lets consider the compound cesium fluoride, csf. –the electro-negativity value (ev) for cs is...
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BONDING
SOL Review
Introduction to Bonding
• Let’s consider the compound Cesium Fluoride, CsF.– The electro-negativity value (EV) for Cs is
0.70; the EV for F is 4.00.– The difference between the two is 3.30, which
falls within the scale of ionic character.• When the electro-negativity difference
between two atoms is greater than 1.7 the bond is mostly ionic.
• Let’s consider the compound Cesium Fluoride, CsF.– The electro-negativity value (EV) for Cs is
0.70; the EV for F is 4.00.– The difference between the two is 3.30, which
falls within the scale of ionic character.• When the electro-negativity difference
between two atoms is greater than 1.7 the bond is mostly ionic.
BondingBonding
Ionic BondingIonic BondingIonic Bonding
• In an Ionic bond:– The electro-negativity difference is
extreme, • So the atom with the stronger
pull doesn’t really share the electron
– Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction
• In an Ionic bond:– The electro-negativity difference is
extreme, • So the atom with the stronger
pull doesn’t really share the electron
– Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction
Ionic BondingIonic Bonding
• When a metal bonds with a nonmetal an: Ionic bond is formed
• An ionic bond contains a positive and negative ion.
• A positive ion is called a cation.• A negative ion is called an anion.• An Ionic bonding always involves the
transfer of an electron from the metal to the nonmetal.
• The cation and anion are held together by electrostatic attraction.
• When a metal bonds with a nonmetal an: Ionic bond is formed
• An ionic bond contains a positive and negative ion.
• A positive ion is called a cation.• A negative ion is called an anion.• An Ionic bonding always involves the
transfer of an electron from the metal to the nonmetal.
• The cation and anion are held together by electrostatic attraction.
Ionic BondingIonic Bonding
• Ionic compounds do not consist of individual molecules. Instead there is a huge network of positive and negative ions that are packed together in a solid brittle crystal lattice.
• Because their bonds are strong, ionic compounds tend to have very high melting and boiling points
• -Ionic compounds are electrolytes, which means they can conduct electricity
• When forming ionic compounds the positive and negative charges must balance
• Ionic crystals cannot conduct electricity because the ions must be able to move.
• Ionic compounds do not consist of individual molecules. Instead there is a huge network of positive and negative ions that are packed together in a solid brittle crystal lattice.
• Because their bonds are strong, ionic compounds tend to have very high melting and boiling points
• -Ionic compounds are electrolytes, which means they can conduct electricity
• When forming ionic compounds the positive and negative charges must balance
• Ionic crystals cannot conduct electricity because the ions must be able to move.
Characteristics of Ionic CompoundsCharacteristics of Ionic Compounds
Sea of ElectronsSea of Electrons
• The take home lesson on electro-negativity and bonding is this:– The closer together the atoms are on the P.T.,
the more evenly their e- interact, and are therefore more likely to form a covalent bond
– The farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond.
• The take home lesson on electro-negativity and bonding is this:– The closer together the atoms are on the P.T.,
the more evenly their e- interact, and are therefore more likely to form a covalent bond
– The farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond.
metal w/nonmetal = ionicmetal w/nonmetal = ionicnonmetal w/nonmetal = covalentnonmetal w/nonmetal = covalent
BondingBonding
Covalent Bonding
• In a covalent bond:– The electro-negativity difference between
the atoms involved is not extreme– So the interaction between the involved
electrons is more like a sharing relationship
– It may not be an equal sharing relationship, but at least the electrons are being “shared”.
• In a covalent bond:– The electro-negativity difference between
the atoms involved is not extreme– So the interaction between the involved
electrons is more like a sharing relationship
– It may not be an equal sharing relationship, but at least the electrons are being “shared”.
Covalent BondingCovalent Bonding
• Covalent Bonding is between two or more non-metals.
• Covalent bonds are formed when electrons are shared between two atoms.
• If they share 2 electrons, the form a single bond; 4 electrons is a double bond;
• If two atoms share 6 electrons, they form a triple bond.
• Covalent Bonding is between two or more non-metals.
• Covalent bonds are formed when electrons are shared between two atoms.
• If they share 2 electrons, the form a single bond; 4 electrons is a double bond;
• If two atoms share 6 electrons, they form a triple bond.
Covalent BondingCovalent Bonding
• Polar bonds usually involve nitrogen, oxygen or fluorine (NOF)
• Non-Polar bonds usually involve carbon-hydrogen bonds
• In polar bonds, the electrons are shared unequally
• In non-polar bonds, the electrons are shared equally.
• Covalent compounds can exist in any state (solid, liquid or gas). They have low melting and boiling points.
• Polar bonds usually involve nitrogen, oxygen or fluorine (NOF)
• Non-Polar bonds usually involve carbon-hydrogen bonds
• In polar bonds, the electrons are shared unequally
• In non-polar bonds, the electrons are shared equally.
• Covalent compounds can exist in any state (solid, liquid or gas). They have low melting and boiling points.
Covalent BondingCovalent Bonding
Lets look at the molecule Cl2Lets look at the molecule Cl2
Covalent BondsCovalent Bonds
ClCl
SharedElectronsShared
Electrons
ClCl ClClClCl
Shared electrons are counted with both
atoms
Shared electrons are counted with both
atoms
Cl ClNotice 8 e-
in each valence shell!!!
ClCl H H ClCl H H
Covalent BondsCovalent BondsHow about the molecule HCl?How about the molecule HCl?
(Polar Covalent) shared, but not evenly
(Polar Covalent) shared, but not evenly
2.12.1 3.03.0
To be stable the two atoms involved in the covalent bond
share their electrons in order to achieve the arrangement of a
Noble Gas.
To be stable the two atoms involved in the covalent bond
share their electrons in order to achieve the arrangement of a
Noble Gas.
So what’s the bottom line?
Drawing Lewis Structures
The Octet Rule
Concept of Formal Charge
Lewis Structures
Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.
The Octet Rule
• The most important requirement for the formation of a stable compound is that the atoms achieve a noble gas electron configuration.
Writing Lewis Structures
PCl31. Find the sum of
valence electrons of all atoms in the polyatomic ion or molecule.– If it is an anion, add one
electron for each negative charge.
– If it is a cation, subtract one electron for each positive charge.
5 + 3(7) = 26
Writing Lewis Structures
2. The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds.
Keep track of the electrons:
26 - 6 = 20
Writing Lewis Structures
3. Fill the octets of the outer atoms.
Keep track of the electrons:
26 - 6 = 20; 20 - 18 = 2
Writing Lewis Structures
4. Fill the octet of the central atom.
Keep track of the electrons:
26 - 6 = 20; 20 - 18 = 2; 2 - 2 = 0
Writing Lewis Structures5. If you run out of electrons
before the central atom has an octet…
…form multiple bonds until it does. Usually occurs with carbon, nitrogen, oxygen
Resonance Structures
More than One Lewis Structure
Resonance
This is the Lewis structure we would draw for ozone, O3. -
Resonance
• But this is at odds with the true, observed structure of ozone, in which…– …both O-O bonds
are the same length.– …both outer
oxygens have a charge of -1/2.
Resonance
• One Lewis structure cannot accurately depict a molecule like ozone.
• We use multiple structures, resonance structures, to describe the molecule.
Resonance
Just as green is a synthesis of blue and yellow…
…ozone is a synthesis of these two resonance structures.
Resonance
• In truth, the electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.
• They are not localized; they are delocalized.
Exceptions to the Octet Rule
Exceptions to the Octet Rule
• There are three types of ions or molecules that do not follow the octet rule:– Ions or molecules with an odd number of
electrons– Ions or molecules with less than an octet– Ions or molecules with more than eight
valence electrons (an expanded octet)
Odd Number of Electrons
Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.
Fewer Than Eight Electrons
• Consider BF3:– Giving boron a filled octet places a negative
charge on the boron and a positive charge on fluorine.
– This would not be an accurate picture of the distribution of electrons in BF3.
Fewer Than Eight Electrons
Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.
Fewer Than Eight Electrons
The lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.
More Than Eight Electrons
• The only way PCl5 can exist is if phosphorus has 10 electrons around it.
• It is allowed to expand the octet of atoms on the 3rd row or below.– Presumably d orbitals in
these atoms participate in bonding.
More Than Eight Electrons
Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.
More Than Eight Electrons
• This eliminates the charge on the phosphorus and the charge on one of the oxygens.
• The lesson is: when the central atom in on the 3rd row or below and expanding its octet eliminates some formal charges, do so.
• In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal.—Some ionic compnds have enough
space between the ions that water molecules can get trapped in between the ions
• Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate.
• In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal.—Some ionic compnds have enough
space between the ions that water molecules can get trapped in between the ions
• Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate.
Hydrate Formation
Trapped WaterMolecules
• Hydrates typically have different properties than their dry versions - A.K.A. anhydrides—Anhydrous CuSO4 is nearly colorless—CuSO4•5 H2O is a bright blue color
• When Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion.—The hydrated name would be Copper
(II) Sulfate Pentahydrate
• Hydrates typically have different properties than their dry versions - A.K.A. anhydrides—Anhydrous CuSO4 is nearly colorless—CuSO4•5 H2O is a bright blue color
• When Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion.—The hydrated name would be Copper
(II) Sulfate Pentahydrate
Hydrate Formation
• Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat?—These crystals are there to absorb
water that might lead to mildew or mold
• The formula of a hydrate is XAYB • Z H2O (Z is a coefficient indicating how many waters are present per formula unit)
• Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat?—These crystals are there to absorb
water that might lead to mildew or mold
• The formula of a hydrate is XAYB • Z H2O (Z is a coefficient indicating how many waters are present per formula unit)
Hydrate Formation
Molecular PolarityMolecular PolarityMolecules will be polar if
a) bonds are polar AND
b) the molecule is NOT “symmetric”
All above are NOT polar