Structure and bonding: 2.20 - Shapes of molecules

The shape of molecule (or ion) is the shape adopted by the atoms in a molecule (or ion) with respect to one another.

Confusion arises occasionally, because the shape adopted by the regions of electronic charge (electron domains) around an atom has to be known before determining the location of the atoms. Students confuse this electronic organisation with the molecular shape. In fact, they may be the same, but often they are not.

Nature of science:

Scientists use models as representations of the real world-the development of the model of molecular shape (VSEPR) to explain observable properties.

The need to regard theories as uncertain - hybridization in valence bond theory can help explain molecular geometries, but is limited. Quantum mechanics involves several theories explaining the same phenomena, depending on specific requirements.

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 form giant covalent/network covalent structures.

Understandings - HL

Essential idea: Larger structures and more in-depth explanations of bonding systems often require more sophisticated concepts and theories of bonding.

Covalent bonds result from the overlap of atomic orbitals. A sigma bond (s) is formed by the direct head-on/end-to-end overlap of atomic orbitals, resulting in electron density concentrated between the nuclei of the bonding atoms. A pi bond (p) is formed by the sideways overlap of atomic orbitals, resulting in electron density above and below the plane of the nuclei of the bonding atoms.

Formal charge (FC) can be used to decide which Lewis (electron dot) structure is preferred from several. The FC is the charge an atom would have if all atoms in the molecule had the same electronegativity. FC = (Number of valence electrons) - ½(Number of bonding electrons) - (Number of non-bonding electrons). The Lewis (electron dot) structure with the atoms having FC values closest to zero is preferred.

Exceptions to the octet rule include some species having incomplete octets and expanded octets.

Delocalization involves electrons that are shared by/between all atoms in a molecule or ion as opposed to being localized between a pair of atoms.

Resonance involves using two or more Lewis (electron dot) structures to represent a particular molecule or ion. A resonance structure is one of two or more alternative Lewis (electron dot) structures for a molecule or ion that cannot be described fully with one Lewis (electron dot) structure alone.

Essential idea: Hybridization results from the mixing of atomic orbitals to form the same number of new equivalent hybrid orbitals that can have the same mean energy as the contributing atomic orbitals.

A hybrid orbital results from the mixing of different types of atomic orbitals on the same atom.

Applications and 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 nonbonding pairs of electrons.

Prediction of molecular polarity from bond polarity and molecular geometry.

Deduction of resonance structures, examples include but are not limited to C₆H₆, CO₃^2- and O₃.

Explanation of the properties of giant covalent compounds in terms of their structures.

Applications and skills - HL

Prediction whether sigma (s) or pi (p) bonds are formed from the linear combination of atomic orbitals.

Deduction of the Lewis (electron dot) structures of molecules and ions showing all valence electrons for up to six electron pairs on each atom.

Application of FC to ascertain which Lewis (electron dot) structure is preferred from different Lewis (electron dot) structures.

Deduction using VSEPR theory of the electron domain geometry and molecular geometry with five and six electron domains and associated bond angles.

Explanation of the wavelength of light required to dissociate oxygen and ozone.

Description of the mechanism of the catalysis of ozone depletion when catalysed by CFCs and NOx.

Explanation of the formation of sp³, sp² and sp hybrid orbitals in methane, ethene and ethyne.

Identification and explanation of the relationships between Lewis (electron dot) structures, electron domains, molecular geometries and types of hybridization.

In Section 2.2