Chemical Bonding. Objectives

Transcription

1 Chemical Bonding Objectives Define chemical bond Explain why most atoms form chemical bonds Describe ionic, non-polar covalent and polar covalent bonds Define and explain the octet rule Explain why most chemical bonding is neither purely ionic nor purely covalent Use electronegativity to predict the charge distribution in molecules and ions and to define the polarity of bonds. 1

2 Atoms interact with one another by transferring or sharing electrons that are furthest from the nucleus. The outer electrons or valence electrons govern the chemical properties of the element. A chemical reaction results in the loss, gain or rearrangement of the valence electrons What is the process of losing an electron is called? Gaining and electron? Oxidation. Reduction. Electrons reorganize to form chemical bonds. Chemical bond reorganization of the valence electrons resulting in a net attractive force between two atoms 2

3 Chemical Bonds 3

4 Ionic bond one or more valence electrons are transferred from one atom to another creating ions (Remember cations and anions?) How can you determine how many electrons will be lost or gained when ions form? Atoms tend to gain or lose or share electrons so that their valence electron configuration is the same as the nearest noble gas. 4

5 Covalent bonding involves sharing of valence electrons between atoms (electron pair) Some examples 5

6 Your Turn Using electron dot diagrams and orbital diagrams, show covalent bonding between two nitrogen atoms Octet Rule Each atom is surrounded by an octet of eight electrons; the tendency of molecules and polyatomic ions to have structures in which eight electrons surround each other 6

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8 Polar covalent bonds are a particular type of covalent bond In a polar covalent bond, the electrons shared by the atoms spend a greater amount of time, on the average, closer to the more electronegative nucleus. This is because of the geometry of the molecule and the great electronegativity difference between the atoms. ξ=2.1 ξ=2.0 ξ=4.1 The result of this pattern of unequal electron distribution is a charge separation in the molecule, where one part of the molecule has a partial negative charge and the other part has a partial positive charge 8

9 Ion-dipole interaction 9

10 Your Turn For each pair of bonds, determine which bond has the greater polarity. H-C or H-P H-C > H-P O-I or O-F O-I > O-F S-O or N-O S-O > N-O N-H or Si-H N-H > Si-H Metallic solids are held together by a metallic bond. Although a number of models for metallic bonding have been proposed, the easiest one to picture has the valence electrons of the atoms contributed to a vast "sea" in which the positive inner atom residues "float". These mobile electrons help to make up for the valence deficiencies since a unit consisting of only metal atoms would generally be far from completing the valence levels for those atoms. They also create the electrical conductivity in the solid state which other structures generally lack. 26a450042effd6174b49257d3b3a69f.gif 10

11 Relevant Physics of Bonding F cohesive vs F disruptive always thermal Intermolecular Forces Secondary Bonding 11

12 A system is the part of the universe being studied, while the surroundings are the rest of the universe that interacts with the system. A system and its surroundings can be as large as the Gobi desert in Asia or as small as the contents of a beaker in a chemistry laboratory. A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a welldefined boundary. Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule Intermolecular vs Intramolecular 41 kj to vaporize 1 mole of water (inter) 930 kj to break all O-H bonds in 1 mole of water (intra) Generally, intermolecular forces are much weaker than intramolecular forces. 12

13 Because the liquid state still involves intermolecular forces at relatively close range, all liquids have some limited physical properties in common. Representative of these are: surface tension capillarity viscosity Measure of intermolecular force boiling point melting point H vap H fus H sub Dipole-Dipole Forces Attractive forces between polar molecules One end of a molecule is permanently positive and one end is permanently negative 13

14 Ion-Dipole Forces Attractive forces between an ion and a polar molecule Dispersion Forces Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules van der Waals Forces Weakest of the weak intermolecular forces Caused by random and temporary movement of electrons Short lived, temporary force More electrons = greater attractions ion-induced dipole interaction dipole-induced dipole interaction 14

15 Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud weekly%20pics/london.gif Dispersion forces usually increase with polarizability. 16/forces.jpg What type(s) of intermolecular forces exist between each of the following molecules? HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 is nonpolar: dispersion forces. SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules. 15

16 Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A weak force of attraction, but strongest compared to the previous two (van der Waals and dipole-dipole) Occurs when the most electronegative elements: N, O,and F; are directly bonded to H. High electronegative elements attract electrons so strongly that the molecule becomes very polar Source: 16