Compound ionic

Compounds of the type [TTF][M(tfd)2] (M = Ni, Pt, Cu, Au) exhibit a range of interesting magnetic properties depending on M. In all cases the charge transfer appears complete, and the compounds are thus best formulated as [TTF]+[M(tfd)2]. On the basis of 

Cu(tfd)2 units by 0.015 A in an orthogonal direction to fill in the space vacated by the TTF molecules. This existence of a progressive dimerization in a regular chain of molecules of spin 1/2 along with the observed activated temperature dependence of the magnetic susceptibility below Tc has been interpreted in terms of a spin-Peierls instability77-78) in a one-dimensional antiferromagnetically coupled chain. 

An additional feature of the temperature dependent X-ray scattering is the persistence above Tc of intensity at the superlattice positions51). This is consistent with a soft phonon mode at a wave vector commensurate with the changes that occur on dimerization. It has been suggested that this low frequency lattice mode may be a requirement for the observation of a spin-Peierls transition. 

Ionic bonding, the bonding that holds the cations and anions together, is one of the two major types of bonding in chemistry. (I describe the other type, covalent bonding, in Chapter 6.) 

An ionic bond occurs between a metal and a nonmetal. The metal loses electrons (to becomes a positively charged cation), and a nonmetal gains those electrons (to become a negatively charged anion). The ions have opposite charges. 

In this section, you look at how ionic bonding works, and you see how to write formulas for and name ionic compounds. 

Making the bond Sodium metal + chlorine a = sodium chloride 

For instance, sodium, a metal, can fill its octet and achieve stability by losing an electron. Chlorine, a nonmetal, can fill its octet by gaining an electron. (See the earlier section Gcdning and losing electrons for details on the octet rule.) If the two are in the same container, then the electron that sodium loses can be the same electron that chlorine gains. The Na cation attracts the Cl anion and forms the compound NaCl, sodium chloride. 

In ionic soHds the particles are charged ions that are bonded together by the Coulomb forces. 

For a positive ion i and a negative ionj separated by a distance ttj the potential of interaction is given by 

The first term on the right side of (15.22) characterizes the Coulomb attraction of the ion pair. The Coulomb interaction is a long-range one since it changes as The second term with n 1 characterizes the repulsion of electron shells at closest approach according to the Pauli principle. In classical approximation, the constants C and n are considered as unknown quantities to be determined from the lattice and compressibihty data.  

In order to calculate the energy of the NaCl-type lattice we summarize interactions over aU ij pairs of ions. To take into account the double counting of interatomic interactions we introduce coeffldent 1/2. Energy per ion is given by 

The Madelung constant plays the leading part in theory of ionic crystals. 

The ionic bond, like the metallic bond, is nondirectional and favors larger coordination numbers than directionally bonded covalent compoimds. The coordination number, however, is limited by Pauling s first rule of ionic crystal formation which requires every ion to be surroimded by a polyhedron of counterions and the stability of the system requires the ion to be in direct contact with each counterion. These requirements establish the allowable coordination number (number of nearest neighbors) based on the ratio of cation/anion size. Since the negatively charged anions have received electrons from the cations, they tend to be the larger of the two. 

Ratio of Cation/Anion Diameter Possible Coordination Number 

The following are examples of the simplest and most common of the many possible structures for ionic compoimds. 

Ionic compounds play an important role in creating the excitement of a Fourth of July celebration. 

Ionic compounds are substances composed of ions attracted to each other by ionic bonds. Let s consider how they play a part in a typical family s Fourth of July. 

Before the family leaves the house to go to the holiday picnic, the kids are sent oflF to brush their teeth and change into clean clothes. Their toothpaste contains sodium fluoride, a common cavity-fighting ionic compound. The white shirts in their red, white, and blue outfits were bleached with the ionic compound sodium hypochlorite, the stains on their red pants were removed by potassium oxalate, and dyes were fixed to their blue hats by aluminum nitrate. 

When our family gets to the park, they find themselves a place on the lawn, which was fertilized with a mixture of ionic compounds, including iron(II) sulfate. They eat their dinner and play in the park until it s time for the fireworks. The safety matches used to light the rockets contain barium chromate, and ionic compounds in the fireworks provide the colors red from strontium chlorate, white from magnesium nitrate, and blue from copper(II) chloride. 

Remember the sodium fluoride in the kids toothpaste It could be made from the reaction of sodium metal with the nonmetallic atoms in fluorine gas. As you discovered in Section 3.2, metallic atoms hold some of their electrons relatively loosely, and as a result, they tend to lose electrons and form cations. In contrast, nonmetallic atoms attract electrons more strongly than metallic atoms, and so nonmetals tend to gain electrons and form anions. 

Ionic compounds are composed of positively charged cations and negatively charged anions. 

Ionic bonds are formed by the transfer of one or more valence electrons from one atom to another. Because electrons are negatively charged, the atom that gives up electrons becomes positively charged, a cation. The atom that receives electrons becomes negatively charged, an anion. The reaction between sodium and chlorine atoms to form sodium chloride (ordinary table salt) is a typical electron-transfer reaction.  

Electropositive atoms give up electrons and form cations. 

Electronegative atoms accept electrons and form anions. 

Write an equation for the reaction of magnesium (Mg) with fluorine (F) atoms. 

Can you explain why network covalent compounds have much higher melting points than molecular covalent compounds  

Groups of atoms of certain elements combine in a rather stable state, making up ionic compounds. An example is CaC03, the compositional formula of limestone, composed of Ca2+ + CO2-. A second example is CaS04, the compositional formula of anhydrite, composed of Ca2+ + S04.  

In the above examples CO2- and SO2- are ionic compounds. Their ionic weights are the algebraic sum of the atomic weights of the atoms involved the ionic weight of CO2- is 12 +(3 x 16) = 60, and the ionic weight of SO2- is 32 +(4 x 16) = 96. The valences of ionic compounds are the algebraic sum of the valences of the atoms involved. Thus the valence of carbonate is 

Ionic compounds commonly dissolved in groundwater include I ICO, (bicarbonate), CO2- (carbonate), SO2- (sulfate), and NO (nitrate). 

The nucleus of an atom is unchanged by chemical processes, but some atoms can readily gain or lose electrons. If electrons are removed from or added to an atom, a charged particle called an ion is formed. An ion with a positive charge is a cation (pronounced CAT-ion) a negatively charged ion is an anion (AN-ion). 

To see how ions form, consider the sodium atom, which has 11 protons and 11 electrons. This atom easily loses one electron. The resulting cation has 11 protons and 10 electrons, which means it has a net charge of l-h. 

The net charge on an ion is represented by a superscript. The superscripts -b, 2-b, and 3-I-, for instance, mean a net charge resulting from the loss of one, two, and three electrons, respectively. The superscripts —, 2—, and 3— represent net charges resulting from the ain of one, two, and three electrons, respectively. Chlorine, with 17 protons and 17 electrons, for example, can gain an electron in chemical reactions, producing the CP ion  

In general, metal atoms tend to lose electrons to form cations and non-metal atoms tend to gain electrons to form anions. Thus, ionic compounds tend to be composed of metals bonded with nonmetals, as in NaCl. 

Give the chemical symbol, induding superscript indicating mass number, for (a) the ion with 22 protons, 

As we will discuss in detail in Chapter 4, an atom consists of a very small, very dense, positively charged nucleus surrounded by a diffuse distribution of negatively charged particles called electrons. The number of positive charges in the nucleus defines the identity of the element to which the atom corresponds. Electrically neutral atoms contain the same number of electrons outside the nucleus as positive charges (protons) within the nucleus. Ions are formed when neutral atoms lose or gain electrons. An Na ion is formed when a sodium atom loses one electron, and a Cl ion is formed when a chlorine atom gains one electron. 

Because there are no molecules of ionic substances, we should not refer to a molecule of sodium chloride, NaCl, for example. Instead, we refer to a formula unit of NaCl, which consists of one Na ion and one Cl ion. Likewise, one formula unit of CaCb consists of one Ca ion and two Cl ions. We speak of the formula unit of all ionic compovmds as the smallest, whole-number ratios of ions that yield neutral representations. It is also acceptable to refer to a formula unit of a molecular compound. One formula unit of propane, C3HS, is the same as one molecule of CsHg it contains three C atoms and eight H atoms bonded together into a group. For the present, we will tell you which substances are ionic and which are molecular when it is important to know. Later you will learn to make the distinction yourself. 

Polyatomic ions are groups of atoms that bear an electric charge. The first atom in the formula is usually the central atom to which the other atoms are bonded to make a stable unit. Examples include tbe ammonium ion, NH/ the sulfate ion, S04 and the 

Owithin the crystal, each chloride Ion 1 surrounded by six sodium Ions. 

O A crystal of sodium chloride consists of an extended array that contains equal numbers of sodium Ions (small spheres) and chloride Ions (large spheres). 

In a molecular compound, covalent bonding happens when two nonmetals share electrons. Covalent bonds are defined not only by the number of electrons shared but also by their length and strength. Covalent bonds have a specific bond length and bond energy. In a homonuclear diatomic molecule where both atoms are identical, the pair or pairs of electrons is/are shared equally between the two atoms. However, in a heteronuclear diatomic molecule, one shared pair(s) will be more attracted to one atom than the other. 

Type (X = atoms bonded to central atom A E = lone electron pairs on central atom) 

There are two types of ions monoatomic (single atom) and polyatomic (two or more atoms possessing an overall charge). The naming protocols for these species are precise, but difficult to learn. The following rules should help you. 

For monoatomic anions replace ending from element name with -ide suffix. Example F (fluorine) becomes F or fluoride 

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Fajans rules Ionic compounds are most readily formed by ... 

The co-ordination number in ionic compounds is determined by the radius ratio - a measure of the necessity to minimize cationic contacts. More subtle effects are the Jahn-Teller effect (distortions due to incomplete occupancy of degenerate orbitals) and metal-metal bonding. 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

The enthalpy of solution is quite small for many simple ionic compounds and can be either positive or negative. It is the difference between two large quantities, the sum of the hydration enthalpies and the lattice energy. 

Prediction of solubility for simple ionic compounds is difficult since we need to know not only values of hydration and lattice enthalpies but also entropy changes on solution before any informed prediction can be given. Even then kinetic factors must be considered. 

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. 

It is soluble in organic solvents (a characteristic of a covalent compound). but dissolves in water and can form hydrates (a characteristic of an ionic compound), hence the hydrated must be... 

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... 

These chlorozincates must not be confused with the non-ionic compounds which quinoline and aniline bases give with neutral zinc chloride the latter have the formulae [(C,H7N)2ZnClt] and [(C,H7N)tZnCl ] respectively, and both are only slightly soluble in water. 

Many elements readily form ionic compounds such as table salt (Na Cl ), iu which the cationic sodium and anionic chlorine are held... 

Whether an element is the source of the cation or anion in an ionic bond depends on several factors for which the periodic table can serve as a guide In forming ionic compounds elements at the left of the periodic table typically lose electrons giving a cation that has the same electron configuration as the nearest noble gas Loss of an elec tron from sodium for example yields Na which has the same electron configuration as neon... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation reaction is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced reaction as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as... 

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. 

Ammonium chloride [12125-02-9] NH Q, ammonium bromide [12124-97-9] NH Br, and ammonium iodide [12027-06-4] NH I, are crystalline, ionic compounds of formula wts 53.49, 97.94, and 144.94, respectively. Their densities d systematically foUow the increase in formula weight 1.53, 2.40, and 2.52. AH three exist in two crystal modifications (10) the chloride, bromide, and iodide have the CsQ stmcture below temperatures of 184.5, 137.8, and — 17.6°C, respectively each reversibly transforms to the NaQ. stmcture at higher temperatures. 

Owing to the strong electropositive character of mbidium, its chemistry is normally restricted to ionic compounds. The element does not readily... 

Beryllium Halides. The properties of the fluoride differ sharply from those of the chloride, bromide, and iodide. BeryUium fluoride is essentiaUy an ionic compound, whereas the other three haUdes are largely covalent. The fluoroberyUate anion is very stable. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

X-ray structural studies have played a major role in transforming chemistry from a descriptive science at the beginning of the twentieth century to one in which the properties of novel compounds can be predicted on theoretical grounds. When W.L. Bragg solved the very first crystal structure, that of rock salt, NaCl, the results completely changed prevalent concepts of bonding forces in ionic compounds. 

Broken Bond Model were developed to describe the process of ionization of ionic compounds, especially under primary oxygen-ion bombardment - or gas admission ("O2... 

Crystal structure, crystal defects and chemical reactions. Most chemical reactions of interest to materials scientists involve at least one reactant in the solid state examples inelude surfaee oxidation, internal oxidation, the photographie process, electrochemieal reaetions in the solid state. All of these are critieally dependent on crystal defects, point defects in particular, and the thermodynamics of these point defeets, especially in ionic compounds, are far more complex than they are in single-component metals. I have spaee only for a superficial overview. 

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

Use of the K p for precipitation information is often complicated by a number of interfering factors including complexation of metallic ions, high ionic strength solutions, and high solids contents. This principle is applicable solely to ionic compounds, i.e., primarily inorganic compounds. 

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