Chemistry – Dr. May Notes
Chemical Bonding
An Introduction to Bonding
Bonding Concepts
A chemical bond is a strong attractive force between atoms or ions in a
compound. The chemical bond is the glue that holds elements in compounds together.
In all the stable compounds, the atoms have acquired the electron configuration
that is isoelectronic with that of a nobel gas element.
Chemical reactions happen when molecules or atoms come in contact with each
other.
The strength of a bond (bond formation or bond breaking) is called Bond Energy.
Ionic Bonds
Ionic bonds are formed from a cation (+) and an anion (). Particles of the
opposite charge attract each other. This attraction is called an electrostatic attraction.
An ionic bond is a chemical bond formed by the electrostatic attraction between
an anion and a cation.
Crystals of ionic compounds exist as three-dimensional arrangements of cations
and anions (called crystal lattices) held together by electrostatic attractions.
Ionic compounds are formed when metals react with nonmetals since a metal can
transfer one or more electron to the nonmetal.
Li 1s22s1 + F 1s22s22p5  Li+ 1s2 + F 1s22s22p6
Fluoride (F) becomes isoionic with neon and lithium ion (Li+) becomes isoionic
with helium.
With the electron dot symbols, the element symbol represents the nucleus and the
dots represent the valence electrons.
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Covalent Bonds
Liquid hydrogen (H2) does not conduct electricity and can’t contain ions. The two
electrons are shared between the two hydrogen nuclei forming a covalent bond. The
distance between the two nuclei is called the bond length.
The electron dot diagram is used to represent the covalent bond as well as the
ionic bond.
Unequal Sharing of Electrons
Some covalent bonds, for example with HCl, form a polar molecule since the
electrons are not shared equally. These bonds are called polar covalent bonds in the same
way that H2 or Cl2 is called a nonpolar covalent bond.
The measure of the attraction an atom has for a pair of shared electrons is called
electronegativity. With HCl the Cl is more electronegative and the partial negative charge
() resides on it. The partial positive charge () resides on the hydrogen.
Electronegativity increases as you move from left to right across the periodic
table. Electronegativity decreases as you move down the columns of the periodic table.
Describing Molecular Structure
The Octet Rule
The octet rule assumes that atoms form bonds to achieve a noble gas electron
configuration. All Lewis electron dot structures place eight valence electrons around each
atom in a compound.
In order to place eight electrons around each atom, we at times need double bonds
(four electrons between the carbon and oxygen in carbon dioxide) or triple bonds (six
electrons between the two nitrogen atoms in N2).
For convenience, electron pairs are represented by a single line. This is especially
useful in organic chemistry. Water is represented at H – O – H and hydrogen cyanide as
H – C  N.
Limitations of the Octet Rule
Boron trifluoride can not be written with eight electrons around the boron atom.
This can be explained by a principal called resonance.
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Shapes and Properties of Molecules
Molecular Shapes
The VSEPR (Valence Shell Electron Pair Repulsion) Theory states that electron
pairs orient themselves so that the repulsion between pairs of electrons are minimized.
Molecular shapes may be linear (BeCl2), trigonal planar (BF3), tetrahedral
(carbon-hydrogen compounds), trigonal pyramidal (ammonia), bent (H2O), trigonal
bipyrimidal (PCl5), and octahedral (SF6).
Dipoles and Molecular Polarity
Water is a polar molecule and behaves as if it has a positive end and a negative
end.
–
O
H
H
+
+
The lower-case Greek letter delta () is used to indicate a partial charge.
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