Download Chapter 10: Chemical Bonding and more Study notes Geometry in PDF only on Docsity!
An understanding of how and why atoms attach together in the manner they do is central to chemistry
Chemical Bonds are the attractive forces that hold atoms
together in more complex units.
Lewis Theory
Emphasizes the importance of valence electrons
VALENCE ELECTRONS - electrons in
the outermost shell - ARE THE ELECTRONS
INVOLVED IN BONDING
Uses dots to represent valence electrons either ON or
SHARED by atoms
Bonding between atoms occurs by either transfer or
sharing of electrons to achieve outer shells with
8 electrons (exceptions: Li, Be and He) Lewis Electron Dot Symbols
Uses symbol of element to represent the nucleus and
inner (core) electrons
(Put one electron on each side first, then pair)
Uses dots around the symbol to represent valence
electrons
Remember that elements in the same group have the same number of valence electrons ; therefore, their Lewis dot symbols will look alike.
Chapter 10: Chemical Bonding
Noble gases are considered stable because they do not
react with other elements. This stability is attributed to
their FULL VALENCE SHELLS - they have a complete
He: 1 s^2 OCTET of ELECTRONS^ (exception: helium)
Ne: 1s^22 s^22 p^6 Ar: 1s^22 s^22 p^63 s^23 p^6 Kr: 1s^22 s^22 p^63 s^23 p^64 s^23 d^104 p^6 Xe: 1s^22 s^22 p^63 s^23 p^64 s^23 d^104 p^6 5s^2 4d^10 5p^6
Atoms of many elements that LACK a complete octet of
electrons in their outer shells react in such a way to attain it.
They may lose or gain electrons depending on the type of element the atom is (metal or nonmetal)
Ion formation occurs when atoms of two
elements (a metal and nonmetal) are present.
Recall: When a neutral atom loses or gains one or more
electrons an ion is formed.
The metal will lose one or more electrons - forms a cation The nonmetal will gain one or more electrons - forms an anion
a) Arsenic b) Iodine c) Silicon
Write the Lewis symbol for:
Formation of Ionic Compounds
Li•
•• : F : Li+^ [: F :]−
••
•• Lithium cation fluoride anion
Ionic Compounds: Electrons Transferred
H
can also be written as (^) H
- • - Cl :
- • The HCl molecule has 1 bonding pair of electrons and 3 nonbonding pairs of electrons (also called "lone pairs")
to the octet rule)
"OCTET RULE" (Note: there are exceptions
Nonmetal atoms share electrons to complete their octet, called:
Completing octets may involve sharing electrons with multiple atoms or sharing multiple pairs of electrons with the same atom.
In Lewis theory:
Nonbonding e-^ pair
Single covalent bonds Atoms share one pair^ of electrons (carbon sharing e-s with multiple hydrogen atoms)
Double covalent bond Atoms share two e-^ pairs Triple covalent bond Atoms share three e-^ pairs
Covalent Lewis Structures
- Decide on the central atom (it will never be H or F). (The central atom is usually the one that is by itself) - Add 1e-^ for a -1 charge, add 2e-^ for a -2 charge, etc... - Subtract 1e-^ for a +1 charge, subtract 2e-^ for +2, etc…
If the structure is an ion:
Determine the total number of valence electrons in the structure.
Form covalent bonds between the central atom and the surrounding atoms - called the "skeletal structure". Count how many electrons have been used to form these bonds.
Subtract electrons used to form covalent bonds from total number of valence electrons in the molecule or ion to determine how many electrons remain (if any).
Any remaining electrons become lone pairs, FIRST ON THE OUTSIDE ATOMS to complete their octets, and then on the central atom.
If any atoms that need an octet of electrons do not have it, form double and triple bonds as necessary by bringing outer atom lone pair electrons down between two atoms so they can share them.
Important: C, N, O and F always follow the octet rule
Write the Lewis structure for NH 3
- Central atom?
- Valence electrons?
- Form skeletal structure
- Remaining electrons?
- Lewis Structure?
Rules for Writing Lewis Structures for Covalent Compounds
Write the Lewis Structures for the following molecules
and polyatomic ions (continued).
e) HCN f) SO 32 - g) NH 2 Br 2 +
Resonance Structures
Write the Lewis structures of:
a) SO 2
In this situation, no one Lewis structure can adequately describe the actual structure of the molecule.
The actual molecule or ion will have characteristics of all the valid Lewis structures that can be drawn. (It is a hybrid of these Lewis structures).
Sometimes we can draw more than one valid Lewis
structure for a molecule or polyatomic ion.
b) HCO 2 -
Resonance Structures
A molecule or ion will be most stable when the
electron pairs or groups are as far apart as
possible.
The most important factor in determining the shape of a
molecule or polyatomic ion is the relative repulsion
between electron pairs.
Number of Electron "Groups" around the Central Atom Example Electron Geometry & Molecular Geometry
3 Trigonal Planar
4 Tetrahedral
(Molecular Geometry = Shape of Molecule)
Shapes of Molecules
Lewis Structure of H 2 O
4 Groups of Electrons
Electron Pair Geometry = Tetrahedral Molecular Geometry = Bent
Determine the Electron and Molecular Geometry of:
a) CCl 4 b) HCN
Determining Molecular Geometry
Determine the Electron and Molecular Geometry of: c) CH 2 S d) SO 2
e) H 2 S f) PH 3
Practice for next class:
Bond Polarity
One atom pulls the electrons in the bond closer to its side. One end of the bond has larger electron density than the other. The result is a (^) POLAR BOND
Bonding between unlike atoms results in unequal sharing of the electrons.
The end with the larger electron density gets a partial negative charge (δδδδ-)and the end that is electron deficient gets a partial positive charge (δδδδ+).
δ+ (^) H : Cl δ−
Example: HCl
Bond Polarity
- Electronegativity difference between zero and 0.
- Many times between two identical atoms
Nonpolar Covalent Bond
- Electronegativity difference between 0.4 and 2
- Between two different NONMETAL atoms
Polar Covalent Bond
Example:
δ+ H — F δ-
- Electronegativity difference is greater than 2
- Primarily exists between METALS and NONMETALS
Ionic Bond
Main Classes of Chemical Bonds
Can show the direction of bond polarity with δδδδ+ and δδδδ-
and/or a special arrow:
Show the direction of bond polarity for the bond in HCl.
AND
It must have polar bonds
It must have an unsymmetrical shape
In order for a MOLECULE to be polar:
If there are no polar bonds, then molecule is NONPOLAR
F F
Molecule is NONPOLAR
If there are polar bonds and the bond dipoles cancel out
Molecule is POLAR
If there are polar bonds and the bond dipoles DO NOT
cancel out,
Polar Molecules
Molecular Polarity Affects Solubility
Polar molecules are attracted
to other polar molecules
(Many ionic compounds dissolve in water as well).
Since water is a polar
molecule, other polar
molecules dissolve well in
water
Nonpolar molecules are
attracted to other nonpolar
molecules and dissolve in
each other
"LIKE DISSOLVES LIKE"
Molecular Polarity and Solubility
