Ch 8.2: Compounds, Chemical Formulas, and Covalent Bonds

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Grade 8 Integrated Science Chapter 8 Lesson 2 on compounds, chemical formulas, and covalent bonds. An introduction to electron sharing.

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Ch 8.2: Compounds, Chemical Formulas, and Covalent Bonds

  1. 1. Chapter 8.2 - Compounds, Chemical Formulas, and Covalent Bonds – p275-282 - page1 Vocabulary • Covalent Bond (277) – a chemical bond formed when two atoms share one or more pairs of valence electrons • Molecule (278) – a group of atoms held together by covalent bonding that acts as a n independent unit • Polar Molecule (279) – A molecule that has a partial positive end and a partial negative end because of unequal sharing of electrons • Chemical Formula (280) – a group of chemical symbols and numbers that represent that elements and the number of atoms of each element that make up a compound Review • Compounds are substances made up of two of more different ________________ • Compounds and the elements that make them up often have different properties • Chemical _____________ join atoms together. • One way atoms from bonds is by sharing ____________ electrons Covalent Bonds – Sharing Electrons • When unstable, nonmetal atoms bond together, they bond by ______________ valence electrons. • A covalent bond is a chemical bond formed when two atoms share one or more ____________ of valence electrons. • The atoms then form a ___________ covalent compound A Noble Gas Electron Arrangement • An atom with less than eight valence electrons becomes stable by forming chemical bonds until it has ________________ valence electrons. • The most chemically stable state for most atoms has the electron configuration of a ________________________. • Look at H2O H2O Bonding
  2. 2. Chapter 8.2 - Compounds, Chemical Formulas, and Covalent Bonds – p275-282 - page2 Shared Electrons • If the oxygen atom and each hydrogen atom share their _______________ valence electrons, they can form two covalent bonds and become a stable covalent compound. • Each covalent bond contains _____________ valence electrons. • Since these electrons are shared, they count as valence electrons for ______________ atoms in the bond. • In H2O, all three atoms combine sharing valence electrons. We know the compound is stable because all ________________ atoms have eight valence electrons. Double and Triple Covalent Bonds • • A single covalent bond exists when two atoms share ___________ pair of valence electrons. • Triple covalent bonds exist when two atoms share ____________ pairs of valence electrons. – Triple bonds are stronger than ___________ bonds. Double bonds exist when two atoms share _________ pairs of valence electrons. – Double bonds are ____________ than single bonds Covalent Compounds • • Covalent compounds usually have __________ melting points and low boiling points. • Covalent compounds are _______________ conductors of thermal energy and electricity They are usually gases or liquids at ________________temperature, but they can also be solids Molecules • A molecule is a group of atoms held together by covalent boning that acts as an independent unit, • Table sugar (C12H22O11) is a __________________ compound. • One grain of sugar is made up of _________________ of sugar molecules. • One molecule of sugar contains _________carbon atoms, ___________hydrogen atoms, and __________oxygen atoms. • The only way to break a molecule of sugar would be to chemically separate the atoms of C, H, and O.
  3. 3. Chapter 8.2 - Compounds, Chemical Formulas, and Covalent Bonds – p275-282 - page3 Water and Other Polar Molecules • In a covalent bond, one atom can __________________ the shared electrons more strongly than the other atoms can. • Consider H2O – The _________________ atom attracts the shared electrons more strongly than each hydrogen atom does. – As a result, the shared electrons are pulled ______________ to the oxygen atom. – Since electrons have a negative charge, the oxygen atom has a partial ___________________ charge. • A molecule that has a partial positive end and a partial negative end because of unequal sharing of electrons is a ________________molecule. Dissolving Molecules • The charges on a polar molecule affect its properties. • Sugar dissolves easily in water because sugar and water are____________________ . • The negative ends of a ____________________ 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 ___________________ from one another and mix with the water molecules.
  4. 4. Chapter 8.2 - Compounds, Chemical Formulas, and Covalent Bonds – p275-282 - page4 Nonpolar molecules • A hydrogen molecule, H2, is a __________________ molecule. • Hydrogen atoms are identical, their attraction for a shared electron is equal. • The carbon dioxide molecule, CO2, is also________________________. • 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_____________________. Chemical Formulas and Molecular Models • A chemical ___________________ 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 formula lists the elements in a compound. • For example, the chemical formula for carbon dioxide is CO2. – CO2 is made up of carbon and _______________ atoms. – The____________________ , or small number after a chemical symbol, shows the number of atoms of each element in the compound. – CO2 contains _______ atoms of oxygen bonded to ________ atom of carbon. • When parentheses (“()”) are used, any subscript applies to all the elements inside the parentheses. Example: Al2(SO4)3 _____ Aluminum _____ Sulfur _____ Oxygen
  5. 5. Chapter 8.2 - Compounds, Chemical Formulas, and Covalent Bonds – p275-282 - page5 Counting Atoms Name each of the following chemical compounds and determine how many atoms of each element are present: 1) CaF2 ___1 Calcium, 2 Flourine_________________________________ 2) Be(OH)2 ____________________________________________________ 3) NO2 ______________________________________________________ 4) Al2(SO4)3 ___________________________________________________ 5) NH4NO3 ____________________________________________________ 6) S2F2 ______________________________________________________ 7) Na2CO3 ____________________________________________________ 8) CH4 ______________________________________________________

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