Bond Formation
• Valence electrons interact to form BONDS
• Bonds form by TRANSFER or SHARING of
valence electrons
• Atoms form bonds to assume a NOBLE GAS
configuration.
• TRANSFER of electrons forms IONIC
compounds (metal non-metal)
• SHARING of electrons form COVALENT
compounds (non-metal non-metal)
Covalent bond
• Shared electrons are in the outer shells of both
atoms
··
H H
• The valence (outer) shells of both of these H
atoms now have 2 electrons.
OCTET RULE:
Atoms form bonds to have 8 electrons (except H2)
Outer S and P orbital electrons are VALENCE
electrons
Very useful for elements up to third period
METALS LOSE ELECTRONS
NON-METALS GAIN ELECTRONS
LEWIS ELECTRON DOT NOTATION
Covalent bond (Sharing)
• Bonding electron pairs
• Nonbonding electron pairs, also called lone pairs
·· ··
·· ··
F F
or
·· ··
–
·· ··
→
··
··
··
··
·
··
··
··
·
··
··
F + F
··
··
F F
Covalent bond
• Both F atoms now have 8 valence electrons
·· ··
–
·· ··
··
··
F F
Multiple bonds
• Atoms can share more than two electrons
·· ··
·· ··
–
·· ··
N≡N
··
N2
O=O
··
triple bond
··
O2
··
double bond
F F
··
F2
··
single bond
Multiple bonds are fatter and stronger
N2
triple bond
O2
F2
double bond
single bond
Ionic bond
• An ionic bond may result from electron transfer
··
··
→ Na+ F
··
··
··
·
··
··
·
Na + F
–
• In an ionic bond, electrons are not shared.
• Atoms are held together by electrostatic
attraction between positive and negative ions.
IONIC COMPOUNDS
The Difference Between Ionic and Covalent
• Ammonia ion plus Nitrate ion (COVALENT)
The difference between ionic and covalent bonds
• Ammonium nitrate, NH4NO3 (NH4+ NO3–)
Trends in electronegativity
Increases because Zeff increases,
which attracts electrons more strongly.
Outermost electrons are in same shell (same n).
Increases because
valence electrons are
closer to the nucleus
(orbitals are smaller)
so they are more
strongly attracted
to the atom.
Practice
• Rank these elements by electronegativity:
Co, O, Si, Sr
Practice
• Rank these elements by electronegativity:
least = Sr < Co < Si < O = most electronegative
Increase
Increase
Going gradually from ionic to covalent bonds
Li–F
Be–O
B–N
C–C
Practice
• Referring only to a periodic table, classify these bonds
as ionic, polar covalent or nonpolar covalent
• S–S
• N–F
• Sn–Si
• C–H
• Si–P
• Ni–S
• Mg–O
• Cs–F
Polar covalent bonds
··
–
··
δ+
δ−
··
H F
Less electronegative
partial positive charge
More electronegative
partial negative charge
These Lewis structures for CO2 are all equivalent
• The placement of lone pairs does not matter, as
long as they are somewhere on the correct atom.
O C O
O C O
O C O
• The direction of bonds does not matter, as long
as there are the correct number of bonds.
O
O C
C O
O
O
C
O