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CHEMICAL BONDS. A Dog’s Tale about an Element’s Search for Happiness (chemical stability). CHEMICAL BONDS. The forces that hold atoms together…often driven by the Octet Rule and stability.

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chemical bonds

CHEMICAL BONDS

A Dog’s Tale about an

Element’s Search for Happiness

(chemical stability)

chemical bonds1
CHEMICAL BONDS

The forces that hold atoms together…often driven by the Octet Rule and stability.

(Mutual electrical attraction between nuclei and valence electrons of different atoms that binds them.)

chemical bonds2
Chemical Bonds

Sometimes it helps to think of bonds (which you can't see) in terms of familiar things you can see.  This is called an analogy. 

Let's use the natural attraction of dogs to bones as an analogy to the attractions that cause chemical bonds.

chemical bonds3
Chemical Bonds

The negatively charged electrons determine how two or more atoms will interact when they are brought near each other.  In a sense, the atoms fight over the available electrons in much the same way two or more dogs will fight over bones. 

The Dog Bone Analogy works quite well for several types of atomic bonds. 

Dogs are the atoms.

Bones are the electrons.

slide5

Octet Rule

Atoms tend to gain, lose, or share electrons in order to acquire a full set of valence electrons.

8 (most of the time)

2 (1st energy level)

or

Happy dogs!

ionic bonds
IONIC BONDS

One big greedy thief dog!Ionic bonding can be best imagined as one big greedy dog stealing the other dog's bone. 

ionic bonds1
IONIC BONDS

The bone represents the electron that is up for grabs.

When the big dog gains an electron he becomes negatively charged, and the little dog who lost the electron becomes positively charged. 

These two ions (that's where the name ionic comes from) are attracted very strongly to each other as a result of the opposite charges.

slide8
IONIC BONDSChemical bonding that results from transfer of electrons and electrical attraction between cations and anions.
  • Form between metals (on the left side of the Periodic Table) and non-metals (on the right side of Periodic Table)
  • Metal loses electron(s) and becomes positively charged…a cation.
  • Non-metal gains electron(s) and becomes negatively charged…an anion.
  • The oppositely charged ions are held together because they are strongly attracted to each other.
  • Determine difference in electronegativity
    • See page 161
    • Difference of greater than 1.7 is often considered ionic
    • Ionic character greater than 50%
ionic bond
IONIC BOND?
  • Cs and F?
  • Page 161 electronegativity chart

F en = 4.0

Cs en = 0.7

--------------

3.3 is the electronegativity difference

  • A bond between Cs and F is ionic, greater than 1.7
ionic compounds
IONIC COMPOUNDS
  • Expressed as “formula units” rather than molecules because they form networks of ions attracted to each other…not independent neutral units.
  • Characteristics:
    • Crystalline structure
    • High melting points
    • Brittle
    • Dissolve in water
    • This separates the ions by breaking the ionic bonds…separated ions move freely making solution of ionic compounds good conductors of electricity.
    • Also conductive when melted.
formula unit is nacl 1 sodium and 1 choride the simplest ratio
Formula unit is NaCl, 1 sodium and 1 choride, the simplest ratio

Ionic compounds form orderly arrangements of the ions , called crystal lattices.

slide13

COVALENT BONDS

The dogs share the bones.

There are two types of sharing:

UNEQUAL

EQUAL

covalent bonds bonds formed when electrons are shared
COVALENT BONDSBonds formed when electrons are shared
  • Covalent bonds form between non-metals(on the right side of the Periodic Table)
  • The electrons are shared by the atoms.
  • Electrons may be shared equally or unequally.
  • Molecules are formed because atoms sharing electrons must be near each other
covalent bonds
COVALENT BONDS
  • Enormous variety of size, shape, physical, and chemical properties.
    • often lower melting point,
    • not necessarily crystalline
    • do not conduct electricity,
    • may or may not dissolve in water
polar covalent bonds
POLAR COVALENT BONDS

Unevenly matched dogs that are willing to share.These bonds can be thought of as two or more dogs that have different desire for bones. They share unequally.

polar covalent bonds1
POLAR COVALENT BONDS

The bigger dog has more strength to possess a larger portion of the bones.  Sharing still takes place but is an uneven sharing. 

polar covalent bonds2
POLAR COVALENT BONDS
  • In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule for both.
  • In a polar covalent bond, the electrons are shared unequally. They are attracted more to the more electronegative element.
  • The electrons spend more time near the more electronegative element, making it seem more negative and the other end of the molecule seem more positive…or “polar”
  • Determine difference in electronegativity
    • See page 161
    • Difference of .3-1.7 is often considered polar covalent
    • Ionic character 5-50%
polar covalent bond
POLAR COVALENT BOND?
  • Unequal sharing of electrons

Partial

negative

charge

Partial positive charge

polar covalent bond1
POLAR COVALENT BOND?
  • H and S?
  • Page 161

S en = 2.5

H en = 2.1

--------------

0.4 is the electronegativity difference

  • A bond between H and S is polar covalent, .3-1.7
polar molecules and polar bonds
Polar molecules and polar bonds

Two sides to the molecules/bonds

Partial positive charge towards less electronegative element

Partial negative charge towards element with greater electronegativity.

S

polamo lecules
Polamolecules

Two sides to Troy Polamalu

Calm, quiet, humble on the sideline

Fearless, crazed, maniacal on the field

polar molecules and polar bonds1
Polar molecules and polar bonds

Two sides to the molecules/bonds

Partial positive charge towards less electronegative element

Partial negative charge towards element with greater electronegativity.

S

non polar covalent bonds
NON-POLAR COVALENT BONDS

Covalent Bonds: Dogs of equal strength share equally.Covalent bonds can be thought of as two or more dogs with equal attraction to the bones. 

They share equally.

non polar covalent bonds1
NON-POLAR COVALENT BONDS

Since the dogs are identical, then the dogs share the bones evenly.  Since one dog does not have more of the bone than the other dog, the bone is equally shared between both dogs. One dog does not have more than the other.

non polar covalent bonds2
NON-POLAR COVALENT BONDS
  • In a covalent bond, the electrons are shared between atoms to fulfill the Octet Rule for both.
  • In a non-polar covalent bond, the electrons are shared equally.
  • The charge is evenly distributed between both atoms.  The molecule is non-polar meaning one side does not have more charge than the other.
  • Determine difference in electronegativity
    • See page 161
    • Difference of less than .3 is often considered nonpolar covalent
    • Ionic character less than 5 %
non polar covalent bond
NON-POLAR COVALENT BOND?
  • Cl and Br?
  • Page 161 for en

Cl en = 3.0

Br en = 2.8

--------------

0.2 is the electronegativity difference

  • A bond between Cl and Br is non=polar covalent, less than 0.3
non polar covalent bond1
NON-POLAR COVALENT BOND?
  • Equal sharing of electrons

Equal

distribution of charge

Equal

distribution of charge

diatomic elements they pair up
Diatomic ElementsThey pair up!
  • Br2
  • I2
  • N2
  • Cl2
  • H2
  • O2
  • F2

The diabolical DIATOMIC BrINClHOF TWINS!

metallic bonds
METALLIC BONDS

Mellow dogs with plenty of bones to go around. They don’t have to worry about sharing!

metallic bonds1
METALLIC BONDS

These bonds are best imagined as a room full of puppies who have plenty of bones to go around and are not possessive of any one particular bone.  This allows the electrons to move through the substance with little restriction.  The model is often described as the "kernels of atoms in a sea of electrons.”

metallic bonds2
METALLIC BONDs
  • Electrons are “delocalized”
  • Electrons move freely around and between atoms in the network of empty orbitals. The electrons move through the substance with little restriction.  The model is often described as the "kernels of atoms in a sea of electrons.”
metallic bonds3
METALLIC BONDS
  • The free movement of electrons explains the properties of metals…
    • Conductivity: free movement of electrons
    • Malleability & ductility: bonding is the same in all directions
    • Luster: energy released as electrons move easily between orbitals
slide35

We will look more closely at the different types of bonds to understand how millions of different compounds form from only about 100 different elements!

lewis dot of ionic compounds electrons taken not shared
Lewis Dot of Ionic Compounds (electrons taken, not shared)
  • Write the dot structures of the neutral atoms
  • Write the ions created by the loss or gain of electrons, in brackets. The metals have no dots. The non-metals have a full outer shell.
  • Show the ion charges as superscripts.
  • Show the ratio of ions needed to create a net charge of zero.
criss cross method to determine ratio of ions
Criss-Cross Method to Determine Ratio of Ions
  • Write the ion symbols and charges (superscripts). Get this info from periodic table for elements, polyatomic ion list for polyatomic ions.
  • Criss-cross the charge (numbers only) to subscripts. This shows the ratio of ions required for a net charge of zero.
  • Simplify. Mg2+ N3-

Mg3N2

why does it work
Why does it work?

The goal is a net charge of 0. The charges, or oxidation numbers, must add up to zero.

Mg3N2

3 x 2+= 6+

2 x 3-= 6-

0 net charge

lewis dot structures of covalent bonds shared electrons
Lewis Dot Structures of Covalent Bonds(shared electrons)
  • Lewis dot structures show atoms near each other sharing pairs of electrons to create bonds and satisfy octet rule for both atoms.
  • They may share up to 3 pairs of electrons
    • 1 SHARED PAIR :SINGLE BOND
    • 2 SHARED PAIRS ::DOUBLE BOND
    • 3SHARED PAIRS :::TRIPLE BOND
  • It does not matter which atom the electrons come from because they are shared.
covalent bonds usually follow the octet rule
Covalent bonds usually follow the octet rule
  • Put all of the valence electrons in a pot and distribute them between the atoms so each atom has a full octet by sharing up to 3 pairs of electrons.

: single bond

:: double bond

:: triple bond