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Formation of Interstellar Polyatomic Substances. Lewis Symbols. Chemical and physical properties of elements are related to the number of electrons in their valence shells For A-families (Main Group elements) the number of valence electrons = Group number

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lewis symbols
Lewis Symbols
  • Chemical and physical properties of elements are related to the number of electrons in their valence shells
  • For A-families (Main Group elements) the number of valence electrons = Group number
  • Lewis symbol -valence electrons are represented by dots around the chemical symbol for an element
slide3

Lewis Symbols

1A 2A 3A 4A 5A 6A 7A 8A

http://www.iun.edu/~cpanhd/C101webnotes/chemical-bond/lewis-electron-dot.html

chemical bonding
Chemical Bonding
  • Chemical bonding - Rearrangement of electron structures of 2 or more atoms
  • Bonding occurs so that each atom attains a more stable electron structure
  • Most common stable electron structure for Main Group elements is that of an inert gas, usually 8e (Octet Rule), except 2e for H, He, and Li
ionic bonding
Ionic Bonding
  • Occurs when a metal combines with a non-metal
  • Sufficient valence electrons are transferred from the metal to the non-metal so that each atom attains an octet (or duet for H)
  • Compound held together by attraction of oppositely charged ions
slide6

Compound formed when Na and Cl combine?

Compound formed when Mg and Cl combine?

http://www.wpbschoolhouse.btinternet.co.uk/page04/4_72bond.htm

slide7

Structure of Salt

http://www.geog.ouc.bc.ca/physgeog/contents/6a.html

covalent bonding
Covalent Bonding
  • Occurs when non-metal atoms combine
  • Pairs of electrons are shared between non-metal atoms so that each atom attains an octet (or duet for H)
  • A shared pair of electrons is called a bonding pair (see hydrogen molecule)

http://www.wpbschoolhouse.btinternet.co.uk/page04/4_72bond.htm

other covalent bonding terms
Other Covalent Bonding Terms
  • Lone pair of electrons - pair of elec-trons not shared between two atoms
  • Central atom - an atom which is bonded to more than one other atom
  • Terminal atom - atom which is bonded to only one other atom

http://www.wpbschoolhouse.btinternet.co.uk/page04/4_72bond.htm

multiple bonds
Multiple Bonds
  • When two atoms share more than one pair of electrons it is called a multiple bond
  • If two pairs of electrons are shared between two atoms it is a double bond and three shared pairs constitute a triple bond
  • Examples
slide11

Draw NO+

http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Drawing/Bond06.htm

chemical formulas
Chemical Formulas
  • Ionic substances are represented by an empirical formula which uses subscripts to indicate the ratio in which the atoms have combined
  • Covalent substances are represented by a molecular formula which uses subscripts to indicate the number of atoms of each type in one molecule of the compound
slide13

Ionic Compound

NaCl is an empirical formula for an ionic compound (metal + non-metal)

Indicates that for every Na there is a Cl in the compound

Covalent Compound

H2O is a molecular formula for a covalent compound (two non-metals)

Indicates that there are 2 H atoms and 1 O atom in a molecule of the compound

molecular weight and mole
Molecular Weight and Mole
  • The sum of the atomic weights of all the atoms in a molecular formula is called the molecular weight (MW).
  • MW of H2O = 2(1.008 amu) + 16.00 amu = 18.02 amu
  • MW expressed in grams,18.02 g, is the mass of one mole of water or 6.02 x 1023 molecules of water
chemical equations
Chemical Equations
  • A chemical equation is a method of representing a chemical change in terms of chemical symbols and formulas
  • General form: A + B => C + D, where A and B are formulas for reactants and C and D are formulas for products
writing chemical equations
Writing Chemical Equations
  • Write formulas for reactants and products in equation in the general form - called skeleton equation
  • Balance skeleton equation by inserting coefficients so as to obtain the same number of atoms of each element on both sides of the equation using fractions if necessary
writing equations con t
Writing Equations (con’t)
  • Multiply coefficients by appropriate factor to clear fractions and obtain the lowest set of integers as coefficients
  • Omit coefficients of 1 in final equation
  • Balance: H2 + O2 => H2O
  • 1 H2 + 1/2 O2 => 1 H2O
  • 2 H2 + O2 => 2 H2O
slide18

Balance: C3H8 + O2 => CO2 + H2O

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookCHEM1.html#Chemical Bonding

interstellar substances
Interstellar Substances
  • Atoms in space combined to form both ionic and covalent substances
  • Many of the substances formed in space do not follow the Octet Rule observed on earth due to the relatively small numbers of collisions of atoms possible in space
slide22

Continued...

http://www.science.gmu.edu/~jmirick/csi904/project.html