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 byH: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