Properties of Oxides Within a Period

One way to compare the properties of the main group elements across a period is to examine the properties of a series of similar compoimds. Because oxygen combines with almost all elements, we will compare the properties of oxides of the third-period elements to see how metals differ from metalloids and nonmetals. Some elements in the third period (P, S, and Cl) form several types of oxides, but for simplicity we will consider only those oxides in which the elements have the highest oxidation ntrmber. Table 7.4 fists a few general characteristics of these oxides and some specific physical properties of the oxides of third-period elements. 

Some Properties of Oxides of the Third-Period Elements 

Melting point (°C) Boiling point (°C) Acid-base nature 

Most oxides can be classified as acidic or basic depending on whether they produce acids or bases when dissolved in water (or whether they react as acids or bases). Some oxides are anqtho-teric, which means that they display both acidic and basic properties. The first two oxides of the third period, Na20 and MgO, are basic oxides. For example, Na20 reacts with water to form the base sodium hydroxide  

Magnesium oxide is quite insoluble it does not react with water to arty appreciable extent. However, it does react with acids in a manner that resembles an acid-base reaction  

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How do the chemical properties of oxides change from left to right across a period From top to bottom within a particular group ... 

For example, carbon and silicon are found within the same group in the periodic table. Considering the trends in a group, we would expect the oxides of these two elements, C02 and Si02, to display similar properties. However, Si02 is a solid with a quartz structure while C02 is a gas that has great importance in the life cycle. What can be the reason for these two compounds being so different ... 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

On the whole the (b) -character thus increases with the number of i-electrons. It follows that, for elements situated within that area of the periodic system where the -shells are still being built up, the (b) -properties of a certain element will be more marked, the lower its oxidation state. Moreover, if an element possesses only five -electrons, only the zero state will be able to unfold ( >)-properties, in accordance with the fact that any oxidation to a higher state would mean a decrease of the number of -electrons below what is generally demanded for the formation of the fairly covalent ( >)-type bond. The creation, or increase, of a positive charge on the acceptor as a consequence of the oxidation will also per se be of great importance for the bond formation, as will be further discussed later on. 

The concept of an atom s oxidation state see Oxidation Number) can provide fundamental information about the stmcture and reactivity of the compound in which the atom is found. In fact, it can be argued that oxidation states provided the basis for Medeleev s initial organization of the periodic table. For the main group elements, the relative stability of lower oxidation states within a given group increases as the atomic number increases. This trend in the periodic table see Periodic Table Trends in the Properties of the Elements) is generally attributable to the presence of an inert s pair see Inert Pair Effect) caused by relativistic effects see Relativistic Effects). 

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