This, like atomic structure, is also one of the fundamental topics in chemistry. A thorough knowledge of it is essential for the understanding of most of the other topics. Associated topics include electronegativity, properties of compounds,
crystal structure and molecular shapes.
Definitions and Laws
The definitions and laws that need to be known and understood are listed below. It is also important to be able to give examples of any of these if required:
- Octet rule and its limitations
- Ionic bond
- Covalent bond (pure and polar)
- Sigma (σ) & pi (π) bonds
- Double bond
- Lone pair
- Hydrogen bond
- Electronegativity and its uses, trends in values
Properties of Compounds
Both ionic and covalent compounds have their own characteristic properties (melting and boiling points, electrical conductivity, solubility, etc), and these properties result from their structure. These properties, and the reasons for them, need to be known.
Predicting Bond Types
A good guide to deciding on the bond type in a compound is the 1.7 rule, i.e., if the difference between the electronegativities of two joined atoms is more than 1.7, the bond is mainly ionic; if the difference is less, it is mainly covalent. However, this rule is only a guide and there are exceptions to it.
Hydrogen Bonding and Related Properties
The abnormal properties of water caused by hydrogen bonding are (i) its very high boiling point, (ii) its high specific latent heats, (iii) its expansion as it freezes, (iv) its excellent solvent powers and (v) the crystalline structure of ice.
Valency and Formula
The valency of an element is equal to the number of electrons which an atom of that element must either gain or lose to attain a noble gas structure. Valency is a measure of combining power and must be considered when working out the formula of a compound. The normal valency of an element is equal to either its group number or 8 minus the group number. Where an element has more than one valency, the appropriate valency is indicated in Roman numerals in brackets immediately after the symbol of the element,
e.g., FeCl2 = iron(II) chloride, FeCl3 = iron(III) chloride.
It is necessary to know what a crystal is, what are the building blocks in ionic crystals and what holds these together. For sodium chloride, which is the most frequently asked example, the building blocks are Na+ and Cl– ions, and they are held together by ionic bonds.
Shapes of Molecules
The shape of a molecule depends mainly on the electron structure of the central atom of the molecule. Using the valence shared electron-pair repulsion (VSEPR) theory, the shape can be predicted by making use of a few simple rules, which will be found in text books, and which should be known. To predict the shape of a molecule:
- Draw its electron structure
- Count the total number of electron pairs (both bonding pairs and lone pairs) in the outer shell of the central atom
- Predict the shape and the bond angle from the appropriate rule
- If there is a lone pair of electrons, discard one bond and reduce the bond angles by 2½°.