Elements non-metals

This is an exothermic process, due largely to the large hydration enthalpy of the proton. However, unlike the metallic elements, non-metallic elements do not usually form hydrated cations when their compounds dissolve in water the process of hydrolysis occurs instead. The reason is probably to be found in the difference in ionisation energies. Compare boron and aluminium in Group III ... 

Ionic compounds generally form following the reaction of metallic elements non-metals rarely have sufficient energy to provide the necessary energy needed to form ions (see p. 123). 

Fluorine is characterized by its extraordinary chemical reactivity— it is the most active of the elements. Non-metals, such as hydrogen, sulfur, iodine, and arsenic, and metalloids, such as silicon, boron, and carbon, combine spontaneously with fluorine, becoming incandescent. All metals are attacked by the gas. The alkali metals and alkaline-earth metals take fire in a stream of the gas at room temperature, whereas the more noble metals react with fluorine when warmed. Fluorine decomposes water, forming hydrogen fluoride and liberating a mixture of oxygen and ozone. 

We now know of the existence of over one hundred elements. A century ago, more than sixty of these were already known, and naturally attempts were made to relate the properties of all these elements in some way. One obvious method was to classify them as metals and non-metals but this clearly did not go far enough. 

Among the non-metals, nitrogen and chlorine, for example, are gases, but phosphorus, which resembles nitrogen chemically, is a solid, as is iodine which chemically resembles chlorine. Clearly we have to consider the physical and chemical properties of the elements and their compounds if we are to establish a meaningful classification. 

By reference to the outline periodic table shown on p. (i) we see that the metals and non-metals occupy fairly distinct regions of the table. The metals can be further sub-divided into (a) soft metals, which are easily deformed and commonly used in moulding, for example, aluminium, lead, mercury, (b) the engineering metals, for example iron, manganese and chromium, many of which are transition elements, and (c) the light metals which have low densities and are found in Groups lA and IIA. 

Essentially the same processes occur when chlorides (for example) of non-metallic elements dissolve in water. Thus, the enthalpy changes for hydration chloride can be represented ... 

The data in Table 7.1 show that, as expected, density, ionic radius, and atomic radius increase with increasing atomic number. However, we should also note the marked differences in m.p. and liquid range of boron compared with the other Group III elements here we have the first indication of the very large difference in properties between boron and the other elements in the group. Boron is in fact a non-metal, whilst the remaining elements are metals with closely related properties. 

In this group the outer quantum level has a full s level and two electrons in the corresponding p level. As the size of the atom increases the ionisation energy changes (see Table 8.1) and these changes are reflected in the gradual change from a typical non-metallic element, carbon, to the weakly metallic element, lead. Hence the oxides of carbon and silicon are acidic whilst those of tin and lead are amphoteric. 

Lead has only one form, a cubic metallic lattice. Thus we can see the change from non-metal to metal in the physical structure of these elements, occurring with increasing atomic weight of the elements carbon, silicon, germanium, tin and lead. 

The change from non-metallic to metallic properties of the Group V elements as the atomic mass of the element increases is shown in their reactions with alkalis. 

These stability changes are in accordance with the change from a non-metal to a weak metal for the Group V elements nitrogen to bismuth. 

Nitrogen is unusual in forming so many oxides. The acidity of the Group V oxides falls from phosphorus, whose oxides are acidic, through arsenic and antimony whose oxides are amphoteric, to the basic oxide ofbismuth. This change is in accordance with the change from the non-metallic element, phosphorus, to the essentially metallic element, bismuth. The +5 oxides are found, in each case, to be more acidic than the corresponding + 3 oxides. 

These elements are generally unaffected by non-oxidising acids (behaviour expected for non-metallic elements) but they do react when heated with concentrated sulphuric and nitric acids, both powerful oxidising agents. Sulphur is oxidised to sulphur dioxide by hot concentrated sulphuric acid,... 

Oxygen is a very reactive element and many metals and non-metals burn in it to give oxides these reactions are dealt with under the individual group headings. 

Sulphur is less reactive than oxygen but still quite a reactive element and when heated it combines directly with the non-metallic elements, oxygen, hydrogen, the halogens (except iodine), carbon and phosphorus, and also with many metals to give sulphides. Selenium and tellurium are less reactive than sulphur but when heated combine directly with many metals and non-metals. 

Oxygen bonds covalently to many non-metals, and in many oxides, both with metals and non-metals, the other element achieves a high oxidation state, for example... 

The other more electronegative elements are non-metals and form oxides which are entirely covalent and usually acidic. For example, sulphur yields the oxides SO2 and SO3, dissolving in bases to form the ions SO3 and SO4" respectively. A few non-metallic oxides are often described as neutral (for example carbon monoxide and dinitrogen oxide) because no directly related acid anion is known to exist. 

Chlorine reacts with most elements, both metals and non-metals except carbon, oxygen and nitrogen, forming chlorides. Sometimes the reaction is catalysed by a trace of water (such as in the case of copper and zinc). If the element attacked exhibits several oxidation states, chlorine, like fluorine, forms compounds of high oxidation state, for example iron forms iron(III) chloride and tin forms tin(IV) chloride. Phosphorus, however, forms first the trichloride, PCI3, and (if excess chlorine is present) the pentachloride PCI5. 

Halides of non-metals are usually prepared by the direct combination of the elements. If the element exhibits more than one oxidation state, excess of the halogen favours the formation of the higher halide whilst excess of the element favours the formation of the lower halide (e.g. PCI5 and PCI3). 

For the majority of metals, the evaporation coefficient is found to be unity, but, as mentioned before, the coefficient of many non-metallic elements with a complex vaporization mechanism such as... 

Fig. 2.29. Formation of surface compounds on Fe-15%Cr alloys by cosegregation of chromium and a non-metallic element [2.160]. (a) nitride, (b) carbide, (c) sulfide.

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