ch 8.2
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Compounds, Chemicals Formulas, and Covalent Bonds
Chapter 8 Lesson 2P275-282
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Review
• Compounds are substances made up of two of more different elements
• Compounds and the elements that make them up often have different properties
• Chemical bonds join atoms together.• One way atoms from bonds is by
sharing valence electrons
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Covalent Bonds – Sharing Electrons
• When unstable, nonmetal atoms bond together, they bond by sharing valence electrons.
• A covalent bond is a chemical bond formed when two atoms share one or more pairs of valence electrons.
• The atoms then form a stable covalent compound
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A Noble Gas Electron Arrangement
• An atoms with less than eight valence electrons becomes stable by forming chemical bonds until it has eight valence electrons.
• The most chemically stable state for most atoms has the electron configuration of a noble gas.
• Look at H2O
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H2O Bonding
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Shared Electrons
• If the oxygen atom and each hydrogen atom share their unpaired valence electrons, they can form two covalent bonds and become a stable covalent compound.
• Each covalent bond contains two valence electrons.
• Since these electrons are shared, they count as valence electrons for both atoms in the bond.
• In H2O, all three atoms combine sharing valence electrons. We know the compound is stable because all three atoms have eight valence electrons.
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Double and Triple Covalent Bonds
• A single covalent bond exists when two atoms share one pair of valence electrons.
• Double bonds exist when two atoms share two pairs of valence electrons.– Double bonds are stronger than single bonds
• Triple covalent bonds exist when two atoms share three pairs of valence electrons.– Triple bonds are stronger than double bonds.
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Covalent Compounds
• Covalent compounds usually have low melting points and low boiling points.
• They are usually gases or liquids at room temperature, but they can also be solids
• Covalent compounds are poor conductors of thermal energy and electricity
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Molecules
• A molecule is a group of atoms held together by covalent boning that acts as an independent unit,
• Table sugar (C12H22O11) is a covalent compound.
• One grain of sugar is made up of trillions of sugar molecules.
• One molecule of sugar contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms.
• The only way to break a molecule of sugar would be to chemically separate the atoms of C, H, and O.
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Water and Other Polar Molecules
• In a covalent bond, one atom can attract the shared electrons more strongly than the other atoms can.
• Consider H2O– The oxygen atom attracts the shared electrons
more strongly than each hydrogen atom does.– As a result, the shared electrons are pulled closer
to the oxygen atom.– Since electrons have a negative charge, the
oxygen atom has a partial negative charge.
• A molecule that has a partial positive end and a partial negative end because of unequal sharing of electrons is a polar molecule.
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Dissolving Molecules
• The charges on a polar molecule affect its properties.
• Sugar dissolves easily in water because sugar and water are polar.
• The negative ends of a water molecule pulls on the positive end of a sugar molecule.
• Also, the positive end of a water molecule pulls on the negative end of the sugar molecule.
• This causes the sugar molecules to separate from one another and mix with the water molecules.
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Nonpolar molecules
• A hydrogen molecule, H2, is a nonpolar molecule.
• Hydrogen atoms are identical, their attraction for a shared electron is equal.
• The carbon dioxide molecule, CO2, is also nonpolar.
• A nonpolar compound will not easily dissolve in a polar compound, but it will dissolve easily in a nonpolar compound.
• Oil is an example of a nonpolar compound. It will not dissolve in water.
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Chemical Formulas and Molecular Models
• A chemical formula is a group of chemical symbols and numbers that represent the elements and the number of atoms of each element that make up a compound.
• Just as a recipe lists ingredients, a chemical carbon dioxide is CO2. formula lists the elements in a compound.
• For example, the chemical formula for – CO2 is made up of carbon and oxygen atoms.– The subscript, or small number after a chemical
symbol, shows the number of atoms of each element in the compound.
– CO2 contains two atoms of oxygen bonded to one atom of carbon.
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Chemical Formulas and Molecular Models
• When parentheses (“()”) are used, any subscript applies to all the elements inside the parentheses.
• Example:– Al2(SO4)3
• 2 Aluminum• 3 Sulfur• 12 Oxygen
•st as a recipe lists ingredients, a chemical formula lists the elements in a compound.For exa
•st as a recipe lists ingredients, a chemical formula lists the elements in a compound.For exa