COVALENT BONDING Chapter 6, Sections 1&2

Electronegativity  A measure of the ability of an atom in a chemical compound to attract electrons from another atom in the compound  The difference between the electronegativities of two atoms in a bond can be used to determine the type of bond

Types of Bonds  Ionic  Electronegativity difference = between 1.7 and 3.3  One atom completely gives up electrons to the other atom  Covalent  Electrons are shared between two atoms  Polar Covalent Electronegativity difference = between 0.3 and 1.7 Atoms attract shared electrons unequally  Nonpolar Covalent Electronegativity difference = less than 0.3 Atoms attract shared electrons equally

Example Problems  H – H  C – O  Na – F

Electron Dot Notation  Shows an atom’s number of valence electrons placed around the atom’s symbol

Examples  C  Na  O

Octet Rule  Compounds form so that each atom has eight electrons in its outer shell  Exceptions:  H and He only have to have two electrons to fill their outer shells  B only has to have six electrons to fill its outer shell  Some elements can have more than eight electrons in their outer shells when they combine with F, O, or Cl

Bond Length & Bond Energy  Bond Length – the average distance between the nuclei of two bonded atoms  Bond Energy – the energy required to break a chemical bond and form neutral isolated atoms  Shorter bond length  Greater bond energy  Stronger bond

Molecular Compounds  Molecule – a neutral group of atoms that are held together by covalent bonds  Molecular Compound – a chemical compound whose simplest units are molecules  Examples  Molecules: H 2 0, CO 2, C 12 H 22 O 11  Molecular compounds: water, carbon dioxide, sugar

Lewis Structures  Way to represent atoms covalently bonded to each other  Similar to electron dot notation, but with more than one atom  Unshared electrons are represented by dots around the symbols  Shared electron pairs are represented by dashes between the symbols

How to Draw a Lewis Structure 1. Decide which atom will be the central atom  If there’s a carbon, that will be it  If there’s no carbon, it will be the least electronegative atom (except for H) 2. Draw the electron dot notation for each atom, arranging them in the form the molecule will be in  Place unpaired electrons next to each other 3. Connect shared electrons with a dash 4. Make sure each atom is surrounded by eight electrons and every unshared electron is paired

Structural Formulas  Another way to represent atoms covalently bonded to each other  Same as a Lewis structure, except without dots

Multiple Bonds  Atoms can share more than one pair of electrons  Double Bond – two pairs of electrons are shared between two atoms  Triple Bond – three pairs of electrons are shared between two atoms  Bond Length  Single bond > Double bond > Triple bond  Bond Energy  Triple bond > Double bond > Single bond

How to Draw a Lewis Structure with a Multiple Bond 1. Do all four steps of drawing a regular Lewis structure 2. If not all unshared electrons are paired, a double or triple bond may be needed 3. Move single unshared electrons from adjacent atoms to form another bond between the two atoms 4. Make sure each atom is surrounded by eight electrons and every unshared electron is paired