Elemental substances

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

Strictly speaking, the size of an atom is a rather nebulous concept The electron cloud surrounding the nucleus does not have a sharp boundary. However, a quantity called the atomic radius can be defined and measured, assuming a spherical atom. Ordinarily, the atomic radius is taken to be one half the distance of closest approach between atoms in an elemental substance (Figure 6.12). 

In the first instance, we must note the processes generating phosphine (PH3) from the elemental substance in aqueous basic medium to be a useful approach with particular types of substrates with which PH3 (or the derived PH21 anion) can act as a nucleophile. Production of a particularly desired type of organophosphorus compound is clearly dependent on both the substrate and the nature of the solvent. 

Example 2.3 Consider the formation of benzene and carbon dioxide from their elemental substances. Find the heat required per unit mole at the standard state 25 °C, 1 atm. 

At one time in history it was thought that air was an elemental substance. Leonardo da Vinci (1452—1519) beheved that air was composed of at least two gases. He was the first to report that air was a mixture of two gases and that one of them supported life and combustion. 

Table 1 Mean Ionization Potentials for Elemental Substances Composing Flue Gas...

Chemists and physicists have collaborated since the middle of the twentieth century to make new elements substances never before seen on Earth. They are expanding the Periodic Table, step by painful step, into uncharted realms where it becomes increasingly hard to predict which elements might form and how they might behave. This is the field of nuclear chemistry. Instead of shuffling elements into new combinations - molecules and compounds - as most chemists do, nuclear chemists are coercing subatomic particles (protons and neutrons) to combine in new liaisons within atomic nuclei. 

Standard states. The standard or reference state of each of the elemental substances is taken to be that physical state (or one of them, if there are two or more) in which the element naturally exists at a pressure, or a fugacity, of one atmosphere and at a temperature of 18°. The isotopic composition of each element in its standard state is understood to be the naturally existing one. For the element carbon, we have selected its form as diamond, C (c, diamond), as the standard state because no other form of solid carbon is at present a reproducible and invariable one. 

Helium One could hardly imagine a simpler elemental substance than helium, which is viewed at the microscopic level as consisting of spherically symmetric closed-shell 2-electron atoms that interact most weakly of all known atoms. It is therefore surprising that helium exhibits thermodynamic phase behavior that is spectacularly unusual and perplexing. 

More than a hundred years passed before an Englishman, Robert Boyle, in 1661, succeeded in killing off the old idea of the four elements. He did it by establishing that there are many elements — substances that cannot be formed by other substances and cannot be broken into other substances. 

The standard free energy of formation of a substance measures its thermodynamic stability with respect to its constituent elements. Substances that have a negative value of AG°f, such as carbon dioxide and water, are stable and do not decompose to their constituent elements under standard-state conditions. Substances that have a positive value of AG°f, such as ethylene and nitrogen dioxide, are thermodynamically unstable with respect to their constituent elements. Once prepared, though, such substances can exist for long periods of time if the rate of their decomposition is slow. 

Lavoisier published a list of elemental substances in 1789. He prepared his list after conducting careful chemical decomposition and recombination reactions. This list of 23 elements is considered by many as the first list of elements. But, he included lime, alumina and silica - stable chemical compounds - and light and heat in his list of elements... 

No such substance as CaCl has been isolated however, the CaCl molecule has been observed spectroscopically in gaseous mixtures at high temperatures, and a number of its molecular constants have been measured. Before asking whether CaCl exists or not, it is necessary to specify whether you are talking about the molecule or the substance. We may presume that the substance calcium(I) chloride would, if isolated, take the form of an ionic solid at room temperature. Thermochemical calculations of the kind described in Chapter 5 show that this substance should be stable with respect to decomposition to the elemental substances. It is definitely unstable, however, towards the disproportionation reaction ... 

You may, for example, see it written that aluminium is one of the most abundant elements in the Earth s crust . This, of course, does not mean that macroscopic particles of the light, silvery metallic substance from which jumbo jets and saucepans are largely fabricated are to be found in nature. Element has become a collective term, and encompasses all the atoms having a particular atomic number, regardless of their state of chemical combination. We must therefore be careful to refer to an elemental substance if that is what we mean. Thus when we say that lead occurs in sulphide minerals , we are referring to an element the statement that lead reacts only slowly with dilute hydrochloric acid obviously... 

All elemental substances are atomic at a sufficiently high temperature the high entropy of an atomic gas will ultimately lead to a TA5 term for atomisation which outweighs the unfavourable enthalpy term. Elemental mercury boils under atmospheric pressure at 357 °C to give a monoatomic gas. Most other elemental substances are molecular in the vapour phase at the boiling point (e.g. Na2, P4, S8) but these molecules break down to smaller fragments and eventually to atoms on heating. 

The most familiar metals are elemental substances such as iron, tin, aluminium etc. However, many compounds are metallic. As well as intermetallic compounds such as AgCd and NaTl, and a huge number of non-stoichiometric alloys, many oxides, sulphides, halides etc. have metallic properties. For details of structure and bonding in metallic substances, see Section 7.5. 

Typical metallic substances have structures which cannot readily be described either in terms of directional covalent bonds or as arrays of cations and anions. Metallic elemental substances exhibit high coordination numbers, and can - to a first approximation - be viewed as arrays of cations embedded in a sea or glue of electrons completely delocalised over the crystal. This model helps to explain the characteristic mechanical, thermal and electrical properties of metals. It is also consistent with... 

In accounts of descriptive inorganic chemistry - especially in more elementary texts - it is common practice to classify elements as metals or nonmetals , with semi-metals or metalloids as a borderline case, according to the nature of the elemental substance. The chemistry of an element is, to some extent, broadly predictable from this classification. Metallic elements tend to form ionic oxides and halides they form... 

The atomic radius of the atom X is defined as half the length of an X-X single bond. This can be obtained experimentally from the structures of elemental substances containing molecules X where the X-X bond order is believed to be unity, e.g. Cl2, P4, S8. It may also be obtained from the X-X distances found in molecules such as HO—OH, H2N—NH2 etc. for atoms which form multiple bonds in the elemental substance. Such atomic radii may be termed covalent radii. For atoms which form metallic elemental substances, metallic radii are obtained. These are usually standardised for 12-coordination of each atom, which is the most common situation in metals. Corrections can be made in the cases of metals which adopt other structures. 

Some writers feel it important to distinguish carefully between covalent and metallic radii. Others suggest that a self-consistent set of atomic radii - some covalent, others metallic - can be devised. Such a collection is presented in Table 4.1. In cases where both a covalent and a metallic radius can be obtained, the agreement is variable. For example, the metallic radii of atoms of the Group 1 elements are 20-30 pm greater than the corresponding covalent radii, taken from the M-M distances in M2(g). The Mn-Mn distance in (CO)5Mn—Mn(CO)5 is 293 pm, which compares with 274 pm calculated from the metallic radius of Mn. The electronic environment of the Mn atom in the carbonyl complex is, of course, very different from that in the elemental substance. 

Towards the end of each of the three series which constitute the d block, we observe a marked increase in radius. For reasons discussed in more detail in Section 7.5, the filled nd subshell tends to weaken the bonding in metallic elemental substances, leading to longer internuclear distances. 

The formation of BaF3(s) from the elemental substances can be broken down as follows ... 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

Since AH° is not very different from AG°, the entropy term is evidently quite small, and we are justified in concentrating on the enthalpy terms in our analysis. The atomisation enthalpy of the elemental substance, the relevant ionisation energies of the gaseous atomic substance and the hydration enthalpy of the cation are obviously the quantities to be compared when looking at different species. The last three steps in the analysis above amount to —439n kJ mol-1. 

Let us then compare zinc with other metallic elemental substances with respect to the formation of M2+(aq) in acid solution at a pH of zero. The relevant data are summarised in Table 5.7. Note that the ° values given refer (in accordance with the European Convention) to reduction potentials for the half-reactions ... 

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