Chemical Bonding I What is a Bond A

Chemical Bonding

I. What is a Bond? A force that holds atoms together in a molecule or a compound Hydrogen atoms bonded together

II. Why are bonds important? A. Almost everything in the world around us is held together by chemical bonds.

B. The way in which atoms are bound together has a great effect on chemical and physical properties of substances. 1. Example: graphite and diamond are both composed solely of carbon yet they are completely different in their properties due to the different types of bonds between the carbon atoms.

Bonding is planar in graphite Bonding is tetrahedral in diamond

C. Molecular bonding and structure play a key role in determining the course of all chemical reactions, many of which are vital to our survival.

Example: The bonds in DNA determine its shape and how it is able to transmit genetic information into protein. DNA side view DNA end view

Example: The bonds in various enzymes determine what molecule they will react with. The amylase enzyme will react only with starch , sucrase only with sucrose and ATP synthetase only with ADP and P.

Sucrose being split Sucrose

Alpha amylase

III. Why do bonds form? A major reason is to give more stable arrangements of electrons within atoms involved in the bond.

IV. What is involved in forming bonds? A. Electron configuration this determines whether an atom will gain, lose or share electrons in forming a bond.

B. Electronegativity 1. Definition - the relative tendency of an atom to attract electrons to itself when it is bonded to another atom. 2. Four factors affecting it include: (1) nuclear charge (2) shielding effect (3) Radius and (4) sublevel

3. Trends in electronegativity: Follows the same trends observed in ionization energy and electron affinity (Decreases down a group and increases left to right across the periodic table). Electronegativity is a unitless value since it is a ratio of the same property for two elements.

C. Electronegativity and Bond Character. 1. The greater the electronegativity difference between the two bonding atoms the stronger the bond is that forms between them. 2. If the difference is greater than 1. 67 an ionic bond forms. If it is less than 1. 67 a covalent bond forms. Most bonds are a mixture of both types.

materials they form. A. Ionic Bonds 1. Characterized by a transfer of electrons. When electrons are transferred between atoms ions are produced having opposite charges. The attraction of oppositely charged ions holds them together. This electrostatic attraction is

2. Characteristics of ionic compounds include high melting points, ability to conduct electricity in the liquid and gaseous states, soluble in water, and they usually crystallize as sharply defined particles. 3. They are typically composed of a metal and a nonmetal.

4. An example of a substance having ionic bonds is Na. Cl, sodium chloride. (Watch the bond being formed in the demonstration below) Na Na Cl Cl

B. Covalent Bonds 1. Characterized by a sharing of electrons. The shared pair of electrons make up the covalent bond. 2. Most covalent compounds are formed between atoms of nonmetals 3. Examples are H 2, H 2 O, NH 3 and CH 4

H H Single bond Triple bond Double bond Quadruple bond

4. Characteristics of Covalent Compounds include (1) low melting points (2) do not conduct electricity in any state (3) solids are brittle. 5. The particle that results from a covalent bond is a molecule.

6. Definitions relating to molecules: a. bond axis - the line joining the nuclei of two bonded atoms b. bond angle - the angle between the bond axes when three or more atoms are bonded together. c. bond length - the distance between nuclei along the bond axis.

Bond Length Bond Angle Bond Axis

7. Bond length is not fixed because the atoms have a variety of movements such as bending, wagging, stretching and rotational vibrations. These movements cause bond angles and bond lengths to vary. What we actually measure is an average.

C. The Metallic Bond 1. Characteristics of metals include high electrical conductivity, luster or shininess, and malleability. 2. Electrons in the metallic bond are said to be delocalized because of an overlap of outer level orbitals between atoms. This allows electrons the freedom to move from atom to atom. These delocalized electrons make up the metallic bond.

Sodium atoms in metallic sodium.

3. The properties of metals are determined by the number of electrons they can contribute to the metallic bond. 4. Group IA metals have only one electron to contribute to the bond are therefore soft. Group IIA with two electrons are a little harder.

5. The hardest metals are found in groups VIB through VIIIB where electrons from partially filled “d” sublevels take part in the metallic bond. Examples of period 4 metals follow in order of increasing atomic number