Chapter 5
Molecular Compounds
Covalent Bonds
• Covalent Molecule – A substance
comprised of only nonmetals and
metalloids
• Covalent Bond
– Formed by the sharing of electrons
• each atom within the bond achieves an octet (8)
except hydrogen (2) and boron (6)
• Minimize the repulsions and maximize the
attractions
Covalent Bonds
• Diatomic molecules
– Br I N Cl H O F
Naming Binary Molecular
Compounds
• The atoms of a molecular formula are
written in the order of the elements
electronegativity
– Least electronegative atom will always be
listed first.
• Electronegativity – indicates how well an
elements nuclei attract the electrons in a
covalent bond
The Periodic Table and
Electronegativity
Naming Covalent molecules
• Translate the elemental symbol into the
elements name
– 1st element – give its name
– 2nd element – gets ending –ide same as found in ionic
compounds
• Because electrons are shared
– Indicate how many of each atom are present using
the prefix system
• All elements will receive a prefix except if there is only 1 of
the first element
Naming Binary Molecular
Componds
The prefix mono- is omitted for the first element.
Problem
• What is the name of PCl3?
– a.
– b.
– c.
– d.
– e.
phosphorus chloride
phosphoric chloride
phosphorus trichlorate
trichlorophosphide
phosphorus trichloride
Problem
• The compound, P4S10, is used in the
manufacture of safety matches. What is its
name?
–
–
–
–
–
a.
b.
c.
d.
e.
phosphorus sulfide
phosphoric sulfide
phosphorus decasulfide
tetraphosphorus decasulfide
phosphorus sulfide
Molecular Formulas
• Translate the words
Covalent Bonds and the Periodic
Table
• Covalent bonds form between nonmetals
– Achieve an octet (noble gas configuration)
• Using electron dot symbols build
– H2, H2O, CH4, O2, N2, HCN, CO2
• Least electronegative atom is central (except H)
Covalent Bonds and the Periodic
Table
• Elements follow octet rule
– H gets 2 electrons – 1 bond
– B has 3 valence electrons – can only form 3
bonds
– Period 3 elements and higher can form more
than an octet because of the empty d-orbitals
Covalent Bonds and the Periodic
Table
Number of bonds formed to achieve octet. Numbers in
parentheses indicate possible numbers of bonds that
result in exceptions to the octet rule.
Multiple Covalent Bonds
• Single bond: A covalent bond formed by
sharing one electron pair.
• Double bond: A covalent bond formed by
sharing two electron pairs.
• Triple bond: A covalent bond formed by
sharing three electron pairs.
Molecular Formulas and Lewis
Structures
• Molecular formula: A formula that shows the
numbers and kinds of atoms in one molecule of
a compound.
• Structural formula: A molecular representation
that shows the connections among atoms by
using lines to represent covalent bonds.
• Lewis structure: A molecular representation
that shows both the connections among atoms
and the locations of lone-pair valence electrons.
Molecular Formulas and Lewis
Structures
► The oxygen atom in H2O
►shares 2 bonding pairs of electrons with two hydrogen
atoms
►Has 2 other pairs of valence electrons that are not shared
in bonds – lone pairs
Drawing Lewis Structures
• Sum up the valence electrons for all atoms in the
molecule
• Identify the atom in the lowest group # (except
H) – Least electronegative atom
– If both atoms in same group # identify the one in the
highest numbered period.
• Write this symbol as the central atom
surrounded by all other atoms
Drawing Lewis Structures
• Place a single bond (-, representing two
electrons) between all external atoms and
the central atom
• Add up the electrons in the bonds and
subtract this from the total valence
electrons
Drawing Lewis Structures
• Place all other electrons around the outer atoms
(in pairs) first giving them an octet (except H)
– Most electronegative atom first
• Place all remaining electrons around the central
atom as lone pairs
• Check to see if all atoms have an octet
– Yes – you are done
– No – make multiple bonds
Drawing Lewis Structures
• H, C, N, O, and halogen atoms usually maintain
consistent bonding patterns:
• H forms one covalent bond.
• C forms four covalent bonds.
• N forms three covalent bonds and has one lone
pair of electrons.
• O forms two covalent bonds and has two lone
pairs of electrons.
• Halogens form one covalent bond and have
three lone pairs of electrons.
The Shapes of the Molecules
• Molecular shapes can be predicted by noting
how many bonds and electron pairs surround
individual atoms and applying what is called the
valence-shell electron-pair repulsion
(VSEPR) model.
• VSEPR model - the negatively charged clouds of
electrons in bonds and lone pairs repel each
other, and keep as far apart as possible.
The Shapes of the Molecules
There are three step to applying the VSEPR model:
• Step 1:
Draw a Lewis structure of the molecule, and
identify the atom whose geometry is of interest.
• Step 2:
Count the number of electron charge clouds
surrounding the atom of interest.
• Step 3:
Predict molecular shape by assuming that the
charge clouds orient in space so that they are as far
away from one another as possible.
The Shapes of the Molecules
• Linear molecules have bond angles of 180°.
• Planar triangular molecules have bond
angles of 120°.
• Tetrahedral molecules have bond angles of
109.5°.
Polar Covalent Bonds and
Electronegativity
• As a rule of thumb
– electronegativity differences of less than 0.5 nonpolar covalent bonds
– differences up to 1.9 indicate polar covalent bonds
– differences of 2 or more indicate ionic bonds.
Problem
• Select the most polar bond amongst the
following.
– a.
– b.
– c.
– d.
– e.
C-O
Si-F
Cl-F
C-F
C-I
Polar Molecules
• If the bonds in a molecule have a polarity
then depending on the shape of the
molecule, the molecule itself can be polar
• Guaranteed nonpolar molecules
– Linear, trigonal planar and tetrahedral if all
peripheral atoms are identical
Characteristics of Molecular
Compounds