1. ELEMENTS, COMPOUNDS AND MIXTURES AND HOW THEY ARE REPRESENTED
8.5D recognize that chemical formulas are used to identify substances and determine the number of atoms of each element in chemical formulas containing substances

2. WHAT ARE ELEMENTS? – RECALL…
Elements are pure substances
Made of only one kind of material
Has definite properties, and
Is the same all throughout
They cannot be broken down into simpler substances without losing their identity
Represented by a symbol (Mg, Na) They’re on the periodic table!

3. WHAT ARE COMPOUNDS?
Made up of 2 or more different elements that are chemically combined.
They are represented by formulas
Ex: H2O, NaCl, C6H12O6, CO2
Compounds have different properties than their original elements (* more on next slide)
They cannot be separated by physical means
Unlike elements, compounds can be broken down to simpler substances through a chemical reaction

4. COMPOUNDS
The properties of the elements that make up a compound are often quite different from the properties of the compound itself
Sodium – Na = highly reactive metal
Chlorine – Cl = poisonous gas
Sodium Chloride = NaCl (table salt)

5. WHAT ARE MIXTURES?
Mixtures - two or more substances that are physically combined and retain the properties of their substances
 Mixture of elements – brass (mixture of copper and zinc)
Mixture of elements and compounds – air
Mixture of compounds – sand, saltwater
Solution – particles are evenly distributed

6. TYPES OF MIXTURES Homogeneous - Entire mixture looks the
same throughout
Ex. Milk, Bronze
Heterogeneous – Parts of the mixture look different
Ex. Fruit Salad, Trail Mix
Both types of mixtures can be separated by a physical
change!

7. ELEMENT, COMPOUND, OR MIXTURE? ? ?

8. QUICK CHECK Element, Compound, or Mixture?
Mixture Mixture Compound
Platinum Pt
Carbon Dioxide CO2
Air O2, N2, and Ar
Brass Alloy of Cu and Zn
5. Glucose
C6H12O6

9. MOLECULES
A molecule is 2 or more atoms chemically bonded that act like one unit
Water-2 atoms of hydrogen and one atom of oxygen-together they form one molecule of H2O
All compounds are molecules but not all molecules are compounds
H2 is a molecule, but not a compound
H2O is both a molecule and a compound (notice the 2 different elements)

10. QUICK CHECK
Which substances are molecules and which substances are both molecules and compounds?
Molecule
Both Both
Both O2
CO2
3. C6H12O6
Cl2
NH3

11. CHEMICAL FORMULAS A shorthand way of representing compounds
If chemical symbols are the “letters,” combined, these are the “words.”
Ex: NH3 = ammonia, C3H7OH = rubbing alcohol
Sometimes, the formula represents a molecule of a single element.
These are called diatomic molecules. This is how that element is naturally found.
O2-Oxygen H2-Hydrogen Cl2-Chlorine

12. CHEMICAL FORMULAS - SUBSCRIPTS
Subscripts are small numbers used in chemical formulas
Shows the elements & number of atoms of each element in a molecule
H2SO4
Subscript
Elements
Hydrogen: 2 atoms
Sulfur: 1 atom
Oxygen: 4 atoms 
7 atoms total

13. COEFFICIENTS (SLIDE 1) A formula may begin with a number
If there is no number, then “1” is understood to be in front of the formula.
This number is called the coefficient
The coefficient represents the number of molecules of that compound or atom needed in the reaction
For example:
2H2SO4 – 2 molecules of Sulfuric Acid
Coefficient

14. COEFFICIENTS (SLIDE 2) 2H2SO4 = 2 molecules of Sulfuric Acid
Note - A coefficient is distributed to ALL elements in a compound
To find the number of atoms:
2 H2 (for a total of 4 H atoms)
2S (for a total of 2 S atoms)
2O4 (for a total of 8 O atoms)

15. BONDING - HOW DO ATOMS JOIN TOGETHER?
A chemical bond is an interaction that holds atoms together – this is known as a molecule (this was mentioned earlier)
When we talked about valence electrons, we learned that when the outer energy level is not full, that atom seeks to either gain or lose electrons to be stable. Atoms form bonds with other elements (or sometimes itself) in order to lose, or gain electrons to be “happy” or stable.
Bonds are formed to get the atoms to a lower energy state.
There are several types of bonds, let’s go over some main ones.

16. COVALENT BONDS
As the attractions bring the atoms together, electrons from each atom are attracted to the nucleus of both atoms, which “share” the electrons.
The sharing of electrons between atoms is called a covalent bond, which holds the atoms together as a molecule.
Atoms covalently bonded as a molecule are more stable than they were as separate atoms
Examples of covalent bonds: H2 (hydrogen), H2O (water), O2 (oxygen), CH4 (methane), and CO2 (carbon dioxide).
Covalent Bonds - animation

17. IONIC BONDS - VIDEO
In ionic bonding, atoms transfer electrons to each other. Ionic bonds require at least one electron donor and one electron acceptor.
Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion.
Common examples: NaCl – table salt, NaBr - sodium bromide

18. IONIC BONDS - VIDEO
Chlorine is a greenish poisonous gas and sodium is a shiny, soft, and very reactive metal. But when they react, they form sodium chloride (table salt). In the video, the drop of water helps expose the atoms at the surface of the sodium so that they can react with the chlorine. The formation of the salt crystals releases a lot of energy.
What is happening?
An electron is transferred from sodium to chlorine. Sodium becomes a positive ion and chlorine becomes a negative ion.
Positive and negative ions attract each other and form an ionic bond and the compound sodium chloride.

19. IONIC BOND IMAGE: The Na gives up the electron to the Cl.
The Na is more stable with 8 valence electrons. The Cl has one more than stable so it has an over all negative charge.

20. METALLIC BONDS
A metallic bond is pretty different from covalent and ionic bonds, but the goal is the same: to achieve a lower energy state. Instead of a bond between just two atoms, a metallic bond is a sharing of electrons between many atoms of a metal element.
The electrons sort of “hover” in between the atoms! Watch this! Bonds (18 min),
And/or: Bonds (9 min)

21. WAYS THAT BONDS ARE REPRESENTED
We can’t “see” bonds, but scientists need to communicate about them, systems have been created to discuss bonding.
Two are typical – model (the dots shown) and “displayed” which shows the bonds.
A displayed formula shows all the bonds in the molecule as individual lines. You need to remember that each line represents a pair of shared electrons.

22. EVERYDAY COMPOUND OR POISON?
HW: due next class