Chapter Eight Molecular Structure The chemical bond is a strong attractive force that exists between certain atoms in a substance. Ionic bond The chemical bond Covalent bond* van der Waals forces Intermolecular forces* hydrogen bonding 2018/11/19

8-3 Intermolecular Forces 8-1 Ionic Bonds 8-2 Covalent Bonds 8-3 Intermolecular Forces 8-2.1 The Lewis Theory 8-2.2 The Valence-Shell Electron-Pair Repulsion (VSEPR) Model 8-2.3 Valence Bond Theory 8-2.4 Hybrid Orbital Theory 8-2.5 Molecular Orbital Theory The Polarity Of Molecules Van Der Waals Forces The Hydrogen Bond 2018/11/19

8-2 Covalent Bonds Covalent bond, is a chemical bond formed by the sharing of a pair of electrons between the two atoms. There are several theories to explain the formation of covalent bond: 8-2.1 The Lewis Theory 8-2.2 The Valence-Shell Electron-Pair Repulsion (VSEPR) Model 8-2.3 Valence Bond Theory 8-2.4 Hybrid Orbital Theory 8-2.5 Molecular Orbital Theory 2018/11/19

8-2.1 The Lewis Theory Gilbert Newton Lewis (1875-1946) was nominated for the Nobel Prize in chemistry over 30 times, and still today, many scientists believe he well deserved it. Lewis became the mentor to 290 Ph.D. recipients and 20 Nobel Prize winners. He directed the experiments that resulted in the discovery of elements 93-106. 2018/11/19

The octet rule, which says that atoms tend to lose, gain or share electrons when bonding to have eight electrons in their valence shells (outer electron shell). (Elements of lowest atomic number tend to lose or gain electrons to give a single shell of two electrons, as in the noble gas Helium.) 2018/11/19

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H : H Electron-dot formula (also called a Lewis formula) is a formula that uses pairs of dots to represent covalent bonds, as well as dots to represent the electrons on individual atoms F-F F2 2018/11/19

Multiple Bonds Single bond: a covalent bond formed by the sharing of a single pair of electrons between two atoms Double bond: a covalent bond formed by the sharing of two pairs of electrons between two atoms Triple bond: a covalent bond formed by the sharing of three pairs of elections between two atoms 2018/11/19

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Lone pair Bonding pair Bonding pair Bonding pair Bonding Pairs and Lone Pairs A bonding pair is an electron pair that is shared between two atoms. A lone pair (or nonbonding pair) is an electron pair that is on one atom, it is not involved in bonding. Lone pair Bonding pair Bonding pair Bonding pair 2018/11/19

+ H Coordinate covalent bond Coordinate covalent bond is formed when both electrons of the bond are donated by one atom + H Coordinate covalent bond 2018/11/19

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8-2.3 Valence Bond Theory 1. The Formation of a Covalent Bond 2018/11/19

8-2. 3 Valence Bond Theory 1. The Formation of a Covalent Bond 84pm ------------------ 84pm 8-2. 3 Valence Bond Theory 1. The Formation of a Covalent Bond 2018/11/19

2. The Central Themes of VB Theory (1) Opposing Spins of the Electron Pair As the exclusion principle prescribes, the molecule formed by the overlapping orbitals has a maximum capacity of two electrons that must have opposite spins. 2018/11/19

(2) The Number of the Covalent Bond Saturation character The number of bonds formed by a given atom to equal the number of unpaired electrons in its valence shell. For example, nitrogen with the out configuration 2s22px12py12pz1, can share its 2p electrons with three hydrogen atom to form three bonds, as in NH3 (ammonia), but it cannot form NH5. 2018/11/19

(3) Maximum Overlap of Bonding Orbitals Direction character An orbital on one atom comes to occupy a portion of the same region of space as an orbital on the other. The two orbitals are said to overlap. the greater the orbital overlap, the stronger (more stable) the bond. 2018/11/19

To maximize overlap in HCl, half-filled H ls and + To maximize overlap in HCl, half-filled H ls and Cl 3p orbitals overlap along the long axis of the 3p orbital involved in bonding. 2018/11/19

3. The classification of covalent bonds Sigma Bond A. Two kinds of bonds Pi Bond Sigma bond (σ) is formed by the overlap of the end of one orbital with the end of the other. It has a cylindrical shape symmetry about the bond axis. Sigma bonds are exemplified by s-s, s-px, and px-px overlaps. (head to head) 2018/11/19

(2) A pi bond (л) is formed by the sidewise overlap of two parallel p orbitals. (side to side) Pi bonds are exemplified by py-py, and pz-pz overlaps. a sidewise overlap will not give so strong a bond as an along-the-axis overlap of two p orbitals. 2018/11/19

The overlap of orbitals to form sigma bond s-s overlap px-px overlap s-px overlap 2018/11/19

The overlap of orbitals to form л bond 2018/11/19

s-s overlap s-px overlap px-px overlap σ bond py-py overlap pz-pz overlap Л bond 2018/11/19

B．Coordinate Covalent Bonds A coordinate covalent bond is a bond in which both electrons of a shared pair come from one of the two atoms involved in the bond. H+ + :NH3 [ H NH3 ]+ ← C O 2018/11/19

4. Bond parameters bond length bond energy bond angle The bond length is defined as the distance between the nuclei of the atoms involved in the bond. 2018/11/19

I-I 2018/11/19

Bond energy is determined by the energy required to break one mole of bonds in a gaseous species. 2018/11/19

2pz 2px 2py The bond angle is the angle formed by the nuclei of two surrounding atoms with the nucleus of the central atom at the vertex. 2pz 2px 2py 2018/11/19