Chapter 4
I.
Chemical Reactions
8th Grade
Atoms, Bonding, and the Periodic Table
A. What determines an element’s chemistry
1. Valence Electrons
a. Electrons at the highest energy level
b. Involved in chemical bonding
c. The number of valence electrons in each atom determines
the chemical properties of that element
2. Electron Dot Diagrams
a. Shows the number of valence electrons (1-8)
b. Includes the symbol for the element surrounded by
dots, one for each valence electron
3. Bonding
a. Atoms with 8 valence electrons are more stable,
nonreactive
b. Atoms try to bond to get 8 electrons & become more
stable
c. Electrons can be transferred or shared
d. Chemical bond: The force of attraction that holds
atoms together as a result of the rearrangement of
electrons
4. Applying the periodic table
a. Valence electrons increase from left to right
b. Groups have the same number of valence electrons
c. Groups have same properties
5. Noble Gases
a. 8 valence electrons
b. unlikely to gain or lose electrons
c. stable
d. do not react easily
6. Metals
a. Middle of periodic table
Chapter 4
Chemical Reactions
8th Grade
b. React by losing electrons
c. Reactivity decreases from left to right
7. Nonmetals
a. Stable when they have 8 electrons by sharing or gaining
b. Combine with metals by gaining electrons
c. Halogens react easily
8. Metalloids
a. Can either lose or share electrons
b. Properties of both metals and nonmetals
9. Hydrogen
a. 1 valence electron
b. nonmetal
c. shares electrons
Chapter 4
II.
Chemical Reactions
8th Grade
Ionic bonds
A. How do Ions form?
1. Ions
a. An atom or group of atoms that has an electric charge?
b. Lose an electron, become +
c. Gain an electron, become –
d. Metals usually lose electrons
e. Nonmetals usually gain electrons
2. Common Ions
a. Polyatomic ions: ions that are made of more than 1
atom
b. Page 142
3. Ionic Bonds
a. The attraction between two oppositely charged ions
b. Ionic compound
i.
Made up of positive and negative ions
ii. Total positive charge = total negative charge
(aka. Neutral charge)
B. How are formulas and names of ionic compounds written?
1. Chemical formula: a group of symbols that shows the ratio of
elements in a compound
2. Formulas of Ionic Compounds
a. Chemical Symbols
b. Subscripts: tell the ratio of elements in a compound
c. To write the formula for an ionic compound, write the
symbol of the positive ion and then the symbol of the
negative ion. Add the subscripts that are needed to
balance the charges.
d. No subscript = 1
e. CaCO3 = calcium carbonate
Chapter 4
Chemical Reactions
8th Grade
3. Naming Ionic Compounds
a. Name the positive ion first
b. Name the negative ion second
i.
if there is only 1 negative ion, ending is –ide
ii. if the negative is polyatomic, ending is –ate or
-ite
C. What are the properties of ionic compounds?
1. In general, ionic compounds form hard, brittle, crystals that have
high melting points. They conduct electric current when
dissolved in water or melted.
2. Ionic Crystals
a. A compound of repeating patterns of ions
b. Crystal: an orderly, 3D arrangement
c. Hard, brittle (because of the bonds)
3. High Melting Points
a. Ions must be broken apart to melt
b. It takes a lot of energy to separate the ions of a crystal
c. Hard to undo the attraction between positive and
negatives
4. Electrical Conductivity
a. The flow of charged particles
b. When dissolved in water ions can move and therefore
conduct current
c. In a solid the ions cannot move easily
d. If charged particles cannot move, there is no current
Chapter 4
III.
Chemical Reactions
8th Grade
Covalent Bonds
A. How are atoms held together in a covalent bond?
1. Covalent bond
a. When 2 atoms share electrons
b. Usually form between nonmetal atoms
2. Electron sharing
a. The attraction between the shared electrons and the
protons in the nucleus of each atom hold the atoms
together in a covalent bond.
b. Molecule: a neutral group of atoms joined by covalent
bonds
3. How many bonds?
a. Double bond – 2 pairs of electrons are shared
b. Triple bond – 3 pairs of electrons are shared
B. What are properties of molecular compounds?
1. Molecular compound: a compound that is made up of
molecules
2. Unlike ionic compounds, molecular compounds usually do not
conduct electric current when melted or dissolved in water.
3. Molecular compounds have lower melting and boiling points
4. Poor Conductivity
a. Do not contain charged particles that are able to move
b. Plastic, rubber, pure water, table sugar
5. Low melting and boiling points
a. Covalent bonds are weaker than ionic bonds
b. Less heat is needed
C. How do bonded atoms become partially charged?
1. Unequal sharing of electrons causes covalently bonded atoms
to have slight electric charges
2. Nonpolar Bonds and Polar Bonds
Chapter 4
Chemical Reactions
8th Grade
a. Nonpolar bond: a covalent bond where electrons are
shared equally
b. Polar bond: electrons are shared unequally creating a
slight charge
3. Polar Bonds in Molecules
a. Has a + charged & - charged end
b. Only some molecules with polar bonds create polar
molecules
4. Attractions Between Molecules
a. Polar molecules are connected by the weak
b. Van der Waals force
c. Polar bonds increase the melting and boiling points
Chapter 4
IV.
Chemical Reactions
8th Grade
Observing Chemical Change
A. How can changes in matter be described?
1. Properties of matter
a. Physical properties are things that can be observed
without changing the substance
b. A chemical property is a characteristic that describes its
ability to change
2. Changes in matter
a. Changes in matter are either physical or chemical
changes
b. Physical change
i.
Changes form or appearance, but doesn’t change
it into something new
ii. Bending, crushing, cutting, melting, freezing,
boiling
c. Chemical Change
i.
A change in matter that produces one or more
new substances
ii. Has new properties
iii. Reactants: substance that undergoes a chemical
change
iv. Product: the new substance formed from a
chemical change
3. Bonding and Chemical Change
a. Chemical changes occur when bonds break or form
b. New substances are formed
B. How do you identify a chemical reaction?
1. Chemical reactions involve changes in properties and changes in
energy that you can often observe
2. Changes in properties
Chapter 4
Chemical Reactions
8th Grade
a. Look for changes in physical properties
b. gas production, color change, texture, hardness
c. precipitate: a solid that forms from liquids during a
chemical reaction
3. Changes in Energy
a. Breaking bonds requires energy
b. Making bonds releases energy
c. Exothermic reaction: energy released is greater than
the energy required to break the bonds
d. Endothermic reaction: more energy is required to
break the bonds than is released in the reaction
Chapter 4
V.
Chemical Reactions
8th Grade
Describing Chemical Reactions
A. What information does a chemical equation contain?
1. Chemical equation: a way to show a chemical reaction using
symbols
2. Formulas in an equation
a. Combination of chemical symbols of elements
b. Water – H2O
3. Structure of an Equation
a. A chemical equation tells you the substances you start in
a reaction and the substances that are formed in the end
b. Reactants yield products
c. The number of each element must remain the same
B. How is mass conserved during a chemical reaction?
1. Law of conservation of mass: during a chemical reaction
matter is neither created nor destroyed
2. In a chemical reaction, all of the atoms present at the start of the
reaction are present at the end of the reaction
3. Open and Closed Systems
a. Open system: matter can enter or escape to the
surroundings
b. Closed system: matter does not enter or leave
4. Balancing Chemical Equations
a. Write the equation
b. Count the atoms
c. Use coefficients (the # in front of a chemical formula) to
balance atoms
d. Check your equation
C. What are three types of chemical reactions?
1. Three types of chemical reactions are synthesis, decomposition,
and replacement
Chapter 4
Chemical Reactions
8th Grade
2. Synthesis
a. When 2 or more elements combine into a more complex
reaction
b. P4 + 3 O2  P4O6
3. Decomposition
a. When a compound breaks down into simpler
substances
b. 2 H2O2  2 H2O2 + O2
4. Replacement
a. When two elements in trade places in compounds
b. 2 Cu2O + C  4 Cu + CO2