Chemical Bonding

Types of Chemical Bonds Ionic Bonding – bonding as a result of atoms giving away or receiving e-; attraction between cations and anions Covalent Bonding – bonding as a result of atoms sharing e-

Q: How do we know whether ionic or covalent? A: Compare electronegativities of the atoms. In general: Small difference = covalent bond Medium difference = polar covalent Large difference = ionic bond

Covalent Bonding & Molecular Compounds

Covalent Bonding & Molecular Compounds Molecule – a neutral group of atoms held together by covalent bonds Molecular Compound – a compound whose simplest units are molecules Molecular Formula – shows the type and number of atoms present Diatomic molecule – molecule with two atoms

Forming Covalent Bonds Atoms almost always favor an arrangement with lower potential energy As atoms get closer, the positively charged nucleus is attracted to the negatively charged electron cloud

Covalent Bond Characteristics Bond length – the average distance between two bonded atoms at minimum potential energy Bond energy – the amount of energy needed to break a chemical bond; measured in kJ/mol Overlapping orbitals

The Octet Rule Chemical compounds tend to form so that each atom has an octet of electrons in its highest energy level Exceptions to the rule to exist; called expanded octets

Electron Dot Notation & Lewis Structures Electron configuration showing only the valence electrons around the chemical symbol Unshared pair (lone pair) – a pair of electrons NOT involved in bonding and belong only to one atom

Drawing Lewis Structures Determine type and number of atoms Find total number of valence electrons Arrange atoms with carbon in the middle; if no carbon present, least electronegative element in the middle; hydrogen is never the central atom Add unshared pairs of electrons forming octets Ex – NH3, H2S, CO2, HCN

Resonance Structures Bonding in molecules or ions that cannot be correctly represented by a single Lewis structure Ex – ozone

Ionic Bonding & Ionic Compounds

Ionic Compounds Ionic compounds – made of positive and negative ions combined so the charges are equal (zero) Formula unit – simplest collection of atoms wich form an ionic compound’s formula Ex – sodium chloride (NaCl)

Ionic Bonding Involve a transfer of electrons making one atom positively charged and the other negatively charged (cations and anions) Form a crystal lattice structure which balances the charges of the ions Lattice energy – amount of energy released when 1 mole of an ionic compound is formed from gaseous ions

Ionic Bonding Electrostatic forces hold ionic compounds together Makes ionic compound very hard but brittle Strong attractions raise boiling and melting points

Polyatomic Ions Charged group of covalently bonded atoms Charge is on the collective group; not any particular atom Caused by an excess or shortage of electrons Lewis structures are written in brackets

Metallic Bonding The chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons Outer-most electrons are free to move between overlapping orbitals This accounts for metals ability to conduct heat and electricity

Metallic Bonding Can absorb a wide range of light wavelengths Immediately lose energy and fall to lower level Accounts for metals shiny appearance Bonding is the same in all directions Accounts for properties of malleability and ductility

Intermolecular Forces (Dipole-Dipole) Created by equal but opposite charges separated by short distances Strongest intermolecular force Represented by an arrow in the negative direction Polar molecules have dipole moments

Intermolecular Forces (Hydrogen Bonding) The intermolecular force in which a hydrogen atom is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule Creates a fairly strong dipole-dipole force

Intermolecular Forces (London Dispersion Forces) Slight attraction between molecules caused by instantaneous dipole moments Weakest intermolecular force Only force acting between noble gases and non-polar molecules