Chapter #3 Chemical Bonding

Transcription

1 Chapter #3 Chemical Bonding Valence Electrons electrons in the last energy level of an atom. Lewis dot symbols Consists of the symbol of an element and one dot for each valence electron in the atom of an element. The dots (representing electrons) are placed on the four sides of the atomic symbol Each side can accommodate up to 2 electrons The number of valence electrons is the same as the column number (1 8) of the element in the periodic table. Li Be B C N O F Ne Dec 7 7:43 AM Octet Rule atoms react by gaining or losing electrons to fill the last energy level with 8 e s, just like in Noble Gases. (Except He) Jan 13 9:42 PM 1

2 Ionic Compound is a compound which is composed of positive and negative ions. The numbers of cations and anions MUST be such that the net charge is ZERO Ionic Bond this type of bond occurs when there is complete transfer (between the two atoms) of the electrons in the bond. Examples: NaCl and MgCl 2. ΔEN above 1.7 Dec 7 7:43 AM Dec 8 10:30 AM 2

3 Salts are made by ionic bonds between a metal and a nonmetal. Salts readily dissolve in water and conduct electricity. They boil at a very high temperatures and freeze at a low temperatures. Dec 12 10:35 AM Covalent Bonds are formed when a pair of electrons is shared between two atoms Dec 12 8:12 AM 3

4 Pure Covalent: This type of bond occurs when there is equal sharing (between the two atoms) of the electrons in the bond. ΔEN is 0 Dec 12 8:22 AM Polar Covalent: This type of bond occurs when there is unequal sharing (between the two atoms) of the electrons in the bond. ΔEN is between Molecules such as: H 2 O, and NH 3 Dec 12 8:27 AM 4

5 Lewis Dot Structure Rules 1. Determine the total number of valence electrons and electron pairs available to the molecule by: a. Sum the valence electrons of all the atoms in the molecule. b. Divide the total number of available electrons by 2 to obtain the number of electron pairs (E.P.) available. 2. Organize the atoms so there is a central atom ( usually the least electronegative) surrounded by outer atoms. Hydrogen is never the central atom. Dec 14 1:57 PM 3. Determine the electron distribution by arranging the electron in pairs (E.P.) until all available pairs have been distributed as bonded pair or unbonded. A few things to remember: Hydrogen will NEVER be a central element C,O,N,S, can double bond C,N, can triple bond Carbon will not have unbonded pairs of electrons Jan 13 9:58 PM 5

6 Molecular Polarity Polarity is a physical property of compounds which relates other physical properties such as melting and boiling points, and solubility. When there are no polar bonds in a molecule, there is no permanent charge difference between one part of the molecule and another, and the molecule is nonpolar. For example, the Cl 2 molecule has no polar bonds because the electron charge is identical on both atoms. It is therefore a nonpolar molecule. A molecule can possess polar bonds and still be nonpolar. If the polar bonds are evenly (or symmetrically) distributed, the bond dipoles cancel and do not create a molecular dipole. No side of the molecule has more negative or positive charge than another side, and so the molecule is nonpolar: Jan 2 9:00 PM A water molecule is polar because its O H bonds are significantly polar, and its bent geometry makes the distribution of those polar bonds asymmetrical. Decide whether the molecules represented by the following formulas are polar or nonpolar. (You may need to draw Lewis structures and geometric sketches to do so.) a. CO 2 b. OF 2 c. CCl 4 d. CH 2 Cl 2 e. HCN Jan 2 9:08 PM 6

7 Metallic Bonds formed by the atoms of metals, in which the outer electrons of the atoms form a common electron cloud or "sea" of electrons. The positive nuclei of the metal atoms are surrounded by free moving or mobile electrons that are attracted by the nuclei. metallic bonds are strong but not rigid, when a force is applied, layers of atoms can slide over one another Dec 15 2:11 PM Alloy is a mixture of two or more metals. They are homogenous mixtures and are prepared by heating and bonding the metals together. The resultant alloy has completely different properties from the starting metals. Sometimes to make an alloy, a non metal is also added. For example for obtaining various grades of steel, carbon is added in varying proportions. Thus an alloy is a uniform mixture of metals and/or metals and nonmetals. Some common alloys are steel, brass, and sterling silver. Jan 13 10:00 PM 7