Topic 4 and 14 – Bonding
Allotrope: Occur when an element can exist in different crystalline forms, such as in carbon, which can exist as graphite, buckminsterfullerene (bucky ball) and diamond. Diamond is exceptionally hard because there is no plane of weakness in the molecule made up of sp3 hybridized carbon atoms. In graphite, the carbon atoms are sp2 hybridized. Remaining e- after the three σ bonds, are delocalized, resulting in the fact that graphite is an excellent lubricant and a good conductor of electricity.
Bond length and strength: Depends on strength of attraction that two nuclei have for the shared e-. Generally, the stronger the bond, the shorter its length.
Bond polarity: A polarity caused by a difference in electronegativity between the elements. The greater the difference, the greater the polarity.
Bond, π: Pi bond. A radial bond formed by the sideways overlap of p orbitals with e- densities concentrated above and below a line drawn through the two nuclei. Double bonds have one π bond, while triple bonds have two which are perpendicular to each other.
Bond, σ: Sigma bond. An axial bond formed by the head on/head-to-head overlap of atomic orbitals from two different atoms along the line drawn through the two nuclei, with e- densities concentrated along the line. Single, double and triple bonds have one σ bond.
Covalent bond: Bonding by the sharing of e-. The e- are shared and attracted by both nuclei resulting in a directional bond between the two atoms.
Dative bond: A bond in which both e- come from one of the atoms. Also known as coordinate bond.
Ionic bond: A bond by which e- are transferred from one atom to another to form ions with complete outer shells. In an ionic compound the + and – ions are attracted to each other by the electrostatic force between them, and build up into a strong lattice. Have relatively high m.p. Ionic bonds occur between elements with a great difference (>1.8) in electronegativity.
Conductivity: The extent to which a substance can conduct electricity. Must possess e- or ions that are free to move.
Delocalization: The sharing of a single e- pair by more than two atoms.
Forces, dipole-dipole: Permanent electrostatic forces of attraction between polar molecules. Stronger than van der Waals’.
Forces, Hydrogen bonding: Occurs when hydrogen is bonded directly to a highly electronegative element (N, F, or O). Stronger than dipole:dipole forces.
Forces, van der Waal’s: Temporary dipole forces due to momentary unevenness in spread of e-. Weakest of intermolecular forces. Increase with increasing molar mass.
Formal charge theory:
If compound is covalent, treat the entire molecule; if ionic, treat each ion separately.
Determine total no. of valence e- available.
Organize atoms such that there is a central atom (usually the most electronegative) surrounded by ligands.
i) One pair of e- between central atom and each ligand atom.
ii)three more pairs on each ligand (except hydrogen).
iii)remaining e- pairs around central atom.
Calculate formal charge on central atom. Should be equal to charge on species, otherwise form a multiple bond with ligand atom that has most – formal charge. Repeat 5.
Formal charge: A hypothetical measure of the number of e- originating from an atom. F = V – (n + b/2), where V is valence e-, n is e- in lone pairs and b is e- shared as bonds.
Hybridization: The combination of orbitals to create new orbitals which are more energetically feasible for bonding.
Lewis structure: Diagram showing arrangement of e- in a molecule. Usually only shows valence shells.
Metallic bonding: The valence e- in metals become detached from the individual atoms so that the metals consist of a closely packed lattice of + ions in a ‘sea’ of delocalized e-. Forces of attraction are between ions and e- and not between the ions themselves, which means that metals are malleable and ductile.
Molecular polarity: Depends on both the bond polarity and the symmetry.
Resonance hybrid: Structures that arise from the possibility to draw a multiple bond in different positions equivalently. Can be better explained by delocalization.
Solubility: The extent to which one substance dissolves in another. ‘Like tends to dissolve like.’
VSEPR theory: Valence Shell Electron Pair Repulsion theory. States that pairs of e- arrange themselves around the central atom so that they are as far apart from each other as possible. Greater repulsion between lone pair of e- than bonded pairs.