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CHEMISTRY Chapter 7 Ionic Compounds Lesson 2 Ionic Bonds and Ionic Compounds. Ms. Ghina Saleh Formation of an ionic Bond: As we have seen before that ions are formed from the gain or lose of valence electrons. Actually, it is transformation of electrons between two atoms. During the reaction of...

CHEMISTRY Chapter 7 Ionic Compounds Lesson 2 Ionic Bonds and Ionic Compounds. Ms. Ghina Saleh Formation of an ionic Bond: As we have seen before that ions are formed from the gain or lose of valence electrons. Actually, it is transformation of electrons between two atoms. During the reaction of chlorine and sodium elements. A sodium (Na) atom transfers its valence electron to a chlorine (Cl) atom and becomes a positive ion. The chlorine atom accepts the electron into its outer energy level and becomes a negative ion. The electrostatic force that holds oppositely charged particles together in an ionic compound is referred to as an ionic bond. Compounds that contain ionic bonds are ionic compounds. If ionic bonds occur between metals and the nonmetal oxygen → oxides form. Most other ionic compounds are called salts. Charge of ionic compounds Ionic compounds are electrically neutral compounds. That is they don’t have a net charge. Consider the formation of the ionic compound calcium fluoride from calcium (Mg) and fluorine (F). Magnesium, a group 2A metal with the electron configuration [Ne]4s2, has two valence electrons. Fluorine, a group 7A nonmetal with the electron configuration [He]2s22p5, must gain one electron to attain the noble gas configuration of neon. Because the number of electrons lost must equal the number of electrons gained, it will take two fluorine atoms to gain the two electrons lost from one magnesium atom. This can be established by criss-cross method EXAMPLE Explain the formation of an ionic compound from the elements aluminum and oxygen. You are given that aluminum and oxygen react to form an ionic compound. Aluminum is a group 3A element with three valence electrons, and oxygen is a group 6A element with six valence electrons. To acquire a noble gas configuration, each aluminum atom must lose three electrons and each oxygen atom must gain two electrons. Remember that the number of electrons lost must equal the number of electrons gained. The smallest number evenly divisible by the three electrons lost by aluminum and the two gained by oxygen is six. Three oxygen atoms are needed to gain the six electrons lost by two aluminum atoms. Properties of Ionic Compounds: Ionic compounds are always solids at room temperature. They form a 3D crystal lattice structure containing regular repeating arrangement of ions, due to the very strong attraction force between positive and negative charged ions. In a crystal lattice, each positive ion is surrounded by negative ions and each negative ion is surrounded by positive ions. ionic crystals have high melting points and boiling points (require a large amount of energy to be broken apart due to strong attractive bonds) These crystals are also hard, rigid, and brittle solids due to the strong attractive forces that hold the ions in place. In the solid state, ionic compounds are nonconductors of electricity because of the fixed positions of the ions. However, in a liquid state or when dissolved in water, ionic compounds are electrical conductors because the ions are free to move. An ionic compound whose aqueous solution conducts an electric current is called an electrolyte. Energy and the ionic bond The formation of ionic compounds from positive and negative ions is always exothermic. When energy is absorbed during a chemical reaction, the reaction is endothermic. If energy is released, it is exothermic. The energy required to separate one mole of the ions of an ionic compound is referred to as the lattice energy. The strength of the forces holding ions in place is reflected by the lattice energy. The more negative the lattice energy, the stronger the force of attraction → greater lattice energy. Lattice energy is directly related to the size of the ions bonded. Smaller ions generally have a more negative value for lattice energy because the nucleus is closer to and thus has more attraction for the valence electrons. Example: the lattice energy of a lithium compound is more negative than that of a potassium compound containing the same anion because the lithium ion is smaller than the potassium ion. LiCl and KCl  The value of lattice energy is also affected by the charge of the ion. The ionic bond formed from the attraction of ions with larger positive or negative charges generally has a more negative lattice energy → greater lattice energy. NaF → Na+ and Cl- The lattice energy of MgO is greater than the lattice + - SrCl2 → Sr and Cl energy of SrCl2 greater than MgO. MgO → Mg2+ and O2-