The following reaction occurs readily. Two of the pi electrons are unpaired in degenerate molecular orbitals creating the highly unstable triplet state. Albites formula is NaAlSi3O8, which we may write Na(AlSi3)O8 to emphasize that both Al3+ and Si4+ occupy the same structural sites. Figure 13.5b, for example, shows the atomic arrangement in CsCl. A similar gap exists in the Ca-Mn-Fe-Mg carbonate system. They have the general formula R2O, and R can be any alkali element. 13.48 Brown zircon crystal from Pakistan. 'pa pdd chac-sb tc-bd bw hbr-20 hbss lpt-25' : 'hdn'">. Second, for some mineral groups we cannot make meaningful generalizations or categorize structures in a useful way. As discussed in Chapter 6, pyroxenes may be orthorhombic or monoclinic. 13.2 Ionic Crystals 13.4 A cation bonded to four anions. A.D.A.M., Inc. is accredited by URAC, for Health Content Provider (www.urac.org). A zinc-carbon dry cell battery (left) and an alkaline battery (right). Cation (+) = positively-charged ions, Other mineral structures are, however, not truly closest packed. However, if they are not balanced, they are called ions. They dominate igneous and metamorphic rocks, and also many sedimentary rocks. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Consequently, oxygen may coordinate to another cation just as strongly as to its coordinating Si4+. The fact that the red/yellow color is held in the center of the ring indicates that the extra electrons of the ion are involved in the aromatic p-electron system. 3) The cyclopropenium ion has 2 pi electrons which follows the 4n + 2 rule when n = 0. Although the closest packing model is a convenient concept, most minerals do not have atoms in closest-packed arrangements. . Because of stronger bonds, they are less easily attacked by water and other weathering agents. The molecule 1,3,5-cycloheptatriene has six pi electrons but is nonaromatic due the presence of an sp3 hybridized -CH2- group which prevents cyclic delocalization. So the marbles in the second layer will not be directly above those in the first (Figure 13.13). In single chain silicates, cations are exclusively tetrahedral or octahedral. The word in the example sentence does not match the entry word. While crystallographers were working on crystals, chemists were developing atomic theory. Figure 13.7 compares average effective ionic radii for common anions and cations. 13.49 Topaz from northern Pakistan. For example, there is only very minor substitution of Fe3+ for Al3+ in corundum, but unlimited substitution of Fe3+ for Al3+ takes place in garnet. LIB, lithium-ion battery; SEI, solid-electrolyte interphase; ZIC, zinc-ion capacitor. In single use, dry cell batteries, zinc is commonly used as the anode, while manganese dioxide is a popular choice for the electrolyte cathode. In three dimensions, other possibilities, including tetrahedral and cubic arrangements, exist. The symbol shows a vacancy. 13.47 Titanite, Gry Parent, Wikimedia-Commons He further argued that stretching a polyhedron to hold a cation larger than ideal might be possible. There are many variables, but we can make some generalizations. where salt water and fresh water mix as a green source of power generation for the future. There are two main situations which need to be considered: The conjugation of a carbocation and the conjugation of a carbanion. Electrolysis : Cation always move towards the cathode, which produces the negative charge. It may be possible to predict whether an atom will form a cation or an anionbased on its position on the periodic table. Note that we cannot list an ionic radius for argon because it is a noble gas and does not ionize to enter ionic structures. You can remember this because the t in Cation ion as a + sign. In sylvite (KCl), six Cl bond to each K+ and the strength of each bond is 1/6, total charge divided by number of bonds (Figure 13.17). The symmetries of the two patterns in Figure 13.11 are equivalent to the symmetries of a hexanet and a square net (Figures 11.23 and 11.27 in Chapter 11). anions and cations Because no other two-dimensional arrangement allows marbles to be closer together, we say the hexagonal arrangement is closest packed. Each bond has a strength of 4/6 (total charge divided by number of bonds) = 2/3. Your email address will not be published. In two dimensions, anions can fit together in symmetrical patterns to form hexagonal or square patterns. Figure 13.34 shows atomic arrangement in diopside, a pyroxene, and wollastonite, a pyroxenoid. 3 These properties could potentially be improved by achieving stimulus control of binding affinity. Individual tetrahedral or octahedral layers in sheet silicates have 3-fold or 6-fold symmetry. If we put another marble on top of them, it slips into the low spot above the center of the triangle, resulting in a tetrahedral structure composed of four marbles (Figure 13.12a). In serpentine, Mg3Si2O5(OH)4, and kaolinite Al2Si2O5(OH)4, T- and O-layers alternate. Thus, tourmaline contains several different kinds of sites and may incorporate just about any element in its structure. The two main polyatomic cations are NH4 + and H3O + . This leads to a charge deficiency. The raw stone in the top of photo is an imperfect orthorhombic prism; a cut and polished tanzanite gemstone is in the bottom of the photo. These data are slightly modified from values in Zoltai and Stout (1984) Mineralogy: Concepts and Principles. The specimen is 8 cm tall. Traditionally, this has been done using superscript Roman numerals. If a bond distribution is mesodesmic, cation-anion bond strength equals exactly half the charge on the anions. The table below lists the typical coordinations for each (with oxygen) and gives example minerals. relative polarisability of an anion: the tendency of the anion to become polarised (polarized) by a cation is known as its polarisability (polarizability). The larger the anion and the greater its negative charge, more easily its outer electron clouds are polarised more easily attracted-distorted towards the cation. Cations Ion Any opinions in the examples do not represent the opinion of the Cambridge Dictionary editors or of Cambridge University Press or its licensors. In albite, for example, Al3+ replaces one-fourth of the tetrahedral Si4+. Where for chloride this results in only a small change in affinity, that of the larger bromide and iodide ions is majorly affected, resulting in altered selectivity. 13.38 Tremolite from Campolungo, Switzerland. Consequently, theytend to lose electrons and form cations. But, we can see a large hole just below the center of the tremolite model in Figure 13.37a. Examples of most common anions We present you a list of the different types of anions. Like the pyroxenes, amphiboles may be orthorhombic or monoclinic. In the tetragonal arrangement, each marble only touches four others. Cubic closest packed (CCP) structures have an ABCABCABC packing sequence; it takes three layers before they repeat. in chemical engineering from Oregon State University in 1922 and a Ph.D. from California Institute of Technology (Cal Tech) in 1925. The amount of electrons lost, and therefore the charge of the ion, is indicated after the chemical symbol, for example silver (Ag) loses one electron to become Ag +, while zinc (Zn) loses two electrons to become in Zn2 +. Minerals containing alkali or alkaline earth elements cannot be closest packed because alkalis and alkaline earths are too large to fit in tetrahedral or octahedral sites. This site complies with the HONcode standard for trustworthy health information: verify here. However, unless we wanted to go into great detail, such a discussion would not be particularly fruitful for several reasons. When you see an element or a compound with a + or minus sign after it, that tells you that it is an ion. Other elements can be in octahedral coordination as well. Olivine and other island silicates are not polymerized at all, and melt at the highest temperatures. We call the pattern in Figure 13.11b tetragonal packing because of the obvious 4-fold symmetry. Decomposition Reaction Displacement Reactions Electrolysis of Aqueous Solutions Electrolysis of Ionic Compounds Energy Changes Extraction of Aluminium Fuel Cells Hydrates Chapter 6 contains additional photos of pyroxenes: jadeite (Figure 6.73), spodumene (Figure 6.74), enstatite (Figure 6.76), diopside (Figure 6.77), and augite (Figure 6.78). In other words, as the radius of the cation increases, so too will the coordination number. Sometimes simple substitutions are described using equations such as Fe2+ = Mg2+ or Fe3+ = Al3+. After completing this section, you should be able to. Mineralogists often call isolated tetrahedral silicates island silicates because tetrahedra do not share oxygen. The specimen is 11 cm wide. Because oxygen and silicon are the two most abundant elements in the Earths crust, and because the (SiO4) tetrahedron is such a stable complex, silicate minerals are extremely stable and abundant in crustal rocks and sediments. A cation is an atom or a molecule which is positively charged, i.e. In case you do not agree to the Different Examples terms, please do not use this website. For example, the radius of Na+ is listed as 1.07, 1.08, and 1.40 ; these difference reflect different numbers of bonds going to the cation. Polyatomic ions. During electrolysis, positively charged cations are drawn to the negatively charged electrode (cathode), while negatively charged anions are drawn to the positively charged electrode (anode). Box 13-2 contains a table with effective ionic radii for the most common ions. Crystallographers have also measured the radius of an Mg atom by using X-ray to learn the distance between neighboring Mg atoms in Mg metal. Lawsonite is similar in many ways to a high-pressure form of anorthite. In pyroxenes the tetrahedra zigzag back and forth along the chain direction. 13.3 Linus Pauling, Pinterest.com 13.38 Tremolite, Didier Descouens, Wikimedia Commons Magnesium and zinc are examples of native metals with hexagonal closest packing. When multiple cations or anions reach their corresponding electrodes, only one of them is discharged. So, the layers are sloping instead of horizontal or vertical. When we think about chemical bonding with ionic compounds, we always have pairs. Although Paulings first two rules are useful guides to crystal structures, they have shortcomings. I have been working since 2016 with online content production. Other sheet silicates, including biotite, K(Mg,Fe)3(AlSi3O10)(OH)2, contain three divalent cations for every four tetrahedra. Sharing of edges or faces by coordinating polyhedra is inherently unstable. Ions consist of nuclei with electron clouds around them. The photos below show muscovite, antigorite (a variety of serpentine), and margarite (a Ca-bearing mica). Because the strength of each S-O bond is greater than half the charge on coordinating oxygen, oxygen bonds to S6+ more tightly than to Ca2+. Ionizing radiation is often used to diagnose or treat a medical condition. Figure 13.37 shows atomic arrangement in tremolite, an amphibole. In Figure 13.22d, they share a face (three oxygen). While being shorter and, perhaps, easier to write, these formulas give little hint of crystal structure. Sulfides and native metals have bonds that range from nearly 100% covalent to 100% metallic, but covalent and metallic radii are nearly the same, and the variations in radii may not be significant. Anions are ions with a negative charge. Removal of a hydride from the -CH2- group in 1,3,5-cycloheptatriene creates an sp2 hybridized carbocation with a vacant p orbital. The atoms in some native metals are in 12-fold coordination. Pyrophyllite, Al2Si4O10(OH)2, and talc, Mg3Si4O10(OH)2, have structures identical to muscovite and biotite except that the K+ layer is absent. His success as a scientist stemmed from his ability to cross traditional discipline boundaries, an uncanny ability to identify key questions, and courage to put forth new, although sometimes incorrect, ideas. The table above shows radii of alkali cations increasing with atomic number (down a row in the Periodic Table). As expected, the radii decrease with charge, reflecting the greater pull nucleus protons have on outer electrons with increasing charge. 13.39 Hornblende, Robert M. Lavinsky, Wikimedia Commons These other cations are commonly Fe2+, Mg2+ or other cations of similar size and charge. Radius Ratio Principle: Cation-anion distances are equal to the sum of their effective ionic radii, and cation coordination numbers are determined by the ratio of cation to anion radii. In all crystals, anions bond to more than one cation. Box 13-3 Why Are Atomic Arrangements Called Hexagonal Closest Packed (HCP) and Cubic Closest Packed (CCP)? Blue spheres show H2O molecules. However, the layers are sightly offset and 3-fold and 6-fold symmetry does not persist in 3D. These difference can occasionally affect coordination number. Although bonds in minerals crystals, such as quartz crystals, may be up to 50% covalent, for most purposes consideration of ions and ionic bonding explains mineral properties. Until now the atoms providing the p orbitals have been neutral sp2 hybridized carbons, however it is also possible for sp2 hybridized carbons to have a charge. Eg: Ca2+, OH 2. The ratio of Si:O, then, indicates silicate subclass because different ratios result from different amounts of oxygen sharing. Figure 13.24 is based on Figure 6.17 (Chapter 6). Ca2+, Mg2+, Fe2+, and sometimes Mn2+ occupy the cubic site. Although this discussion of Paulings third rule is focused on tetrahedra, the same principles apply to octahedra and other polyhedra. Analysis Identification of Anionic Degradation Products by IC As is the case with cations, the charge on the anion is indicated by a superscript following the symbol. Fe3+ and Al3+ replace each other in minerals such as garnet and spinel. As this ionic substance reacts with the electrodes, it generates electrical current. In three dimensions, marbles (or anions) can be closest packed in two ways. The alkalis are in column 1 (Group 1) of the Periodic Table. Sphalerite (ZnS), halite (NaCl), and native metals gold, silver, platinum, and copper are all examples of cubic closest-packed minerals. Native iron has atoms with a body-centered cubic arrangement (Figure 13.14c) with atoms at the corners and one in the center. There is tremendous interest in the development of artificial anion receptors for their sensing, 1 extraction, 2 and transport properties. Quartz has the lowest melting temperature, Kfeldspar the second lowest, followed by muscovite, biotite, amphiboles, pyroxenes, and finally olivine. Salts are inorganic molecules (lacking carbon, hydrogen, or both) that dissolve well in water, separating into anions and cations. To maintain charge balance, Al3+ replaces Si4+ at the same time, and we describe the coupled substitution as Ca2+Al3+ = Na+Si4+ or just CaAl = NaSi. The zinc anode also acts as the battery container in zinc-carbon batteries, so as it oxidizes during use, the contents can start to leak over time. Pyroxenoids, which are triclinic, are much less common than pyroxenes. Also plays a role in the alkalinity of water. More accurate models of crystal structures could better reflect variations in ionic radii, bond length, and the way that ions fit together. The most abundant crustal cations include Si4+, Al3+, Fe2+, Fe3+, Mg2+, Mn2+, Na+, Ca2+, and K+. Cation vs. Anion: Definition, Explanation, & Examples The middle layer (green) contains three touching atoms forming an equilateral triangle. In still other silicates, Al3+ replaces some tetrahedral Si4+. Ions If CCP, atoms from four layers combine to form a unit cell with the shape of a cube. This is the essence of Paulings third rule. For example, OCl is the hypochlorite ion, and HOCl is hypochlorous acid; NO 2 is the nitrite ion, and HNO 2 is nitrous acid; and SO 32 is the sulfite ion, and H 2 SO 3 is sulfurous acid. 13.43 Crystals of lawsonite from Mendocino County, California, 13.44 Cubes of gehlenite from Monte Monzoni, Italy. We can use Paulings first two rules to analyze a more complicated mineral, rutile (TiO2). The spacing between them, and the way they pack together, are functions of their size. The inset shows the layers rotated to be horizontal. So we say it is in 4-fold coordination.. After placing these 4 pi electrons into the molecular orbital for a cyclic 5 p orbital species the bonding molecular orbitals remain unfilled. If two oxygen on each tetrahedron link to other tetrahedra, we get single-chain silicates (inosilicates) or ring silicates (cyclosilicates). Halite, sylvite, fluorite, and rutile are relatively simple minerals; they each contain one cation and one anion and, therefore, one kind of bond. Anions are packed in a repetitive structure, with the cations at regular intervals throughout. The geometric shapes in the bottom of Figure 13.12 are different views of tetrahedra and octahedra created by joining the centers of the anions. More complex substitutions in micas involve the replacement of (OH) by O2-, which requires some compensatory substitution to maintain charge balance. Consequently, K+ and other large cations cannot be present. For example, fluorite has mostly ionic bonds. 9.2Cd calcium ion, Ca (orange-red) 9.2Ce copper ion, Cu (blue-green) 9.3Cb calcium ion, Ca 9.4C Describe the chemical test for ammonia For opposite reasons, Na+ and K+ enjoy highly polymerized structures because of the large sites that easily accommodate monovalent cations. The difference is because Cs+ is larger than Na+. It may be possible to predict whether an atom will form a cation or an anion, based on its position on the periodic table. Silicate crystal structures may be complex. Figure 6.90 (Chapter 6) shows a different example of schorl. A cation is an ion with a positive charge. Called an anion.The oppositely charged atoms attract and you have an ionic bond.We also have what are called polyatomic ions. The drawing on the left does a better job of showing the relative sizes of the cation and anions but the one on the right is more clear about bond angles and distances. We may write magnetites as FeIVFeVI2O4 to show that iron occupies two differently coordinated sites. Quartz is the most common framework silicate. Three adjacent marbles in a hexagonal closest-packed sheet make a triangle. They are trioctahedral. Perhaps the most common pyroxene species is augite (Figure 13.36), a dark green to black pyroxene that typifies mafic and some other igneous rocks. Cations Ion Chromatography (IC) provides ion-exchange separations of anionic (or cationic) sample components. The drawing of cubic closest packing (Figure 13.14b) is a bit more difficult to see. Anion always move towards the anode, which produces the positive charge. Important in biological processes, such as DNA and ATP. Bowens reaction series is based on melting temperatures determined by laboratory experiments, but it mirrors the degree to which silicate minerals are polymerized (which is also a reflection of how much silica they contain). Terms Of Use Paulings fourth rule is an extension of his third, stating that small cations with high charges do not share anions easily with other cations. Both are equivalent to piling hexagonal packed sheets one above another. It was not long after the Braggss X-ray studies led to the first crystal structure determination that scientists found exceptions. Anion: Definition & Examples - Video & Lesson Transcript 13.27 Amethyst (quartz) from Veracruz, Mexico. As shown in the enlarged views, each Na+ bonds to six Cl and vice versa. The repeat distance for layers in clinochlore is about 14 ; in clay minerals may it be 16 or greater. Amphiboles (double chain silicates) and pyroxenes (single chain silicates) are still less polymerized, and melt at even higher temperatures. Different Examples reserves the right to alter the terms of use at any time. One consequence is that sheet silicates can accommodate cations of all sizes. In the 1930s and 1940s, Pauling turned his attention to molecular chemistry, producing significant papers concerning blood, proteins, and sickle cell anemia. The crystal is about 3 cm long. As an example, Ca2+, Mn2+, Fe2+, and Mg2+ substitute for each other in many silicates (and other minerals), including garnets and pyroxenes. The specimen is 5.3 cm tall. 13.33 Margarite, David J. Eicher, mindat.org Qualitative analysis: test for ions. 13.35 Diopside crystals on calcite. 13.35 Diopside, worthpoint.com Like the pyroxenes, chains in amphiboles point up and down. Cation vs. Anion | Chemistry Coach Twinning, both simple and complex, is common for both groups of minerals. The vacant p orbital does not change the pi electron count so the tropylium cation has 6 pi electrons which obeys the 4n + 2 rule for aromaticity. Since each O2 has a total charge of -2, it must be bonded to three Ti4+ to satisfy Rule 2, so O2- is in triangular coordination (Figure 13.18). What are cations and anions? Explain with examples. As coordination number increases, the space inside a polyhedron increases to accommodate larger cations. 13.33 Silver-pink margarite from Chester, Massachusetts. In sodalite, Na+ occupies the large site. . Anions have a negative sign. We call 3-fold coordination triangular because the anions form a triangle. Chemistry Terms: Table of Common Cations - ThoughtCo This means that atomic structures are relatively simple and ordered. Many normal substances exist in the body as ions. is among the first to achieve this important distinction for online health information and services. Since the radius of O2- is common to all alkali oxides, variations in bond length must be due to variations in the radii of the alkalis. Edge- and even sometimes face-sharing, however is more common in larger polyhedra than in tetrahedra because there is more room for cations to stay apart. His studies of chemical bonding resulted in the publication of his book The Nature of the Chemical Bond in 1939. Figure 13.2 shows the arrangement of atoms in anhydrite (CaSO4). For an anion to form, one or more electrons must be obtained, typically distant from other atoms with a weaker affinity for them. As an example of application of Paulings first rule, lets take another look at halite. Clay minerals, in particular, have complicated multilayered structures involving interlayers of H2O. Pyroxenes and pyroxenoids are both single-chain silicates. Copyright 1997-2023, A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited without authorization. For consistency, values from different sources should not be combined. Therefore, when the atoms of a metallic and a non-metallic element combine, the non-metallic atoms tend to extract one or more electrons from the metallic atoms to form ions. And tourmaline comes in many different colors because of its highly variable chemistry. Besides halite (NaCl), other alkalis combine with chlorine to produce alkali chlorides. 0 && stateHdr.searchDesk ? Consider the mineral halite, NaCl, which contains an equal number of Na and Cl atoms (Figure 13.5a). In Figure 13.22c, adjacent tetrahedra share an edge (two oxygen). If concentrations of dissolved Na+ and Cl reach high enough levels, perhaps due to evaporation, halite may precipitate from solution. For a long time, all structures were thought to have simple repetitive closest packing. It is related to diopside by the coupled substitution Li+Al3+ = Ca2+Mg2+. Each marble is in contact with three in the adjacent layer but you may have to look closely to convince yourself that this is the case. Also, it only has 4 pi electrons which does not follow Hckel's 4n + 2 rule. One of the effects of the aromaticity of the cyclopentadienyl anion is that the acidity of 1,3-cyclopentadiene is unusually strong. Yet, because the Si4+ are all positively charged, we expect them to repel each other. The photos below show two examples of amphiboles. For example, the average ionic radius of Mg, Additionally, ionic size varies depending on the number of bonds (called the, Another complication may arise because in some structures ions become, A final ambiguity arises because we assume a value for the radius of O. It has 10-12 fold coordination. About DifferentExample has more number of protons than electrons. The first number in every entry is the coordination number. To use the sharing features on this page, please enable JavaScript. The table below summarizes the different limiting ratios for different coordinations. Common examples include sodium, potassium, calcium, chloride, and bicarbonate. Both the 1H and 13C NMR spectrum would only have one signal. This means that every atom in the ring must have a. is important to understand how charged carbons in these compounds will affect the determination of aromaticity. Due to the ionic properties of salt water, scientists are now striving to exploit the, ionic electricity generating potential of salinity gradients. The lone pair electrons and the negative charge are conjugated about the entire ring making each carbon in the cyclopentadienyl anion equivalent with 1/5 the negative charge. Draw the resonance structures for cycloheptatriene anion. The octahedral site normally contains Al3+, but sometimes by Fe3+ or Cr3+. In micas and amphiboles, for example, F or Cl may replace (OH). Other photos of serpentine can be seen in Figures 8.72, 8.73, and 8.74 (Chapter 8). The subscript 0-1 tells us that the A site may not be fully occupied by K+ and Na+. Grainger & Allison's Diagnostic Radiology. In anorthite, another feldspar, Al3+ replaces half the Si4+, resulting in the formula Ca(Al2Si2)O8. About Me No matter the kind of bonding, (wholly or partially) positively charged ions alternate with (wholly or partially) negatively charged ions. If the third layer is above the first layer, we call the structure hexagonal closest packed. Today we know that some minerals have complicated structures, and that many minerals have polyhedral frameworks that are not closest packed at all. Electrostatic Valency Principle: The strength of an ionic bond is equal to ionic charge divided by coordination number. In monticellite, CaMgSiO4 (a member of the olivine group), the larger octahedral site contains Ca2+. Other silicates, such as kyanite, and titanite, CaTi(SiO4)O, contain O2- ions unassociated with the (SiO4)4- tetrahedra. Unlike micas, in chlorites, additional brucite layers (octahedral) separate the TOT sandwiches. Zoisite, vesuvianite, and epidote, for example, contain both SiO4 and Si2O7 groups. This is a coupled substitution, described as Al3+Al3+ = (Mg,Fe)2+Si4+ earlier in this chapter. Layers of SiO4 tetrahedra alternate with layers of Mg octahedra in brucite layers. Completed complementary courses involving information management, intellectual property, laboratory techniques, and operation of chemical processes. Compositional variations in silicate minerals are directly related to the nature of the crystallographic sites in their structures. These are made up of several atoms bonded together. before we may make calculations (see Box 13-3). When 1,3,5-cycloheptatriene is reacted with a reagent that can remove a hydride ion (H:-), the 1,3,5-cycloheptatrienyl cation, which is commonly known as the tropylium cation, is formed with unexpected ease. 13.31 Muscovite from Minas Gerais, Brazil. Cations (positively-charged ions) and anions (negatively-charged ions) are formed when a metal loses electrons, and a nonmetal gains those electrons. There are two main types of ions, cations and anions.When you see an element or a compound with a + or minus sign after it, that tells you that it is an ion.Cations have positive signs after them. Philadelphia, PA: Elsevier; 2021:chap 38. They break easily when salts dissolve in water, releasing free alkalis and Cl ions. Anion vs Cation - Difference and Comparison | Diffen Other photos of quartz were in Chapters 1, 3, and 6: Figure 1.8, Figure 3.44, Figure 3.61, Figure 6.26, and Figure 6.27. Figure 13.15c shows similar calculations for a cation in cubic coordination. Yet, minerals often contain other kinds of bonds. Second, bonds in minerals are rarely completely ionic, and ionic radius varies with the nature of the bond. For example, Figure 13.22 shows several ways silica tetrahedra might be associated in an atomic structure. Si4+, for example, nearly always bonds to four O2- anions in minerals, and therefore has a C.N. Ions with a negative charge are called anions. Silicate minerals belong to a special group of compounds that are mesodesmic. Video 13-1: Further discussion about why ionic radii can vary, even for the same element, Most minerals, except native elements such as gold or copper, contain anions or anionic molecules.