covalent bonds – the nice bonds that share!. how do we know the type of bond that will be formed?...
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Covalent Bonds – The nice bonds that share!
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How do we know the type of bond that will be formed?
• Simplistically, Ionic bonds occur when electrons are given and taken, and covalent bonds occur when electrons are shared. However… there is a bit more to the story. There’s really a spectrum of bonding, from taking to partial sharing, to unequal sharing, to complete sharing of valence electrons.
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What determines what really happens to electrons in a
chemical bond?
Electronegativity of an atom determines what happens to electrons in a bond! (Remember electronegativity = the ability of an atom to attract electrons)
To determine the bond type, you have to find the electronegativity difference between the atoms.
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Bond Polarity
• Most bonds are a blend of ionic and covalent characteristics.
• Difference in electronegativity determines bond type.
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How do I find Electronegativity difference?
Use your green electronegativity chart to determine the difference of electronegativity between atoms (remember to subtract big-small, and that the electronegativities for all atoms are listed on the purple sheet)
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Electronegativity differences and their bond type
Bond Type Electronegativity Difference
Ionic Greater than 1.7
Polar Covalent 1.7 ≥X > 0.3 (less than or equal to 1.7, greater than 0.3)
Covalent (pure /non-polar) <0.3 (less than or equal to 0.3
Once you know the electronegativity difference, you can determine the type of bond the atoms will form. See the chart below:
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Bond Polarity
Examples:
• NaCl
• HCl
• Cl2
3.0-0.9=2.1Ionic
3.0-2.1=0.9Polar
3.0-3.0=0.0Non-Polar
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Lets Classify some bond types based on electronegativity.
Ni and Sb
C and Br
H and O
K and Cl
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General properties of Covalent Compounds
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Molecules and Molecular Compounds
• Compounds that are NOT held together by an electrical attraction, but instead by a sharing of electrons.
• Atoms held together by a sharing of electrons are joined by a covalent bond.
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Molecules and Molecular Compounds
• A molecule is a neutral group of atoms joined together covalent bonds. A compound composed of molecules is called a molecular compound.
• The chemical formula for a molecule is called the molecular formula.
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Properties of Molecular Compounds
• Composed of two or more nonmetals.
• Usually gases or liquids at room temperature.
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Properties of Molecular Compounds
• Molecular compounds tend to have a relatively lower melting and boiling point than ionic compounds.– Reason: There are no (or few and weak)
bonds holding the molecules together in molecular compounds.
Ionic CompoundMolecular Compound
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Properties of Molecular Compounds
• Do not conduct electricity. They form nonelectrolytes in solution.– Reason: Molecular compounds do not break
apart into ions in solutions.
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IONIC COVALENTBond Formation
Type of Structure
Electrical Conductivity
OtherProperties
e- are transferred from metal to nonmetal
high
yes (solution or liquid)
e- are shared between two nonmetals
low
no
MeltingPoint
crystal lattice true molecules
Comparing Bonds
Physical State solid liquid or gas
Odorous and volatileLow volatility
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Properties of Polar Covalent bonds
• Unequal sharing of a pair of electrons
• Form a molecule, but since unequal sharing, one atom has a stronger pull on the electrons than on the other.
• Intermediate strength: 88% of all bonds (Ionic bonds are strongest, 10% of all bonds)
• Soft, solids, and liquids.
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Properties of Polar Covalent Bonds
• Even though the electrons are shared, the fact that the electrons are more attracted to the Cl atom results in a partial negative charge.
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+ -
Bond Polarity
• Polar Covalent Bond– e- are shared unequally– asymmetrical e- density– results in partial charges (dipole)
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Water is a polar molecule– you need to know this handy fact!
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Properties of Non-Polar Bonds
• Equal sharing of electrons
• Weakest bond type, about 2% of all bonds
• Usually identical atoms or nonmetals very close on periodic table.
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Oil is a non-polar compound. You need to know this!
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LIKE DISSOLVES LIKE
• The rule “like dissolves like” refers to the fact that nonpolar substances do NOT mix with polar substances.
• Polar mixes with polar. Ionic compounds also mix with polar substnaces
• Nonpolar mixes with nonpolar.
• Example: Oil is nonpolar and does not mix with water which is polar.
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Nonpolar
Polar
Ionic
View Bonding Animations.
Bond Polarity
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Covalent Bonding and the Octet Rule
• Compounds that are NOT held together by an electrical attraction, but instead by a sharing of electrons.
• Atoms attain an octet (also called noble gas electron configurations) by sharing electrons.
• The bonds that form from this sharing can be single, double or triple. Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds.
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To draw Lewis structures for covalent bonds, use the NAS method:
– N (Needed): Find the number of electrons needed to form full octets for all elements involved. For most nonmetals, the number needed is 8. Hydrogen is the exception, it needs only 2.
– A (Available): Find the number of electrons available by adding up all of the valence electrons for all elements involved.
– S (Shared): Subtract the two numbers. S= N-A A bond is formed with two electrons, so divide by two to tell you how many bonds to draw between the elements.
– Draw the molecule. Put first atom in the center. H’s are always outside. Draw in the bonds, then fill in the rest of the electrons.
– Check to ensure all atoms have a full octet.
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H• •H
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Example #1: CH4
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Example #2: CO2
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Example #3: N2
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Naming Binary Molecular Compounds (2 elements)
• When naming molecular compounds Greek prefixes (listed on your yellow periodic table) are used. The least electronegative element is named first and the most electronegative element is named second.
• Put prefixes in front of element names to tell how many atoms are there and change the ending to –ide. The prefix mono is only used with oxygen.
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Example #1: N2O4 :_________________
Example #2: SF6 :___________________
Example #3: diphosphorus pentoxide:______
Example #4: phosphorus pentachloride :_______Greek Prefixes
Mono – 1(Oxygen only) Hexa – 6Di – 2 Hepta – 7Tri – 3 Octa – 8Tetra – 4 Nona – 9Penta – 5 Deca – 10