively charged ions and delocalized electrons.
  • The strength of a metallic bond depends on the charge of the ions and radius of the metal ion.
  • Alloys usually contain more than one metal and have enhanced properties.

 

Applications & Skills:

  • Explanation of electrical conductivity and malleability in metals.
  • Explanation of trends in melting points of metals.
  • Explanation of the properties of alloys in terms of non-directional bonding.

 

Guidance:

  • Trends should be limited to s- and p-block elements.
  • Examples of various alloys should be covered.

 

International-mindedness:

  • The availability of metal resources, and the means to extract them, varies greatly in different countries, and is a factor in determining national wealth. As technologies develop, the demands for different metals change and careful strategies are needed to manage the supply of these finite resources.