The structure of atoms is discussed in terms of the occupation of atomic orbitals by the electrons in that atom. Similarly, the structure of a molecule can be discussed in terms of molecular orbitals. The electronic structure of an atom is based on the approximation that the overall wavefunction describing the atom is the product of the wavefunctions … Read more
The Valence Shell Electron Pair Repulsion Theory is a way of predicting the shape of a molecule based on the number of bonding and lone pairs of electrons in a polyatomic species. It is based on the fact that these electrons pairs interact and repel each other due the electrostatic repulsion. In doing so, they adopt a … Read more
The Lewis scheme of bonding involves the formation of a covalent bond by the sharing of two electrons between two species. Double and triple covalent bonds have two and three sets of shared electron pairs, whilst lone pairs are valence electron pairs which do not contribute to bonding. The structures adopted by many compounds can be accounted for by the octet rule. This states … Read more
Trends in solid compounds: the importance of lattice enthalpy Solid compounds of Group 1 and Group 2 elements: All these elements form stable simple binary hydrides, halides, oxides and hydroxides with the group oxidation state. The +1 oxidation state of Group 2 is unstable with respect to disproportionation. The stability of compounds with small anions (F–, H–, OH–,O2-) decrease down the groups, … Read more
The Born-Mayer equation gives the total electrostatic interaction energy for a given structure in terms of the Madelung constant, A, for that structure. Something else that varies from structure to structure is the number of ions within the formula unit for a given structure. Structure Coordination Number Madelung constant, A A/ν ν is the number of ions in the … Read more
When a crystalline solid is broken up and the gas phase ions formed, the enthalpy change accompanying the process is known as the Lattice Enthalpy. It takes energy to counteract the coulombic forces which hold the solid together, and so the lattice enthalpy is endothermic. The enthalpy of reaction for this process is the lattice enthalpy, ΔHL. In general, the crystal structure adopted by a given compound is … Read more
In the ionic model, the bonding is described as the electrostatic interaction between charged spheres, whose sizes are given by the ionic radius. In determining an ionic radius, it is necessary to split up the internuclear separation into a contribution from the anion and a contribution from the cation. This is most often done by assuming the value of the … Read more
Metals have structures which may be discussed in terms of the close packing of spheres, and as a result have the high coordination numbers of the close packed systems. Ionic solids, however, have lower coordination numbers, and the discussion of the structure simply in terms of close packed species has to be adapted. The idea … Read more
When systems crystallize, they adopt the structure whose configuration has the lowest energy crystal structure. In ionic solids, this is mainly determined by the coulombic interaction between the ions: a maximization of the attractive forces between oppositely charged ions and a minimization of the repulsion between like charged ions. The arrangement of ions in a crystal is known as … Read more
The very small size of the Li+ and Be2+ ions means that they have a large charge to size ratio and so show anomalous behaviour. The Li+ ion has a charge to size ratio comparable to that of Mg2+, and hence the properties of some lithium compounds resemble those of magnesium compounds. Many of these anomalous properties arise because … Read more