Drawing and Naming Molecules
Lewis Dot Structures and Multiple
Bonds
Lewis Electron-Dot Structures
• Both ionic and covalent bonds involve valence
electrons
• A Lewis Structure is a structural formula in
which electrons are represented by dots; dot
pairs or dashes between two atomic symbols
represent pairs of covalent bonds.
Lewis Structures
• Lewis Structures show only the valence
electrons in an atom or molecule.
• The rest of the electrons and stuff inside the
nucleus is denoted by the chemical symbol.
• What is the maximum number of electrons
that can be in the valence shell??
Lewis Structures
• Hydrogen has _________ valence electron.
• The Lewis Structure for hydrogen then is:
H·
• The inner electrons, neutrons, and protons are
represented by the “H”, the valence electron is
denoted by the one lone dot.
Lewis Structures
• Hydrogen wants to have ___________ valence
electrons.
• To satisfy this, two hydrogens must come
together and “share” their one electrons, forming
a ____________ bond.
• This can be written out with a Lewis Dot by
H:H
Lewis Dot Structures for Others
LDS Show Valence Electrons
• Chlorine has seven valence electrons:
..
:Cl·
..
• Diatomic Cl (Cl2) has seven valence e-’s each:
.. ..
:Cl:Cl:
.. ..
• The two e-’s in the middle are shared
electrons, indicating a single bond.
LDS Show Valence Electrons
• Diatomic Cl (Cl2) has seven valence e-’s each:
.. ..
:Cl:Cl:
.. ..
• Each chlorine atom has _______ unshared
electrons or lone pairs.
• The same can be represented by
.. ..
:Cl
.. – Cl:
..
• The dash indicates a single bond also
When Writing LDS
• Remember with filling in orbitals, you must
put one electron in each bonding site before
putting in the second.
• The same is true for Lewis Dot Structures
• Draw out the Dot Structure for Carbon
Drawing Molecules
• Draw out the Lewis Dot Structures for the
following:
HCl
CH4
Br2
Radon
Oxygen
Phosphorous
Drawing Lewis Structures with Many
Atoms:
1. Gather information.
– Draw a Lewis structure for each atoms in the
compound. When placing valence electrons
around an atom, place one electron on each side
before pairing any electrons.
– Determine the total number of valence electrons
in the compound
Drawing Lewis Structures with Many
Atoms:
2. Arrange the atoms.
– Arrange the Lewis structure to show how the atoms
bond in the molecule.
– Halogen and hydrogen atoms often bind to only one
other atom and are usually at the end of the
molecule.
– Carbon is often placed in the center of the molecule.
– You will find that, with the exception of carbon, the
atom with the lowest electronegativity is often the
central atom.
Drawing Lewis Structures with Many
Atoms:
3. Distribute the dots.
– Distribute the electron dots so that each atoms,
except for hydrogen, beryllium, and boron,
satisfies the octet rule.
Drawing Lewis Structures with Many
Atoms:
4. Draw the bonds.
– Change each pair of dots that represents a shared
pair of electrons to a long dash.
5. Verify the structure.
– Count the number of electrons surrounding each
atom. Except for H, Be, and B, all atoms must
satisfy the octet rule. Check that the number of
valence electrons is still the same number you
determined in step 1.
Try the Rules, page 202
• Draw a Lewis structure for CH3I
Lewis Structures for Polyatomic Ions
• Try drawing the Lewis structure for the
ammonium ion, NH4+ **Same rules apply**
• Draw it out using square brackets with the
charge on the outside, letting us know the
charge on the polyatomic ion.
Multiple Bonds
• Atoms can share more than one pair of
electrons in a covalent bond.
• Draw out the Lewis structure for oxygen gas.
• Try drawing out the Lewis structure for C2H4
Multiple Bonds
• A covalent bond in which two atoms share
two pairs of electrons is known as a Double
Bond.
• A covalent bond in which two atoms share
three pairs of electrons is known as a
_____________________. Draw out the
Lewis structure for nitrogen gas.
Resonance Structures
• Some molecules, such as ozone, cannot be
represented by a single Lewis structure.
Ozone has two:
• When a molecule has two or more possible
Lewis structures, the structures are called
resonance structures.
• You place a double headed arrow between the
structures to show that the actual molecule is
an average of the two possible structures.
Another Resonance Structure
• Nitrogen dioxide also has two resonance
structures, try drawing them out.
• The actual structure of NO2 is an average, or a
resonance hybrid of the two structures.
How Come?
• This does not actually mean the bonds are
switching back and forth.
• It means that there is a mixture of the
resonance structures in the sample.
• The actual bonding is a mixture of the two
extremes represented by the Lewis structures.
Naming Covalent Compounds
• Review naming on pages 206 – 207 (type C)
• Uses prefixes.
• Might help you out.
Molecular Shapes
tomorrow (end of
chapter).
Test on Thursday