Chapter 6 Chemical Bonding：6章化学键.docx

Chapter 6: Chemical Bonding Section 3: Ionic and Metallic Bonding Pages 176- 182RBQs #15,17,18,20,21,22,55,59 As previously seen an ionic bond is made from a metal and a non-metal. Metals form positive ions called _ Nonmetals form negative ions called _ When positive and negative ions combine so that the number of positive and negative charges are equal (making the compound neutral) an ionic compound is formed. For example, if a cation with a +1 charge is present, it needs an anion with a -1 charge to combine with Or if a cation with a +2 charge is present, it needs either an anion with a -2 charge to combine with, or 2 anions each with a -1 charge Recall that the chemical formula shows the relative number and types of atoms present In ionic compounds shows the chemical formula shows the ratio of the ions present Where the molecular formula(chemical formula of a covalent compound) shows the number of atoms in each molecule, the chemical formula of an ionic substance shows the number of ions in each formula unit A formula unit is the simplest collection of atoms from which an ionic compound formula can be established. In ionic bonding, electrons are transferred from one atom to another to form ions How do the electron affinities of metals and nonmetals compare? How do the ionization energies of metals and nonmetals compareExample of Ionic Bonding Why do atoms form chemical bonds? Ionic bonds minimize potential energy by combining in an orderly arrangement known as a crystal lattice A crystal lattice is a three dimensional network of positive and negative ions mutually attracted to one another The repulsive and attractive forces keep the distance between the ions in the crystal arrangement To compare bond strengths, lattice energy is examined Lattice energy is the energy needed to separate one mole of an ionic crystalline compound into formed from gaseous ions Or like bond energy, the amount of energy needed to break the bonds in a crystal lattice; again, a measure of bond strength Ionic bonds and covalent bonds have comparable strengths, depending on the atoms involved in the bond Properties such Melting point, boiling point, and hardness depend on how strong a compounds basic units are attracted Or in other words, how strongly the atoms in a bond are held together determines how much heat is needed to melt and boil them Ionic compounds are hard but brittle. One slight shift causes a buildup of repulsive forces. When dissolved in water ionic compounds conduct electricity For an electric current to be conducted, there must be free charged particles (like ions) that can move around This is what happens when ionic substances are dissolved in water Why dont solid ionic compounds conduct electricity? Several ions have already been seen: Al3+, Cl-, F-, etc. Each of these ions consists of only one atom, but there are also ions that consist of more than one atom These are called polyatomic ions; what are some examples? Polyatomic ions bond with ions of opposite charge to form ionic compounds. EXAMPLE: Na+ + NO3- NaNO3 Chemical bonding in metals (metallic bonding) is different from ionic and covalent bonds The difference is evident in the unique properties of metals (luster, good conductivity, etc.) The highest energy levels in metals are occupied by very few electrons, such as in titanium Ar _ _ _ _ _ _ 4s 3d 3d 3d 3d 3d The vacant orbitals overlap allowing the electrons to move freely around the metal The electrons are said to be delocalized, meaning that they do not belong to just one atom but move throughout the metals network of empty atomic orbitals These mobile electrons form what is known as the “sea of electrons” The chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons is called metallic bonding Metallic bonding (each metal bonds in all directions) and the sea of electrons are what give metals their characteristics What are the characteristics of metals? Conductivity: What is needed to make something conductive? (think back to the ionic solutions dissolved in water) What makes metals conductive? Luster: This is what makes metals so shiny and pretty. What happens to metal atoms when their atoms absorb light? Two important properties of metals are ductility and malleability. Malleability is the ability of a substance to be hammered or beaten down to thin sheets. Ductility is the ability of a substance to be drawn, pulled or extruded through a small opening to produce a wire. Both of these properties are possible because metallic bonding is the same in all directions, so the atoms stay bonded with each other and bond with new atoms when rearranged Metallic bond