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									 Physical Science
Chapter 20: Chemical Bonds
Compounds
 A compound is a combination of atoms
  from 2 or more different elements, in a
  definite ratio
 The properties of a compound may be
  completely different from those of the
  elements which make it up.

   Example: water (H2O)
            Chemical Formulas

• A short-hand way to write the
  name of a chemical compound
  is by using a chemical formula,
  which is like a "recipe" for a
  compound.
• It tells what elements are in the
  compound, and how many
  atoms of each element.
• The # of atoms of each element is shown
  using a subscript.

• Example: Sulfuric acid, H2SO4

         Hydrogen - 2 atoms
         Sulfur - 1 atom (understood subscript)
         Oxygen - 4 atoms
• Molecule - smallest sample of a
  compound.
• Molecular mass - the mass of a single
  molecule of a compound.

• Ex.   H2O      (2x1) + (1x16) = 18
• Why do you think table salt sinks
  when you put it into water?
• C6H12O6 is the chemical formula for a
  simple sugar. What is the molecular
  mass of this compound?
         Chemical stability
• Some elements do not join together with
  other elements to form compounds.
  These un-reactive elements are said to
  already be chemically stable.

• All of the elements in G18 are this way.
• The way that an element reacts with other
  elements is determined by it's outer shell electrons.
  (Outer shell electrons refers to the electrons that are
  farthest out from the nucleus in the electron cloud)

• Atoms which react with each other do so by either
  gaining, losing, or sharing electrons with other atoms.
• Think of it like an egg carton. It can only
  hold a certain number of eggs. And when
  the carton is full, then it doesn’t need any
  more.
• Likewise, atoms of different elements can only
  have a certain # of electrons in their electron
  clouds, and if they are full then they are stable,
  and won’t react with other elements.
• Elements with 8 o.s.e.’s, or a full first level
  (2 e-’s) are stable, and will not react.
• However, elements with only a partially-filled
  outer shell may react with certain other
  elements.
• An electron dot diagram can show how many
  outer shell electrons an atom has, and
  whether or not it can hold any more, and
  therefore whether that atom will react or not.
       Electron Dot Diagrams
• (Note: The only elements we will concern
  ourselves with now are the G1 & G2 metals,
  and the non-metals, since these are the most
  common reactive elements)
• An element with 8 electrons in the outer shell is
  considered full, and stable (un-reactive). This is
  called the Octet Rule.


Ex. He-2 ose's, Ne-8, Ar-8, Kr-8
• A quick way to determine the # of outer
  shell electrons (G1, G2, and the non-metals) is to
  use the last # of the group #.

For example: Mg          G2         2 o.s.e.’s
             N           G15        5 o.s.e.’s
             Cl          G17        7 o.s.e.’s
             Kr          G18        8 o.s.e.’s
• A dot diagram shows an elements
  chemical symbol with 1 dot for each
  outer shell electron up to 8.
 So.....magnesium has 2 ose's, and would
 therefore have 2 dots surrounding it.



                  Mg
   Examples:


    H       He      Al   C



    N           O   Cl   Ne
• Noble gases have 8 o.s.e.’s, which is
  why they are stable and un-reactive.
              Chemical Bonds
• When 2 atoms do react with
  each other to form a new
  substance, this new substance
  is held together with chemical
  bonds. A chemical bond is a
  force holding the atoms in a
  compound together.
• Think of bonds like the
  connecting sticks in a
  Tinkertoy set.
            Types of Bonds
• There are 2 main types of chemical bonds:

1. Ionic - bonds created by the attraction of
           opposite charges (+, -) like a magnet
2. Covalent - bonds created by the sharing of
              electrons
                Ionic Bonds
• When 2 atoms react with each other,
  they both want to have a stable
  relationship (i.e. have 8 outer shell
  electrons)

• Sometimes, one atom will "steal"
  electrons from another atom in order to
  become stable.
• For example, sodium (Na) has 1 outer shell
  electron and chlorine (Cl) has 7.
 If chlorine were to gain 1 electron, then it
 would have 8, and be stable.
 So it "steals" sodium's electron to become
 stable.
 Sodium, however, now drops down an
 energy level and ends up with 8 outer shell
 electrons also. So everybody's happy.
• When 1 atoms steals (gains) electrons, it
  will take on a negative charge since that is
  the charge of the electrons.
• Likewise, the atom which gave up
  electrons will take on a positive charge,
  since it lost negatively charged electrons.
• So………the 2 atoms now have opposite
  charges which attract each other, just like
  a magnet.
• When the atoms join together, the positive
  and negative charges will cancel each
  other out, forming a neutral molecule.

• Ions are individual atoms with a positive or
  negative charge due to gained or lost
  electrons

• Ionic bonds form when the oppositely
  charged ions attract each other
           Covalent Bonds
• A covalent bond is formed when atoms
  combine together by sharing electrons.

• Covalent bonds usually occur between 2
  non-metals

• Example: H has 1 o.s.e. and Cl has 7.
     By sharing, H will have 2 (stable)
     and Cl will have 8 (stable)
• A diatomic molecule is one with 2 atoms of
  the same element covalently bonded.

• Ex.: Cl2 (chlorine gas)

       O2 (oxygen gas)
         Binary Compounds
• A binary compound is one composed of
  atoms from exactly 2 different elements.

• In order to determine the ratio between 2
  elements which will produce a compound
  it is often useful to use oxidation numbers
  (numbers which show how many electrons
  are gained, lost, or shared when 1 atom
  combines with another).
Using oxidation numbers to determine
         chemical formulas.

Oxidation numbers show the number of
electrons gained, lost, or shared when an atom
combines.
Oxidation numbers are the same for elements in
the same groups.*

Ex.: G17 elements gain 1 electron when
     bonding, and thus have an ox. # of -1.
+1 +2   +3 +4 -3 -2   -1   0
             The criss-cross method
 Oxidation numbers can be written as a
 superscript for each of the elements in a binary
 compound. Those numbers are then criss-
 crossed down and used as subscripts for the
 other element.
• Example:
                Mg+2      N-3
 Cross the +2 and the -3 down to the subscript
 of the other element, drop the signs, and you
 end up with Mg3N2.
• Example #2:

 Calcium has an
 oxidation # of +2,
 chlorine is -1,
 what is the
 formula for a
 compound of
 calcium and
 chlorine?
   Naming Binary Compounds
• To name a binary compound, you simply
  say the name of the first element, and then
  add the suffix -ide to the root of the
  second element.

• Example: Na + Cl = sodium chloride
            Al + N = aluminum nitride
            Ca + P = calcium phosphide
     Toxic and Corrosive Materials
• Many compounds are completely
  harmless to humans, while some
  are very harmful.
• Corrosive materials attack and/or
  destroy metals, living tissue, and
  other materials.
  Acids are corrosive, although their
  relative strength will vary greatly.
• Toxic materials are poisonous.
• If toxic materials are ingested,
  vomiting is often induced to remove
  the materials from the body.
  This is usually not the case with
  corrosive materials. Why?
          Sample Problems
Write formulas, give names, and calculate
molecular masses for each of the following
compounds:
              Al + Cl
              Ca + O
              Na + N
              H+S
              K+F
• The following covalently bonded
  diatomic molecules are listed in order
  from most stable to least stable. What
  do you think is the main determining
  factor of stability in these molecules?

•   He
•   N2
•   O2
•   Cl2
              Dot structure
                  for each    Criss-Cross   Molecular   Compound   Compound
Reactants         reactant        Method       Mass        Name      Formula

Potassium &
    Oxygen

 Sodium &
    Sulfur

 Calcium &
   Nitrogen
Magnesium
    and
   Oxygen

 Carbon &
   Sulfide


Hydrogen &
   Phospho
     rus
               Dot structure
                   for each    Criss-Cross   Molecular   Compound   Compound
Reactants          reactant        Method       Mass        Name      Formula

Beryllium &
   Iodine

Lithium &
    Fluorine

Barium &
   Chlorine

Cesium &
  Nitrogen

Calcium &
   Bromine
Magnesium
  &
  Nitrogen

Hydrogen &
   Sulfur
• Identify 2 specific materials for each of
  the following:

 A.   Corrosive
 B.   Toxic
 C.   Corrosive, but not toxic
 D.   Toxic, but not corrosive

 (assume materials are at everyday common strengths)
+1 +2   +3 +4 -3 -2   -1   0
• Identify the following:

-P3 element with 6 outer shell electrons
-the lightest inert element
-the lightest binary compound (17 a.m.u.)
-the lightest diatomic molecule
• Provide the following information for a binary
  compound composed of hydrogen and sulfur:

• Compound name
• Compound formula
• Molecular mass
Give 1 example for each of the following:

• A diatomic molecule which is very stable

• A compound formula which reduces down to
  a 1:1 ratio

• An element which loses 2 electrons when
  ionically bonding

• An element which does not follow the Octet
  Rule
  Physical Science
Ch. 21: Chemical Reactions
• A chemical reaction is where 1
  or more substances are
  chemically changed into new
  substance(s)

• Example: Changing hydrogen
  gas and oxygen gas into water
   In order for a chemical reaction
    to take place, you need both
    reactants and products.

   Reactants are the substances
    used to produce the reaction.

   Products are the new
    substances formed as a result
    of the reaction.
       Chemical Equations
A short-hand way to show a chemical reaction by
using formulas and symbols is called a chemical
equation.


Example:        H2 + O2      g H20
In a chemical equation, the reactant(s) are
always on the left hand side, and the product(s)
are always on the right.
• Is it necessary to have the same # of
  reactants as products?




                  Hmmm......
• No! You just saw that the formation of water
  creates 1 product from 2 reactants.

                  H2 + O2 g H20

• So how come there are 2 atoms oxygen on the
  left, but we only end up with 1 on the right?
  We'll get to that later.
• There are several symbols used in
  chemical equations which you will need to
  be familiar with.

 Those symbols are on pg. 635 in your
 book.
  Law of Conservation of Mass
• In a chemical reaction, mass can neither
  be created or destroyed, it can only
  change form.
 Therefore, the mass of the reactants before the
 reaction occurs must be exactly equal to the
 mass of the products following the reaction.
• For example, if a log were placed in a sealed
  metal box and set on fire. The products
  (smoke, ash, gases, etc.) would have the
  exact same mass as the log before it were
  burnt.

• If the box were sitting on a scale as the log
  burned, the weight would not change at all.
         Molecular Mass

• Molecular mass is the mass of 1
  molecule of a given compound.
 Example: Calcium Chloride (CaCl2)

    Calcium atomic mass = 40 a.m.u.
    Chlorine atomic mass = 35 a.m.u. (x2)
    40 + (35 x 2 atoms) = 110 a.m.u.
• Some chemicals when mixed produce
  very mild reactions, some produce very
  violent ones.

 Certain chemicals, like strong acids and
 bases, are just not made to be mixed
 together (by sane people), unless under highly
 controlled conditions.
• For example, some household
  cleaners like Windex contain
  ammonia (NH4), a strong base.
• Others, like Liquid Plumber,
  contain sulfuric acid (H2SO4).
  Mixing these can produce a
  violent reaction.
• Or even worse, mixing
  ammonia with bleach can
  result in the production of
  poisonous chlorine gas (Cl2).
        Types of Reactions
• A synthesis reaction is one where 2 or more
  reactants will combine to produce 1 product.

           A + B    g     AB

 A decomposition reaction is where 1
 reactant will break down into 2 or more
 products.

          AB    g       A + B
              Thermal Reactions
• An exothermic reaction is one where energy (heat,
  light, or electricity) is given off during the reaction.
  Example: burning, pocket hand warmers,
           glowsticks, batteries, electric eels
• In an endothermic
  reaction, heat is taken in
  during the reaction. This
  causes the surrounding
  area to be cooled.

 Example: chemical cold
 pack
                  Catalysts
• A catalyst is a substance which will speed
  up the rate of a chemical reaction, without
  changing the product(s).
• Ex: blowing on a fire, moisture speeding
      up the rate of corrosion, heating Luminol
          Inhibitor
•An inhibitor is a substance
 which slows down a chemical
 reaction without changing the
 product(s).

 Ex: lemon juice on an apple,
     food preservatives, CO2
     on a fire, cooling Luminol
• Consider a chemical reaction where
  sodium oxide and hydrogen chloride
  combine to produce sodium chloride
  and hydrogen oxide. Provide the
  following:

• Formulas for all four compounds
• A balanced chemical equation for the
  reaction
• The molecular mass of the reactants
• Bud decided to mix
  sodium chloride (salt)
  and carbon dioxide gas
  together in a beaker.
  Nothing happened. But
  when he heated it up, a
  reaction occurred
  which produced carbon
  chloride and sodium
  oxide.
  Write a balanced
  chemical equation for
  this reaction.

								
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