Chemical Bonding and Molecular Structure

Unit 3: Chemical Bonding and Molecular Structure : Unit 3: Chemical Bonding and Molecular Structure Cartoon courtesy of NearingZero.net

Bonds : Bonds Forces that hold groups of atoms together and make them function as a unit. Ionic bonds – transfer of electrons Covalent bonds – sharing of electrons

Electron Dot Notation : Electron Dot Notation

The Octet Rule – Ionic Compounds : The Octet Rule – Ionic Compounds Ionic compounds tend to form so that each atom, by gaining or losing electrons, has an octet of electrons in its highest occupied energy level.

Ionic Bonding:The Formation of Sodium Chloride : Ionic Bonding:The Formation of Sodium Chloride Sodium has 1 valence electron Cl 1s22s22p63s23p5 Na 1s22s22p63s1 Chlorine has 7 valence electrons An electron transferred gives each an octet

Ionic Bonding:The Formation of Sodium Chloride : Ionic Bonding:The Formation of Sodium Chloride Cl- 1s22s22p63s23p6 Na+ 1s22s22p6 This transfer forms ions, each with an octet:

Ionic Bonding:The Formation of Sodium Chloride : Ionic Bonding:The Formation of Sodium Chloride Cl- Na+ The resulting ions come together due to electrostatic attraction (opposites attract): The net charge on the compound must equal zero

Examples of Ionic compounds : Examples of Ionic compounds All salts, which are composed of metals bonded to nonmetals, are ionic compounds and form ionic crystals. Examples: Ba2+S2- Mg2+Cl-2 Na+2O2- K+I- Ca2+O2- Li+F-

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Properties of Ionic Compounds : Properties of Ionic Compounds

Sodium Chloride Crystal Lattice : Sodium Chloride Crystal Lattice Ionic compounds form solids at ordinary temperatures. Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions.

Representation of Components in an Ionic Solid : Representation of Components in an Ionic Solid Lattice: A 3-dimensional system of points designating the centers of components (atoms, ions, or molecules) that make up the substance.

Metallic Bonding : Metallic Bonding The chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons Vacant p and d orbitals in metal's outer energy levels overlap, and allow outer electrons to move freely throughout the metal Valence electrons do not belong to any one atom

Properties of Metals : Properties of Metals Metals are good conductors of heat and electricity Metals are malleable Metals are ductile Metals have high tensile strength Metals have luster

Packing in Metals : Packing in Metals Model: Packing uniform, hard spheres to best use available space. This is called closest packing. Each atom has 12 nearest neighbors.

Metal Alloys : Metal Alloys Substitutional Alloy: some metal atoms replaced by others of similar size.

Metal Alloys : Metal Alloys Interstitial Alloy: Interstices (holes) in closest packed metal structure are occupied by small atoms.

The Octet Rule – Covalent Compounds : The Octet Rule – Covalent Compounds Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level. Diatomic Fluorine

Hydrogen Chloride by the Octet Rule : Hydrogen Chloride by the Octet Rule

Formation of Water by the Octet Rule : Formation of Water by the Octet Rule

Comments About the Octet Rule : Comments About the Octet Rule 2nd row elements C, N, O, F observe the octet rule. 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive. 3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals. When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

Lewis Structures : Shows how valence electrons are arranged among atoms in a molecule. Reflects central idea that stability of a compound relates to noble gas electron configuration. Lewis Structures

Completing a Lewis Structure -CH3Cl : C H H H Cl .. .. .. .. Completing a Lewis Structure -CH3Cl Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14 Join peripheral atoms to the central atom with electron pairs. Complete octets on atoms other than hydrogen with remaining electrons Make carbon the central atom .. .. ..

Multiple Covalent Bonds:Double bonds : Multiple Covalent Bonds:Double bonds Two pairs of shared electrons

Multiple Covalent Bonds:Triple bonds : Multiple Covalent Bonds:Triple bonds Three pairs of shared electrons

Bond Length and Bond Energy : Bond Length and Bond Energy

Resonance : Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule. These are resonance structures. The actual structure is an average of the resonance structures.

Resonance in Ozone : Resonance in Ozone Neither structure is correct.

Models : Models Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Models can be physical as with this DNA model Models can be mathematical Models can be theoretical or philosophical

Fundamental Properties of Models : Fundamental Properties of Models A model does not equal reality. Models are oversimplifications, and are therefore often wrong. Models become more complicated as they age. We must understand the underlying assumptions in a model so that we don’t misuse it.

VSEPR Model : VSEPR Model The structure around a given atom is determined principally by minimizing electron pair repulsions. (Valence Shell Electron Pair Repulsion)

Predicting a VSEPR Structure : Predicting a VSEPR Structure Draw Lewis structure. Put pairs as far apart as possible. Determine positions of atoms from the way electron pairs are shared. Determine the name of molecular structure from positions of the atoms.

Table – VSEPR Structures : Table – VSEPR Structures

Polarity : Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment.

Hydrogen Bonding : Hydrogen Bonding Hydrogen bonding in Kevlar, a strong polymer used in bullet-proof vests. Bonding between hydrogen and more electronegative neighboring atoms such as oxygen and nitrogen

Hydrogen Bonding in Water : Hydrogen Bonding in Water

Hydrogen Bonding between Ammonia and Water : Hydrogen Bonding between Ammonia and Water

Dipole-Dipole Attractions : Dipole-Dipole Attractions Attraction between oppositely charged regions of neighboring molecules.

The water dipole : The water dipole

Relative magnitudes of forces : Relative magnitudes of forces The types of bonding forces vary in their strength as measured by average bond energy. Covalent bonds (400 kcal) Hydrogen bonding (12-16 kcal ) Dipole-dipole interactions (2-0.5 kcal) London forces (less than 1 kcal) Strongest Weakest