Molecules ionic compounds

Chemical bond, atoms, molecules, ionic compounds, valence electrons... 

The small lithium Li" and beryllium Be ions have high charge-radius ratios and consequently exert particularly strong attractions on other ions and on polar molecules. These attractions result in both high lattice and hydration energies and it is these high energies which account for many of the abnormal properties of the ionic compounds of lithium and beryllium. 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

After being formed as a spray, many of the droplets contain some excess positive (or negative) electric charge. Solvent (S) evaporates from the droplets to form smaller ones until, eventually, ions (MH+, SH+) from the sample M and solvent begins to evaporate to leave even smaller drops and clusters (S H+ n = I, 2, 3, etc,). Later, collisions between ions and molecules (Cl) leave [M + H]" ions, which proceed on into the mass analyzer. Ion yield can be enhanced by including a volatile ionic compound (e.g., ammonium acetate) in the initial solution before it reaches the spraying zone. 

The mp and bp increase in the order of nonpolar to polar to ionic compounds. Associated liquids, in which the molecules are held together by hydrogen bonds, show higher bp than nonassociated polar compounds of similar molecular w eights. 

Ionic compounds often separate from water solution with molecules of water incorporated into the solid. Such compounds are referred to as hydrates. An example is hydrated copper sulfate, which contains five moles of H20 for every mole of CuS04. Its formula is CuS04- 5H20 a dot is used to separate the formulas of the two compounds CuS04 and H20. A Greek prefix is used to show the number of moles of water the systematic name of CuS04- 5H20 is copper(ll) sulfate pentahydrate. 

We do not usually speak of a molecule of an ionic compound. However, it is useful to be able to refer to a group of ions having the number of atoms given by... 

Ionic compounds are named by starting with the name of the cation (with its oxidation number if more than one charge is possible), followed by the name of the anion hydrates are named by adding the word hydrate, preceded by a Greek prefix indicating the number of water molecules in the formula unit. 

The molar mass of a molecular compound is the mass per mole of its molecules. The molar mass of an ionic compound is the mass per mole of its formula units. 

The hydrated ion [Cu(H20)6]2+ is an example of a complex, a species consisting of a central metal atom or ion to which a number of molecules or ions are attached by coordinate covalent bonds. A coordination compound is an electrically neutral compound in which at least one of the ions present is a complex. However, the terms coordination compound (the overall neutral compound) and complex (one or more of the ions or neutral species present in the compound) are often used interchangeably. Coordination compounds include complexes in which the central metal atom is electrically neutral, such as Ni(CO)4, and ionic compounds, such as K4[Fe(CN)6]. 

For a given molecule and a given intemuclear separation a would have a definite value, such as to make the energy level for P+ lie as low as possible. If a happens to be nearly 1 for the equilibrium state of the molecule, it would be convenient to say that the bond is an electron-pair bond if a is nearly zero, it could be called an ionic bond. This definition is somewhat unsatisfactory in that it does not depend on easily observable quantities. For example, a compound which is ionic by the above definition might dissociate adiabatically into neutral atoms, the value of a changing from nearly zero to unity as the nuclei separate, and it would do this in case the electron affinity of X were less than the ionization potential of M. HF is an example of such a compound. There is evidence, given bdow, that the normal molecule approximates an ionic compound yet it would dissociate adiabatically into neutral F and H.13... 

A solid that contains cations and anions in balanced whole-number ratios is called an ionic compound. Sodium chloride, commonly known as table salt, is a simple example. Sodium chloride can form through the vigorous chemical reaction of elemental sodium and elemental chlorine. The appearance and composition of these substances are very different, as Figure 2-24 shows. Sodium is a soft, silver-colored metal that is an array of Na atoms packed closely together. Chlorine is a faintly yellow-green toxic gas made up of diatomic, neutral CI2 molecules. When these two elements react, they form colorless ciystals of NaCl that contain Na and Cl" ions in a 1 1 ratio. 

Ionic compounds are named using the same guidelines used for naming binary molecules, except that the cation name aiways precedes the anion name. Thus, NH4 NO3 is ammonium nitrate, Na2 CO3 is sodium carbonate, and Ca3 (P04)2 is caicium phosphate. The subscripts are not specified in these names because the fixed ionic charges determine the cation-anion ratios unambiguously. Example 3-6 reinforces these guidelines by showing how to construct chemicai formulas from chemical names. 

Many ionic compounds can have water molecules incorporated into their solid structures. Such compounds are called hydrates. To emphasize the presence of discrete water molecules in the chemical structure, the formula of any hydrate shows the waters of hydration separated from the rest of the chemical formula by a dot. A coefficient before H2 O indicates the number of water molecules in the formula. Copper(II) sulfate pentahydrate is a good example. The formula of this beautiful deep blue solid is C11SO4 5 H2 O, indicating that five water molecules are associated with each CuSOq unit. Upon prolonged heating, CuSOq 5 H2 O loses its waters of hydration along with its color. Other examples of hydrates include aluminum nitrate nonahydrate, A1 (N03)3 9 H2 O,... 

In contrast to sugar, solid sodium chloride dissolves in water to give a liquid that conducts electricity. Figure 3-19 shows that a solution of NaCl is a good conductor. When an ionic compound dissolves in water, its component cations and anions are free to move about in the solution. Mixing leads to a uniform distribution of Na and Cl ions through the entire solution, with each ion surrounded by a sheath of water molecules as shown in Figure 3-20. 

A limited number of elements form ionic compounds. As we describe in the next two chapters, most substances contain neutral molecules rather than charged ions. The trends in ionization energies and electron affinities indicate which elements tend to form ions. Ionic compounds form when the stabilization gained through ionic attraction... 

Other metallic elements form ionic compounds with cation charges ranging from -F1 to + 3. Aluminum nitrate nonahydrate, A1 (N03)3 9 H2 O, is composed of cations, NO3 anions, and water molecules. Silver nitrate (AgNO ), which contains Ag cations, is a soluble silver salt that is used in silver plating. 

Sodium bromide and silver nitrate are both soluble ionic compounds, so the major species are Na , Ag, Br, NO3, and H3 O molecules. The solubility guidelines presented in Section 4- identify the... 

Many ionic compounds contain what used to be referred to as water of crystallization . For example, magnesium chloride can exist as a fully hydrated salt which was formerly written MgCla.bHjO, but is more appropriately written Mg(OH2)eCl2, since the water molecules occupy coordination sites around the magnesium ions. This is typical. In most compounds that contain water of crystallization, the water molecules are bound to the cation in an aquo complex in the manner originally proposed by Alfred Werner (1866-1919) in 1893 (Kauffman, 1981). Such an arrangement has been confirmed in numerous cases by X-ray diffraction techniques. 

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