1. Chapter 8 Earth Chemistry Section 2

2. 8.2 – Atomic Number ► An atom of a specific element is different from other elements by the number of protons it has ► Atomic Number is the number of protons in an atom. ► All atoms of any element have exactly the same number of protons

3. 8.2 – Atomic Number ► The number of protons also equals the number of electrons in a neutral atom ► So, the atomic number tells us how many protons which equals how many electrons ► Example: Oxygen’s atomic number is 8 ► It has 8 protons and 8 electrons

4. 8.2 – Atomic Number ► The periodic table is a list of all the known elements in order by their atomic numbers ► It is used to classify the elements into groups

5. 8.2 – Mass Number ► Each atom has a mass number. ► Mass number is the number of protons plus the number of neutrons in the atom ► The mass number is not on the periodic table

6. 8.2 – Mass Number ► You can use the atomic number and mass number to find the total number of protons, neutrons, and electrons ► Mass # - Atomic # = # Neutrons

7. 8.2 – Combinations of Atoms ► Elements rarely occur in pure form in the Earth’s crust ► They are generally found in combination with other elements ► A compound is a substance made of two or more elements

8. 8.2 – Combinations of Atoms ► A compound is a new substance ► It has different properties than the elements that make it. ► Example: Water is made of when 2 atoms of hydrogen combine with 1 atom of oxygen. ► Water is a liquid, while hydrogen and oxygen are colorless gases.

9. 8.2 – Combinations of Atoms ► The smallest unit of a compound is called a molecule ► Molecules are made of 2 or more atoms ► Example: There are millions of water molecules in a bottle of water

10. 8.2 – Combinations of Atoms ► Atoms join together to become more stable. ► An unstable metal atom will join with an unstable nonmetal atom to become a stable compound ► An unstable atom will always combine with another unstable atom to be more stable

11. 8.2 - Combinations of Atoms ► When looking at a chemical formula for a compound, each capital letter represents a new element ► Subscripts tell us how many atoms of that element are in the compound ► If there are no subscripts, then there is only one atom of that element

12. 8.2 - Combinations of Atoms ► Example: C 2 H 4 ► It has 2 Carbon atoms and 4 Hydrogen atoms