Element electropositive

Salt-like. These are the hydrides of the most electropositive elements (e.g. Na) and contain H " ions. 

Only the carbon atom can gain four electrons this only happens when it is combined with extremely electropositive elements and this state may be regarded as exceptional. Bonding in carbides is almost invariably predominantly covalent. 

Bromine has a lower electron affinity and electrode potential than chlorine but is still a very reactive element. It combines violently with alkali metals and reacts spontaneously with phosphorus, arsenic and antimony. When heated it reacts with many other elements, including gold, but it does not attack platinum, and silver forms a protective film of silver bromide. Because of the strong oxidising properties, bromine, like fluorine and chlorine, tends to form compounds with the electropositive element in a high oxidation state. 

These are halides formed by highly electropositive elements (for example those of Groups I and II, except for beryllium and lithium). They have ionic lattices, are non-volatile solids, and conduct when molten they are usually soluble in polar solvents in which they produce conducting solutions, indicating the presence of ions. 

These are formed by less electropositive elements. They are characterised by the existence of discrete molecules which exist even in the solid state. They have generally lower melting and boiling points than the ionic halides, are more volatile and dissolve in non-polar solvents. 

In empirical formulas of inorganic compounds, electropositive elements are listed first [3]. The stoichiometry of the element symbols is indicated at the lower right-hand side by index numbers. If necessary, the charges of ions are placed at the top right-hand side next to the element symbol (e.g., S "). In ions of complexes, the central atom is specified before the ligands are listed in alphabetical order, the complex ion is set in square brackets (e.g., Na2[Sn(OH)+]). 

Binary Compounds of Hydrogen. Binary compounds of hydrogen with the more electropositive elements are designated hydrides (NaH, sodium hydride). 

Ternary compounds are also named by citing the more electropositive constituent first. The various oxidation states of the more electropositive element are designated by a system of prefixes and terminations added to a stem characteristic of the element, except in the case of coordination compounds (qv). Examples are as follows (see Chlorine oxygen acids and salts) ... 

Phosphides. Compounds of phosphoms containing the more electropositive elements are generally called phosphides. A large number of binary phosphides as well as many ternary mixed-metal phosphides, metal phosphide nitrides, etc, are known. Some binary phosphides, such as those of nickel, exhibit a variety of stoichiometries (Ni P, Ni P2, Nq2P5, Ni2P, Ni P, NiP, NiP2, NiP ), whereas others, such as aluminum, form only one (AlP). Metalloids such as B and Si also form phosphides. 

Preparation of Uranium Metal. Uranium is a highly electropositive element, and extremely difficult to reduce. As such, elemental uranium caimot be prepared by reduction with hydrogen. Instead, uranium metal must be prepared using a number of rather forcing conditions. Uranium metal can be prepared by reduction of uranium oxides (UO2 [1344-59-8] or UO [1344-58-7] with strongly electropositive elements (Ca, Mg, Na), reduction of uranium halides (UCl [10025-93-1], UCl [10026-10-5] UF [10049-14-6] with electropositive metals (Li, Na, Mg, Ca, Ba), electro deposition from molten... 

The oxygen ion beam diameter is limited to 0.5 pm by the duoplasmatron source used. For mapping electropositive elements this drawback must be tolerated because of the chemical enhancement effect. 

Other electropositive elements have been used (e.g. Li, Na, K, Be, Ca, Al, Fe), but the product is generally amorphous and contaminated with refractory impurities such as metal borides. Massive crystalline boron (96%) has been prepared by reacting BCI3 with zinc in a flow system at 900°C. 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

The product of reaction of BRj" with element halides depends on the electropositivity of the element. Halides of the electropositive elements tend to form the corresponding M(BH4)jc,... 

This type of transhalogenation reaction, which is common amongst the halides of main group elements, always proceeds in the direction which pairs the most electropositive element with the most electronegative, since the greatest amount of energy is evolved with this combination. 

Arsine, AsHs, is formed when many As-containing compounds are reduced with nascent hydrogen and its decomposition on a heated glass surface to form a metallic mirror formed the basis of Marsh s test for the element. The low-temperature reduction of AsCls with LiAlH4 in diethyl ether solution gives good yields of the gas as does the dilute acid hydrolysis of many arsenides of electropositive elements (Na, Mg, Zn, etc.). Similar reactions yield stibine, e.g. ... 

Finally, sulfur occurs in many localities as the sulfates of electropositive elements (see Chapters 4 and 5) and to a lesser extent as sulfates of Al, Fe, Cu and Pb, etc. Gypsum (CaS04.2H20) and anhydrite (CaSO ) are particularly notable but are little used as a source of sulfur because of high capital and operating costs. Similarly, by far the largest untapped source of sulfur is in the oceans as the dissolved sulfates of Mg, Ca and K. It has been estimated that there are some 1.5 x 10 cubic km of water in the oceans of the world and that 1 cubic km of sea-water contains approximately 1 million tonnes of sulfur combined as sulfate. 

The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes... 

Sulfuric acid forms salts (sulfates and hydrogen sulfates) with many metals. These are frequently very stable and, indeed, they are the most important mineral compounds of. several of the more electropositive elements. They have been discussed in detail under the appropriate elements. Sulfates can be prepared by ... 

Two types of chemical bonds, ionic and covalent, are found in chemical compounds. An ionic bond results from the transfer of valence electrons from the atom of an electropositive element (M) to the atom(s) of an electronegative element (X). It is due to coulombic (electrostatic) attraction between the oppositely charged ions, M (cation) and X (anion). Such ionic bonds are typical of the stable salts formed by combination of the metallic elements (Na, K, Li, Mg, etc.) with the nonmetallic elements (F, Cl, Br, etc.). As an example, the formation of the magnesium chloride molecule from its elemental atoms is shown by the following sequence ... 

Alkali metals are strongly electropositive elements with low (2-3 eV) work function and low ionization potential. Upon adsorption on other metal surfaces they cause a severe (up to 3 eV) lowering of the metal work function, as already established by Langmuir in the early 1920 s. 

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