Ch 10 - Covalent Bonds

bonds

forces that hold groups of atoms together and make them function as a unit

electronegativity

a measure of the tendency of an atom to attract electrons (or electron density towards itself

dipole moment

a measure of the separation of positive and negative electrical charges within a system

What does an electronegativity difference < 0.4 mean?

pure covalent

What does an electronegativity difference between 0.4 and 1.8 mean? 

polar covalent 

What doos an electronegativity difference >1.8 mean?

ionic

lewis structure

depicts the arrangement of valence electrons among atoms in a molecule

lone pairs (LPs)

pairs of electrons localized on an atom

bonding pairs (BPs)

electron pairs participating in bonding

what does electron counting for each electron involve?

BP + LP = octet (most cases)

octet rule

the tendency of main group atoms (except the first row) to form enough bonds to obtain 8 valence electrons,

what criteria does the central atom of a lewis structure have to meet?

• needs to make more bonds to become stable 

• lower electronegativity 

resonance forms

when two lewis structures are equally plausible , only electrons can be shifted not atoms 

formal charge

hypothetical charge the atom would have if we could redistribute the electron int he bonds evenly between atoms

How to do calculate the formal change?

the number of valence electrons - the number of lone electrons - ½ (the number of electrons)

How many bonds does Beryllium’s valence electrons make?

4

How many bonds does Boron’s valence electrons make?

6

electron deficient

odd electron species (free radicals), paramagnetic

expanded octets

elements that are capable of surrounding themselves with more than 4 pairs of electrons (i.e. sulfate, phosphate pentachloride, phosphate, sulfur hexafluoride 

bond enthalpy

the enthalpy change associated when breaking a specific bond in one mol of gaseous molecules

bond breaking =

endothermic

bond making =

exothermic

the equation for estimating the reaction enthalpy from bond enthalpies

(detla H of broken bonds) - (delta H of bonds formed)

bond length

the distance between the nuclei of two atoms that share electrons covalently

What happens as bond length increase?

bond enthalpy decrease