1. Warm-up
Explain how a person can determine the number of valence electrons in an atom of a given element such as Oxygen?

2. Review Suppose you have an atom of Oxygen (element # 8)
How do you know how many electrons are in that element?
The number of electrons will be the same as the atomic # of the element.

3. How many shells does oxygen have?
Since oxygen has 8 electrons we know that it has 2 energy levels.
This can also be determined by the period (row) the element is found.
All elements in the second row of the Periodic Table have 2 energy levels. In the 3rd row they have three and so on. …

4. How many electrons does oxygen have in the first and second energy level?
2 electrons in the first level
6 in the second.
So how many valence electrons does it have?
6 valence electrons
This can also be determined by the Group the element is found in. Oxygen is found in Group 16 all the elements in this group have 6 valence electrons.

6. Compounds & Their Formulas
Key Question:
What can we determine from a chemical formula?

7. Introduction to Compounds
What is a compound?
Compounds are the result of two or more atoms of different elements bonding together.
Most matter is made up of compounds or mixtures of compounds.

8. Examples of Simple Compounds:
The water you drink
The salt you put on your food
The sugar you put on your cereal.

9. Why would it be helpful to know the number of valence electrons in an element?
An elements valence electrons, help determine what elements will combine with it, to form compounds.
Oxygen has 6 valence electrons and needs 2 more to have a full energy level (shell).
Hydrogen atoms have one valence electron each. Two can combine with one oxygen atom to form the compound water.

10. Writing Chemical formulas for compounds
The chemical formula of a compound describes the number of atoms of each element with a subscript written after the symbol for the element
No subscript is written when a compound contains only one atom of an element.

11. Examples: Water = H2O two atoms Hydrogen one atom Oxygen
There is a 2:1 ratio of Hydrogen to Oxygen
Plumbic Hydroxide = Pb(OH)4.
one atom Lead
four atoms Oxygen
four atoms Hydrogen
There is a1:4:4 ratio of Lead to Oxygen to Hydrogen

12. Do compounds have the same properties as the elements that make them up?
No!
Example:
sodium chloride or table salt is a compound we can eat but separately chlorine forms a deadly gas and pure sodium (Na) will burst into flame the moment it touches water.

13. What is a Molecule?
A molecule is the smallest particle that has any of the properties of a compound.
If we have more than one molecule of the same compound we can represent it in a chemical formula by a coefficient written in front of the formula.
25H2O2 = 25 molecules H2O2
2H2O = 2 molecules of H2O

14. 5H2O For the following formula calculate: The number of molecules
The number of atoms of each element
The ratio to atoms to each other
5H2O
We have a total of:
5 molecules
10 Hydrogen atoms
5 Oxygen atoms
The ratio would still be 2:1