In most ionic compounds, the anions are much larger than the cations, and it is the anions which form the crystal array. The smaller cations reside in the holes between the anions. Basic Concepts: 1. Ions are assumed to be charged, incompressible, nonpolarizable spheres. 2. Ions try to surround themselves with as many ions of opposite charge as closely as possible. Usually in the packing arrangement, the cation is just large enough to allow te anions to surround it without touching one another. 3. The cation to anion ratio must reflect the stoichiometry of the compound.
For MgCl2 the lattice must be an array of chloride anions with only half that number of magnesium ion. An ionic lattice is a structure of millions of atomic formations of an ionic substance, structured like building blocks into one three-dimensional formation. Ionic Bond A chemical bond is a mutual attraction between the nuclei and valence electrons of two different atoms. This attraction results in the two atoms binding together. An ionic bond, also called an electron-transfer bond, is a type of chemical bond that is a result of the electromagnetic attraction between ions of opposite charges, i. . , a cation (a positively charged ion) and an anion (a negatively charged ion). An ion is an atom or group of atoms that has acquired an electrical charge due to the loss or gain of electrons. In an ionic bond, an atom gives or receives electrons from another atom. This is in contrast to covalent bonding, where two atoms share electron pairs between them. An ionic compound consists of anions and cations combined such that the total charge of the molecule is zero. All salts are ionic compounds. One characteristic that both ionic and covalent compounds share is that they adhere to the octet rule.
The octet rule is the principle that describes the bonding in atoms. Individual atoms are unstable unless they have an octet of electrons in their highest energy level. The electrons in this level are called valence electrons. When atoms gain, lose, or share electrons with other atoms, they satisfy the octet rule and form chemical compounds. Ionic bonding occurs when one atom transfers electrons to another atom. In doing so, the atoms may achieve a complete outer energy level, satisfying the octet rule. During the formation of an ionic bond, one atom gains electrons and the other atom loses electrons.
As a result, the atoms gain an electric charge. The atom that gains electrons gains a negative charge, becoming an anion. The atom that loses electrons gains a positive charge, becoming a cation. An example of this is the ionic bond that is formed between sodium (Na) and fluorine (F) to make sodium fluoride, NaF. A fluorine atom has seven valence electrons. It needs one more to have a complete outer energy level and satisfy the octet rule. If it gains this electron, it will become a negatively charged fluorine anion (F-). A sodium atom has only one valence electron.
If it loses this electron, it is left with a complete outermost energy level that satisfies the octet rule. At the same time, it becomes a positively charged sodium cation (Na+). As the sodium atom loses its electron, the fluorine atom picks it up. The two ions now have opposite charges and attract each other. This electromagnetic attraction is quite strong and holds the ions together, forming sodium fluoride. This binding of the two ions is called an ionic bond. Energy is required for an atom to lose an electron. The process of losing an electron and forming an ion is called ionization.
The energy that is needed for ionization to occur is called the ionization energy. The ionization energy needs to be sufficient to overcome the attraction between the positively charged nucleus and the negatively charged electron. If an atom has only a few valence electrons, the ionization energy is low. The removal of a small number of electrons does not require much energy. Atoms with few valence electrons tend to lose these electrons easily and become cations. Sodium is an example of this phenomenon. A sodium atom only has one valence electron, which it loses quite easily. Thus, a sodium atom has a low ionization energy.
In contrast, atoms with many valence electrons have high ionization energies. The removal of several electrons requires more energy. These atoms do not lose electrons easily, instead, they tend to gain electrons and become anions. An atom that gains electrons easily is said to have electron affinity. Fluorine is an example of an atom with electron affinity. A fluorine atom has seven valence electrons, so it does not lose electrons easily. Instead, it tends to gain an electron in order to complete its outermost energy level. Most bonds are not completely ionic nor are they completely covalent.
How ionic or covalent a chemical bond is depends on how strongly the atoms of each element attract electrons. Electronegativity (a measure of an atom’s tendency to attract electrons) can be used to predict whether a bond will be a nonpolar covalent bond, a polar covalent bond, or an ionic bond. If the difference between the electronegativities of two atoms is 2. 0 or greater, the bond is an ionic bond. If the difference is 0. 4 or less, the bond is nonpolar. If the electronegativity difference between two atoms is between 0. 4 and 2. 0, the bond is considered to be a polar covalent bond.
The greater the electronegativity difference, the greater the polarity, and the greater the ionic character of the bond. Many substances are formed through ionic bonding. As mentioned above, all salts are formed with ionic bonds. A familiar example of an ionic compound is table salt, found in nature as rock salt. Table salt is sodium chloride (NaCl). A sodium ion, Na+, has a charge of 1+. A chloride ion, Cl-, has a charge of 1-. When a sodium atom gives up an electron to become Na+, and a chlorine atom gains this electron to become Cl-, these atoms combine as NaCl, forming an ionic compound with no electrical charge.
The majority of the rocks and minerals found on the Earth are formed using ionic bonding. An ionic compound has a specific structure. Most ionic compounds are crystalline solids. An ionic compound is composed of a network of ions that results in a three-dimensional matrix of cations and anions. This crystalline structure is an orderly arrangement of ions known as a crystal lattice. A crystal lattice structure minimizes an ion’s potential energy. Therefore, this structure is energetically favorable. The arrangement of ions in a crystal lattice represent the optimum balance between the forces f attraction among oppositely charged ions and the forces of repulsion among ions of the same charge. The physical arrangement of ions in a crystal lattice depends upon the number of ions as well as the sizes and charges of the ions. An ionic compound cannot be isolated into individual, neutral units, like a molecular compound can. The chemical formula of an ionic compound, therefore, does not represent the formula for one molecule of the substance. Instead, it represents the simplest ratio of the ions that gives the compound no net electrical charge.
The chemical formula of an ionic compound represents one formula unit of the compound, i. e. , the simplest combination of atoms that gives the compound electrical neutrality. The ratio of ions in a formula unit depends on the charges of the ions in the compound. For example, the ionic compound calcium fluoride is composed of calcium cations (Ca2+) and fluorine anions (F-). In order to have a net charge of zero, two fluorine atoms must combine for every calcium atom. This relationship is represented in the compound’s chemical formula, CaF2.
The forces of attraction between ions in an ionic compound are very strong. The forces involved in ionic bonding are much stronger than the forces in a covalent bond. This difference in strength gives ionic compounds different properties than covalent compounds. The strong forces that hold ions together cause ionic compounds to have higher melting and boiling points than covalent compounds. Ionic compounds also do not vaporize as easily at room temperature as covalent compounds. Ionic compounds are very hard and also quite brittle, due to the crystal lattice structure.
The ions in the crystal lattice cannot move very much without disturbing the overall balance between negative and positive charges. If one layer of atoms is moved, the result is a buildup of repulsive forces within the crystal structure, and the entire structure falls apart. Ionic compounds are good conductors of electricity when melted or dissolved in water because the ions dissociate in these states and are free to move and carry electrical current. In the solid state, the ions are not free to move, and the solid ionic compound does not conduct electricity.
Ionic compounds are found extensively in the Earth’s crust. They are very strong structures with unique properties. The properties of ionic compounds, for example, electrical conductivity, can be exploited for use in science and technology. Research into the nature of ionic bonding continues in order to find new uses for these compounds. Ionic lattice, or crystal lattice, has to do with how many of the same molecule orient themselves in a solid. This is not the same as ionic bonding. Ionic bonding is the bonding of two atoms of different ionic charges, Na+ and Cl- = NaCl.
There are many different kinds of ionic lattices and the patterns they make depend on the complexity of the molecules and how much space they take up in orientation to another duplicate molecule in the solid. Ionic Bonding Key Concepts • An ionic solid is made up of positive ions (cations) and negative ions (anions) held together by electrostatic forces in a rigid array or lattice. • Ionic bonding refers to the electrostatic attraction between cations and anions. • The physical properties of ionic compounds are: o High melting and boiling points o Ionic solids do not conduct electricity (they are insulators). When molten (liquid) ionic compounds conduct electricity. o When dissolved in water to form an aqueous solution ionic compounds conduct electricity. o Hard o Brittle Physical Properties of Ionic Compounds Melting Point Ionic compounds have high melting points. The electrostatic attraction (ionic bond) between cations and anions is strong. It takes a lot of energy to overcome this attraction in order to allow the ions to move more freely and form a liquid. The factors which affect the melting point of an ionic compound are: • The charge on the ions.
In general, the greater the charge, the greater the electrostatic attraction, the stronger the ionic bond, the higher the melting point. The table below compares the melting point and ion charges for sodium chloride and magnesium oxide. |Ionic Compound |Melting Point (oC) |Cation Charge |Anion Charge | |NaCl |801 |+1 |-1 | |MgO |2800 |+2 |-2 | •
MgO has a higher melting point than NaCl because 2 electrons are transferred from magnesium to oxygen to form MgO while only 1 electron is transferred from sodium to chlorine to form NaCl. • The size of the ions. Smaller ions can pack closer together than larger ions so the electrostatic attraction is greater, the ionic bond is stronger, the melting point is higher. The melting point of Group IA (alkali) metal fluorides is compared to the ionic radius of the cation in the table below. Ionic Compound |Melting Point (oC) |Cation Radius (pm) | |NaF |992 |99 | |KF |857 |136 | |RbF |775 |148 | |CsF |683 |169 | •
As the radius of the cations increases down Group I from Na+ to Cs+, the melting points of the fluorides decrease. Conductivity In order for a substance to conduct electricity it must contain mobile particles capable of carrying charge. | |Ionic Solid |Ionic Liquid |Aqueous Solution | |Mobility of Ions |very poor |good |good | |Electrical Conductivity |very poor |good |good |
Solid ionic compounds do not conduct electricity because the ions (charged particles) are locked into a rigid lattice or array. The ions cannot move out of the lattice, so the solid cannot conduct electricity. When molten, the ions are free to move out of the lattice structure. • Cations (positive ions) move towards the negative electrode (cathode) M+ + e —–> M • Anions (negative ions) move towards the positive electrode (anode) X- —–> X + e
When an ionic solid is dissolved in water to form an aqueous solution, the ions are released from the lattice structure and are free to move so the solution conducts electricity just like the molten (liquid) ionic compound. Brittleness Ionic solids are brittle. When a stress is applied to the ionic lattice, the layers shift slightly. The layers are arranged so that each cation is surrounded by anions in the lattice. If the layers shift then ions of the same charge will be brought closer together. Ions of the same charge will repel each other, so the lattice structure breaks down into smaller pieces.
