Ch. 10 - The Formation of Covalent Bonds

octet rule

atoms gain, lose, or share e- to achieve a stable noble gas e- configuration

duet rule

atoms form only one bond in covalent compounds; atoms achieve stability by having 2 e- in the outermost shell

• ex. H and He

lewis structure

diagram of atom

lone pairs

unshared pairs of e-

single bond

a bond consisting of two atoms sharing one pair of e-

double bond

a bond consisting of two atoms sharing two pairs of e-

triple bond

a bond consisting of two atoms sharing three pairs of e-

resonance structures

multiple, equivalent lewis structures

delocalized bond

multiple binds in resonance structures that are spread out over more than one location

resonance hybrid

a single structure with delocalized bonds that represents the average of the separate resonance structures

radical

molecule that has single, unpaired e- in lewis structure 

expanded valence shell

elements from periods 3-7 can form more than four bonds when making compounds, expanding beyond the octet rule. 

electronegativity 

measure of the electron-attracting ability of an atom in a covalent bond 

formal charge

compares an atom’s e- status within a molecule to the number of valence e- of the atom

formal charge formula

VE - 1/2BE - lone pairs e-

octet rule exceptions

• H has 2 e-

• Be has 4 e-

• B has 6 e-

• period 3 elements and group 7 can have more than 8 e- ONLY as the CENTRAL atom

nonpolar covalent bond

a bond resulting from equal sharing of e-

polar covalent bond

a bond resulting from unequal sharing of e- and characterized by a separation of partial charges 

ionic character

measured as a percent comparing the observed charge separation to 100% charge separation

dipole

a bond that exhibits separate areas of opposite charge of equal magnitude 

bond enthalpy

the enthalpy change associated with breaking a specific bond in 1 mol of gaseous molecule 

bond breaking

• energy must be added to break bonds

• endothermic

bond forming

• energy is released when bonds are formed

• exothermic

bond length increases

bond enthalpy decreases

longer bonds

• weaker than shorter bonds

• have smaller bond enthalpies than shorter bonds

endothermic reaction

if it takes more energy to break the bonds in the reactant molecules than is released when making the bonds in the product molecules, then the rxn absorbs energy

exothermic reactions

if more energy is released in bonds making than was used in bond breaking, then the rxn releases energy