Mr. D. Scott; CHS Chemical Bonding

Transcription

1 Chemical Bonding Chemical compounds are formed by the joining of two or more atoms. Chemical compounds consisting of two or more different chemical elements joined by a chemical bond; combined in a fixed ratio; have a unique and defined set of chemical properties; held together in a defined spatial arrangement (shape). All types of bonds are the result of two or more atoms interacting with each other s outermost (valence) electrons. These interactions are crudely divided into either sharing or transferring of the valence electrons. Covalent bond: bond in which one or more pairs of electrons are shared by two atoms. Ionic bond: bond in which one or more electrons from one atom are removed and attached to another atom (transferred), resulting in positive and negative ions which attract each other. These two categories represent extremely opposite actions. In reality, most chemical bonds are somewhere in between and involve some degree of sharing of the valence electrons.

2 Other types of atomic bonds include metallic bonds however, while metallic bonding is the method by which atoms are held together in metals, it does not result in the formation of Chemical Compounds. Therefore it is not a type of chemical bonding and will be dealt with in a later section of these notes. Let s remember three main ideas from our last topic: Octet Rule = atoms tend to gain or lose electrons in such a way that they each have eight electrons in their valence level, giving them the same electronic configuration as a noble gas. Electronegativity = the ability of an atom to pull electrons toward itself while bonded to another atom. Metals tend to lose electrons; Non-metals tend to gain electrons. We ll look first at Ionic Bonding.

3 Let s choose a couple of atom combinations and examine the situation. We ll begin with sodium and chlorine: Na X Cl For the sake of simplicity we ll crudely represent only their valence electrons. Note that the chlorine is 1 electron short of an octet or filled outer s 2 p 6. The one valence electron of sodium (3s 1 ) sits on top of a hidden filled s 2 p 6 beneath it. Now, let s compare their electronegativities. Na = 0.93 Cl = 3.16 Since these values represent the pull on electrons we ll picture a tug-of-war contest between them. e - Na = 0.93 Cl = 3.16

4 As you can clearly see, the chlorine will overpower the sodium in a battle for the sodium s valence electron. Na The electron will be transferred. AND.. X Cl + a cation and an anion will form. And then.. + Opposite charges attract, so the ions will then stay close together due to their mutual electrostatic attraction.

5 As more sodium and chlorine atoms react in this manner, an extended network of ions called a lattice structure will form. The resulting solid will have a definite crystalline shape due to the manner in which the ions stack together. For sodium chloride, this shape is cubic.

6 Here is some greater detail for this situation: Na = 1 st ionization energy is 496 kj/mol Cl = electron affinity is 349 kj/mol Therefore, it takes 496 kj/mol to remove the valence electron from sodium and chlorine releases 349 kj/mol when the electron is added to it. With only 147 kj/mol of outside energy, the electron can be moved from the sodium to the chlorine. Furthermore, as the two resulting ions join together in space, the potential energy released will easily provide this energy to the next two atoms that will react. This joining of ions energy is called lattice energy (LE). The overall reaction between Na + Cl is very exothermic. The LE of NaCl is 788 kj/mol; therefore a net energy of 641 kj/mol is released to the surroundings by this reaction. View the Reaction ( ) View the Reaction ( )

7 Overview of Ionic Bonding This is the type of bonding when there is a metal element + a non-metal element. Usually the electronegativity difference is more than about 2.1 between the two elements. Ionic bonding results in the formation of SALTS SALT = Compound consisting of a combination of cations and anions. Salts are all solids at room temperature. The ions in salts combine in a crystal lattice arrangement. Salt crystals have definite geometric shapes. Crystal shapes arise from the way the ions are stacked together and in what ratio they are combined. Lattice Energy = energy released as gaseous ions form into the solid salt crystal. (Equal to the energy required to take apart the crystal.)