- Explain how an ionic bond is formed when electrons are transferred from one atom to another in terms of the resulting electrostatic attraction. Draw diagrams showing this process.
- Describe the structural arrangements of ions in a crystal, including coordination number and its relationship to a given compound’s formula unit.
- Explain how various physical properties result from the ionic crystal lattice, such as strength, hardness, high melting points, brittleness, and electrical conductivity.
- coordination number
- formula unit
- ionic bond
- ionic compound
Check Your Understanding
Recalling Prior Knowledge
- What types of charged particles attract each other and what types repel each other?
- How is an empirical formula different from a molecular formula?
- What is a crystal lattice?
Most of the rocks and minerals that make up the Earth’s crust are composed of positive and negative ions held together by ionic bonding. An ionic compound is an electrically neutral compound consisting of positive and negative ions. You are very familiar with some ionic compounds, such as sodium chloride (NaCl). A sodium chloride crystal contains equal numbers of positive sodium ions (Na+) and negative chloride ions (Cl-).
Oppositely charged particles attract each other. This attractive force is often referred to as an electrostatic force. An ionic bond is the electrostatic force that holds ions together in an ionic compound. The strength of an ionic bond is directly dependent upon the magnitudes of the charges and inversely dependent on the distance between the charged particles. For example, a cation with a 2+ charge will make a stronger ionic bond than a cation with a 1+ charge. Additionally, a larger ion will form a weaker ionic bond than a smaller ion, due to the larger distance between its nucleus and the electrons of the oppositely charged ion.
Watch an animation of ionic bonding at http://www.dlt.ncssm.edu/core/Chapter9-Bonding_and_Geometry/Chapter9-Animations/IonicBonding.html.
- How does this animation represent the transfer of electrons?
- How do the sodium chloride units join together?
Electron Dot Diagrams
We will use sodium chloride as an example to demonstrate the nature of the ionic bond and how it forms. As you know, sodium is a metal, and it can lose its one valence electron to become a cation. Chlorine is a nonmetal, and it gains one electron to become an anion. By forming ions in this way, both atoms achieve a noble gas electron configuration. However, electrons cannot be simply “lost” to nowhere in particular, nor can they be "gained" without a source. In the case of sodium chloride, a single electron is transferred from the sodium atom to the chlorine atom, as shown below.
The ionic bond is the attraction between the Na+ ion and the Cl- ion. It is conventional to show the cation without any dots around the symbol, since the energy level that originally contained the valence electron(s) is now empty. The anion is now shown with a complete octet of electrons.
For a compound such as magnesium chloride, the two elements are not combined in a 1:1 ratio. Because magnesium has two valence electrons, it needs to lose both to achieve a noble gas configuration. Since chlorine only has room for one more electron in its valence level, two chlorine atoms must be present as electron acceptors in order to form each Mg2+ ion.
The final formula for magnesium chloride is MgCl2.
The formula unit is the lowest whole number ratio of the ions present in an ionic compound. The formula unit of sodium chloride is NaCl, while the formula unit of magnesium chloride is MgCl2. The formula unit of an ionic compound is always an empirical formula. In a previous chapter, Chemical Nomenclature, you learned how to write correct formula units for ionic compounds by employing the crisscross method.
The electron dot diagrams show the nature of the electron transfer that takes place between metal and nonmetal atoms. However, ionic compounds do not exist as discrete molecules, as the dot diagrams may suggest. In order to minimize the potential energy of the system, as nature prefers, ionic compounds take on the form of an extended three-dimensional array of alternating cations and anions. This maximizes the attractive forces between the oppositely charges ions. The figure below (Figure below) shows two different ways of representing the ionic crystal lattice. A ball and stick model makes it easier to see how individual ions are oriented with respect to one another. A space filling diagram is a more accurate representation of how the ions pack together in the crystal.
Two models of a sodium chloride crystal are shown. The purple spheres represent the Na+ ions, while the green spheres represent the Cl- ions. (A) In an expanded view, the distances between ions are exaggerated, more easily showing the coordination numbers of each ion. (B) In a space filling model, the electron clouds of the ions are in contact with each other.
The coordination number is the number of ions that immediately surround an ion of the opposite charge within a crystal lattice. If you examine the figure above (Figure above) (A), you will see that there are six chloride ions immediately surrounding a single sodium ion, so the coordination number of sodium is 6. Likewise, six sodium ions immediately surround each chloride ion, making the coordination number of chloride also equal to 6. Because the formula unit of sodium chloride displays a 1:1 ratio between the ions, the coordination numbers must be the same.
The formula unit for cesium chloride is CsCl, also a 1:1 ratio. However, as shown below (Figure below), the coordination numbers are not 6, like they are in NaCl. The center Cs+ ion is surrounded by eight Cl- ions at the corners of the cube. Each Cl- ion is also surrounded by eight Cs+ ions. The coordination numbers in this type of crystal are both 8. CsCl and NaCl do not adopt identical crystal packing arrangements because the Cs+ ion is considerably larger than the Na+ ion.
In a cesium chloride crystal, the cesium ion (orange) occupies the center, while the chloride ions (green) occupy each corner of the cube. The coordination number for both ions is eight.
Another type of crystal is illustrated by titanium(IV) oxide, TiO2, which is commonly known as rutile. The rutile crystal is shown below (Figure below).
Titanium(IV) oxide forms tetragonal crystals. The coordination number of the Ti4+ ions (gray) is 6, while the coordination number of the O2- ions (red) is 3.
The gray Ti4+ ions are each surrounded by six red O2- ions. The O2- ions are each surrounded by three Ti4+ ions. The coordination of the titanium(IV) cation is 6, which is twice the coordination number of the oxide anion, which is 3. This fits with the formula unit of TiO2, since there are twice as many O2- ions as Ti4+ ions.
The crystal structure of any ionic compound must reflect its formula unit. For example, in a crystal of iron(III) chloride, FeCl3, there are three times as many chloride ions as iron(III) ions.
View an animated example at http://www.dlt.ncssm.edu/core/Chapter9-Bonding_and_Geometry/Chapter9-Animations/IonicBonding.html
- How does this animation represent the transfer of electrons?
- How do the sodium chloride units join together?
To better understand the structures and properties of ionic compounds, go to http://www.chemguide.co.uk/atoms/structures/ionicstruct.html.
Physical Properties of Ionic Compounds
Pictured below (Figure below) are a few examples of the color and brilliance of naturally occurring ionic crystals.
In nature, the ordered arrangement of ionic solids gives rise to beautiful crystals. (A) Amethyst—a form of quartz, SiO2, whose purple color comes from iron ions. (B) Cinnabar—the common name for mercury(II) sulfide (HgS), which is the primary ore from which mercury is obtained. (C) Azurite—a copper-containing mineral, Cu3(CO3)2(OH)2. (D) Vanadinite—the primary ore from which vanadium is obtained, Pb5(VO4)3Cl.
The regular and orderly arrangement of ions in the crystal lattice is responsible for the various shapes of these crystals, while transition metal ions give rise to the colors.
Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about 800°C.
Ionic compounds are generally hard but brittle. Why? It takes a large amount of mechanical force, such as striking a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens, it brings ions of the same charge next to each other (Figure below). The repulsive forces between ions of the same charge causes the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions.
(A) A two-dimensional cross section of a sodium chloride crystal is shown. (B) When struck by a hammer, the negatively-charged chloride ions are forced near each other, and the repulsive force causes the crystal to shatter.
Another characteristic property of ionic compounds is their electrical conductivity. The figure below (Figure below) shows three experiments in which two electrodes that are connected to a light bulb are placed in beakers containing three different substances.
(A) Distilled water does not conduct electricity. (B) A solid ionic compound is also not very conductive. (C) A solution of an ionic compound dissolved in water conducts electricity well.
In the first beaker, distilled water does not conduct a current because water is a molecular compound. In the second beaker, solid sodium chloride also does not conduct a current. Despite being ionic and thus composed of charges particles, the solid crystal lattice does not allow the ions to move between the electrodes. Mobile charged particles are required for the circuit to be complete and the light bulb to light up. In the third beaker, the NaCl has been dissolved into the distilled water. Now the crystal lattice has been broken apart and the individual positive and negative ions can move. Cations move to one electrode, while anions move to the other, allowing electricity to flow (Figure below). Melting an ionic compound also frees the ions to conduct a current. Ionic compounds conduct an electric current when melted or dissolved in water.
In an ionic solution, the A+ ions migrate toward the negative electrode, while the B- ions migrate toward the positive electrode.
- One or more electrons are transferred from a metal atom to a nonmetal atom to form ions. Ionic bonds are the electrostatic attractions between positive and negative ions.
- An ionic compound is a three-dimensional network of alternating cations and anions that are mutually attracted to one another. The coordination number of an ion is the number of nearest neighbors that it has within the crystal lattice.
- Ionic compounds are hard and have high melting points. They are difficult to break but are also very brittle. They conduct electricity only when melted or dissolved in water to form a solution.
Lesson Review Questions
- Draw an electron dot diagrams for the following atoms and ions.
- Which of the following pairs of atoms would be expected to combine chemically to form an ionic compound? Explain.
- Li and O
- N and H
- Al and S
- Cl and F
- Sr and Br
- Zn and I
- Explain why most ionic compounds are strong and hard, yet brittle.
- Explain why potassium fluoride does not conduct an electric current as a solid, but it does conduct after being dissolved in water.
- Use electron dot diagrams to demonstrate the formation of ionic compounds involving the following elements. Use arrows to show the transfer of electron(s) from one atom to another.
- K and O
- Ca and N
- Ba and S
- Write the formula units for each of the ionic compounds from question number 5.
- Answer the following:
- What is a coordination number?
- If the coordination numbers for each of the two ions in a crystal lattice are identical, what must be true about the formula unit of the compound?
- An ionic compound forms between metal A and nonmetal B. The coordination number of the cation of element A is 4 and the coordination number of the anion of element B is 8. Write the chemical formula of the compound.
- A general ionic bond forms between a cation X+ and an anion Y-. How will the strength of the ionic bond change if the following changes are made? In other words, will the resulting bond be stronger or weaker?
- The charge of the cation is doubled.
- The size of the cation is increased.
Further Reading / Supplemental Links
Points to Consider
Metals are also crystalline materials. For a pure metal, each lattice point is an atom of the metal, rather than cations and anions as in an ionic crystal lattice. The familiar properties of metals result from its crystalline structure.
- What are the physical properties of metals?
- What is an alloy?