slide 1 of 18 chemistry 8.1. © copyright pearson prentice hall slide 2 of 18 molecular compounds...

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Chemistry 8.1

© Copyright Pearson Prentice Hall

Slide 2 of 18

Molecular Compounds

These toy models are made from circular pieces joined together in units by sticks. Atoms can also be arranged in different ways to make a variety of products.

8.1


Molecular Compounds >

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Molecules and Molecular Compounds

In nature, matter takes many forms. The noble gases, including helium and neon, are monatomic. That means they exist as single atoms.

In Ch 7, you learned about ionic compounds that are composed of ions held together by the electrostatic attraction of opposite charges.

8.1




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In some compounds the atoms are held together by sharing electrons. . . . .

This is called a covalent bond.

8.1

A molecule is a neutral group of atoms joined together by covalent bonds.

A diatomic molecule is a molecule consisting of two atoms. An oxygen molecule is a diatomic molecule.



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A compound joined by covalent bonds is called a molecular compound.

Water and carbon monoxide are molecular compounds.

8.1

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Bonding Theories >



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Molecules and Molecular Compounds8.1



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Molecules and Molecular Changes8.1

Ethane, a component of natural gas, is also a molecular compound.



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Molecular Formulas8.1

A molecular formula is the chemical formula of a molecular compound.

A molecular formula shows how many atoms of each element a molecule contains.

C6H12O6

6 carbon atoms + 12 hydrogen atoms + 6 oxygen atoms



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Molecular Formulas

Formulas of Some Molecular Compounds

8.1



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Representing Molecules8.1



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8.2 Representing Molecules

• An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots.

• A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms.



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8.2 The Octet Rule in Covalent Bonding

Why do atoms share electrons?

To attain the electron configurations of noble gases.

Which is . . . . . an octet of valence electrons.

(Does this make them a noble gas??? NO! )



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The Octet Rule in Covalent Bonding

Molecular compounds can share

one, ****two, ** **** **

or three ** ** ** ** ** ** pairs

of electrons in order to achieve an octet of 8 electrons in their valence shell.



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Single Covalent Bonds

Two atoms held together by sharing a pair of electrons

are joined by a single covalent bond.

8.2Single Covalent Bonds



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The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example.

8.2



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The hydrogen and oxygen atoms attain noble-gas configurations by sharing electrons.

8.2



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The ammonia molecule has one unshared pair of electrons.

8.2A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair.



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Methane has no unshared pairs of electrons.

8.2



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8.2 Double and Triple Covalent Bonds

Double and Triple Covalent Bonds

How do atoms form double or triple covalent bonds?

Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons.

2 shared pairs = double covalent bond.

3 shared pairs = triple covalent bond.



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These oxygen atoms share 2 pairs of electrons:

8.2



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These nitrogen atoms share 3 pairs of electrons:

8.2



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Bond Dissociation Energies

Bond Dissociation Energy: The energy required to break the bond between two covalently bonded atoms.

8.2

• A large bond dissociation energy corresponds to a strong covalent bond.

A carbon–carbon single bond: 347 kJ/mol.

Carbon–carbon double bonds: 657 kJ/mol.

Carbon- carbon triple bonds: 908 kJ/mol.

Strong carbon–carbon bonds help explain the stability of carbon compounds.



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Covalent bonds in polyatomic ions

A polyatomic ion, such as NH4+, is a tightly bound

group of atoms that has a positive or negative charge and behaves as a unit. It is held together by covalent bonds.

8.2

The polyatomic ammonium ion (NH4+), present in ammonium sulfate, is an important component of

fertilizer for field crops, home gardens, and potted plants.



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Resonance

Ozone, O3, in the upper atmosphere blocks harmful ultraviolet radiation from the sun.

At lower elevations, it contributes to smog.

8.2



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Resonance

A resonance structure is a structure that occurs when there are two or more valid electron dot structures for a molecule.

The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms.

8.2

Notice that it is possible to draw two valid electron dot structures for ozone:



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Exceptions to the Octet Rule

Exceptions to the Octet Rule (Oh, nooo!!!!!)

Sometimes there will exist molecules with atoms that have fewer or more than an octet of valence electrons.

8.2



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NO2 is produced naturally by lightning strikes.

8.2



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In PCl5, phosphorus has ten valence electrons.

In SF6, sulfur has twelve valence electrons!!

8.2



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How to Draw Lewis Dot Structures1. Sum the valence electrons from all atoms.

If the molecule is a polyatomic anion, add 1 e- for each (-) charge.

If the molecule is a polyatomic cation, subtract 1 e- for each (+) charge.

2. Identify the central and terminal atoms and write the skeleton structure.

The central atom is usually the one with the lowest electronegativity (except H)

Tips: (H is always terminal) (C is always a central atom, it always has 4 bonds)

3. Draw a bond between each pair of atoms. (each uses 2 electrons)

4. Place remaining e- pairs around terminal elements until octet rule has been met (duet rule for H), or until you run out of electrons.

5. If there are leftover e-, place them on the central atoms to complete octets.

6. If all e- are used up and the central atoms do not have octets, move one or more lone pairs to form a double or triple bond.

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