What elements form basic and acidic oxides

Lecture: Structural Chemistry of Oxides

There are very different reasons to motivate a lecture Structural chemistry of oxides call. These include:
  • Frequency of oxygen
    Oxygen is the most abundant element on the earth's surface. It makes up 23 percent by mass of the atmosphere, 85 percent of the hydrosphere (H.2O) and 46 percent of the lithosphere. In the lithosphere it is in the form of compounds that are solid (structural chemistry!) Under normal conditions.
  • Formation of connections with oxygen
    Almost all elements (with the exception of the light noble gases) form binary oxides, and most elements even form several different ones. The variety of ternary or quaternary connections is correspondingly greater.
  • Physical Properties
    The physical properties of the oxides range from gases that are difficult to condense (e.g. CO) to the most heat-resistant materials (e.g. ZrO2). The electrical properties vary from insulators (e.g. MgO) to semiconductors (e.g. NiO) to metallic conductors (e.g. ReO3).
  • Technical importance
    Solid oxides are of great interest technically as basic materials (e.g. pigments, ceramics, glasses), in the chemical industry (e.g. catalysts), materials technology (hard materials) and electronics (components, magnetics, ferroelectrics, HT superconductors).
  • Diverse structural chemistry
    The structural chemistry of the oxides is characterized by different bond characters, which range from ionic to covalent to metallic in character. In addition to crystalline compounds, there are glasses and ceramics (non-crystalline solids). There are examples of stoichiometric and non-stoichiometric compounds. The coordination numbers at oxygen cover a wide range from 1 (e.g. in CO) to 8 (e.g. in Li2O)
The points mentioned indicate the oxides as an interesting and diverse class of substances (not only in terms of their structural chemistry). The aim of the lecture is therefore not only to repeat and expand the knowledge of one of the most important compound classes, but also to deepen the principles and ways of thinking of inorganic solid-state chemistry using the example of a group of compounds.
For reasons of time, the treatment of many ternary and higher oxides (e.g. silicates) must be avoided. The treatment of binary oxides (chapters 2 and 3) is structured as follows: The elements highlighted in red form predominantly covalent oxides, which are described in chap. 2 - arranged according to main groups - can be considered. With a few exceptions, the oxides of the elements highlighted in blue can be treated according to their composition, see Chap. 3. The elements shown in black form oxides which can be assigned to the field of molecular chemistry and which are gases or liquids under normal conditions, so that their structural chemistry is only of subordinate importance here. In chap. 4, selected non-stoichiometric oxides (crystallographic shearings, etc.) are dealt with. In chap. 5 are structure field diagrams using the example of compounds of the composition AB2O4 discussed.
The complete table of contents is available on template 0.
A chemical classification the oxides can, for example, be based on the acid-base properties. A distinction can be made between:
  • acidic oxides = most non-metal oxides (e.g. CO2, P4O10, SO3)
  • basic oxides = oxides of electropositive elements (e.g. Na2O, Tl2O, La2O3)
  • amphoteric oxides = oxides of less electropositive elements (e.g. BeO, Al2O3)
  • neutral oxides = no reaction with water (e.g. CO, NO)
With regard to these properties, the following tendencies emerge in the PSE:
  • In a period the acidic character increases:
    N / A2O < mgo=""><>2O3 <>2 <>4O10 <>3
    basic - amphot. - sour - strongly sour
  • For an element, the acidity increases with increasing oxidation number:
    MnO <>2O3 <>2 <>2O7
  • In the main groups, the basicity increases with increasing atomic number:
    BeO < mgo="">< cao="">< sro="">< bao="">
Corresponding regular courses can also be seen in the PSE with regard to many physical properties (e.g. the melting points).

The structural chemical classification the oxides can be made according to the components / structural features or the nature of the bond. On the basis of the linking of the MOn-Polyhedra, oxides form a wide variety of structural elements:

  • molecular units (e.g. RuO4, Sb4O6)
  • Chains (e.g. HgO, SeO2, CrO3)
  • Layers (MoO3, PbO, Re2O7)
  • Networks (SiO2, typical salts)
As for all chemical compounds, the type of structures and the bonding relationships that are formed in the solid state initially depend on the sum of the electronegativities of the bonding partners and the difference in the electronegativities (see also Section 1.2 of the lecture Inorganic Structural Chemistry Oxygen with an EN of 3.5 is the second strongest electronegative element, i.e. the sum of the EN is always large in oxygen compounds (with the exception of the metal-rich suboxides), but the difference can either be small (non-metals = covalent oxides with directed bonds) or large (metals = Ionic crystals with undirected bonds) According to this, two essential classes of oxides can be differentiated according to their bond character, whereby the transitions between the two are of course fluid.
  1. predominantly covalent oxides
    In these compounds, the hybridization of the oxygen determines the stereochemistry at the O:
    • sp = linear (e.g. [Cl5RuIV-O-RuCl5]4-) (2 double bonds)
    • sp2 = angled, ideally 120 (e.g. O3) (1 double bond)
    • sp3 = angled, ideal 109 (e.g. H.3O+) (0 double bonds)
    The occurrence of the various structure types is therefore dependent on the stoichiometry and the formation of pi-pi double bonds to the oxygen. The relative atomic sizes are only of secondary importance.
  2. predominantly ionic oxides
    consist of O2-Ions and metal cations. As with all salts, the ionic charge and the proportions are the primary structure-determining factors. In concrete terms, this means for oxygen: the ionic radius of the oxide ion O2 with a coordination number (CN) of 6 is approx. 140 pm and is therefore larger than the typical cation radii. Therefore, one very often finds densely packed oxide sublattices with filled octahedral and tetrahedral gaps. In this context, it is extremely helpful to consider the closest packing.
  3. Oxides with metallic bond components
    are only formed with the heavy alkali metals and some transition metals (suboxides). In these compounds there is an ionic and metallic bond character - spatially separated from one another.