Download Chemical Bonding Review

Chemical Bonding Review
Compounds differ in composition as
well as chemical and physical
properties. Two major categories of
compounds are ionic compounds and
molecular (covalent) compounds.
• Ionic compounds:
metal + non-metal or
polyatomic ion
• Covalent compounds:
all non-metals
Recent Ionic Bonding Questions
Compounds and Elements
Chemical bonds are formed when
valence electrons are:
– Transferred from one atom to another (ionic)
• A metal from the left and a non-metal from
the right.
– Shared between atoms (covalent)
• Can be polar (unequal sharing), non-polar
(equal sharing)
– Mobile within a metal (metallic)
• The phrase “mobile valence electrons” is often
used.
Ionic Compounds can contain covalent bonds
In a multiple covalent bond, more than
one pair of electrons is shared
between two atoms. Unsaturated
organic compounds contain at least
one double or triple bond.
• Single, double, or triple bonds – each bond is
a shared pair of electrons
• Double and triple bonds are usually
represented by two or three lines between
atoms. For dot diagrams, two or three pairs of
dots are between the elements.
Molecular polarity can be determined by
the shape and the distribution of charge.
• Examples of symmetrical (nonpolar) molecules
include CO2, CH4, and the diatomic elements.
• Examples of asymmetrical (polar) molecules
include HCl, NH3 and H2O.
• These examples are used year after year.
Symmetrical molecules with polar bonds (the
C=O double bond is very polar) are non-polar.
• You should be able to draw or recognize Lewis
structures for all of the molecules above.
When an atom gains one or more
electrons, it becomes a negative ion
and its radius increases.
When an atom loses one or more
electrons, it becomes a positive ion
and its radius decreases.
When a bond is broken, energy is
absorbed. When a bond is formed, energy
is released.
• Absorbing energy – endothermic
• Releasing energy – exothermic
ΔH is (+)
ΔH is (-)
Making and Breaking Bonds
Atoms tend to bond so that a stable
valence electron configuration, like
that of a noble gas, is achieved.
• The octet rule governs much of chemistry.
• Sometimes they will ask about isoelectronic
atoms and ions; they have the same number
of electrons; for example, O2- = Ne = Mg2+ = 10
electrons (eight in the second energy level)
Electron dot diagrams (lewis
structures) can represent the valence
electron arrangement in elements and
compounds.
• You must be able to distinguish ionic and covalent
compounds to write Lewis structures successfully!
• Ionic compounds have brackets and charges. Covalent
compounds have shared electron pairs.
NaCl right: [Na]+ [: Cl : ]1-
wrong: Na:Cl :
HCl
wrong: [H]+ [: Cl : ]1-
right: H:Cl:
Electronegativity indicates how
strongly an atom of an element
attracts electrons in a chemical bond.
Electronegativity values are assigned
according to arbitrary scales.
• The definition of electronegativity is a common test
question. Sometimes they do not use the word
“electronegativity” in the question at all – you have to
recognize that they are talking about it!
• Electronegativity values for the elements are found in
Table S.
Electronegativity Values
The electronegativity difference between two bonded
atoms is used to assess the degree of polarity in the bond.
• Little or no difference: non-polar bond; large difference: polar
bond.
Random Bonding Questions