1. Introduction to Atomic Structure

2. ATOMS Considered the building blocks of matter
Made up of three main subatomic particles:
Protons
Neutrons
Electrons .

3. Atomic Mass Units Mass of proton = 1 amu (atomic mass unit)
Neutrons have slightly more mass than protons but they are considered equal.
Mass of neutron still = 1 a.m.u.
Mass of electron = 0 a.m.u

4. Atomic Number
Number of protons in nucleus determines the atomic number of an element.
This identifies the element!
Hydrogen--Atomic number 1 = 1 proton
Helium--Atomic number 2 = 2 protons
Oxygen--Atomic Number 8 = 8 protons
All atoms with 8 protons MUST be oxygen!
The # of protons in an atom CANNOT be changed (otherwise, you will have a different element!)

5. Number of protons = number of electrons
As the number of electrons is equal to the number of protons in an atom, each oxygen atom contains 8 electrons.

6. Isotopes
Even though the number of protons will never change, the number of neutrons can vary from atom to atom.
Atoms of the same element that have the same number of protons, but different number of neutrons are called isotopes.

7. Isotopes Hydrogen has 3 different isotopes (all of them have 1 proton)
H-1 (no neutrons)
H-2 (1 neutron)
H-3 (2 neutrons)

8. Mass number
Mass Number of an atom is the sum of the protons and neutrons in its nucleus.
The electrons are ignored because they are so small!
The mass number varies for different isotopes of an element.

9. Mass number Carbon has two known isotopes:
Carbon-12 (6 protons and 6 neutrons) (6 electrons)
Carbon-14 (6 protons and 8 neutrons) (6 electrons)

10. Using the mass number to find the number of neutrons.
You can find the number of neutrons by subtracting the atomic number from the mass number.
Example Carbon-14
Mass number is 14 ( Protons + neutrons)
Carbon has 6 protons
So = 8 ( the number of neutrons)

11. Atomic Mass Most elements contain a mixture of 2 or more isotopes.
The atomic mass of an element is the average mass of all the isotopes of that element. Therefore, the atomic mass is usually not a whole number.

12. Atomic Mass For example, we have 2 isotopes of copper in nature.
69% Copper-63 atoms and 31% copper-65 atoms.
Therefore, the atomic mass of copper is the weighted average of both masses.

13. Calculating the mass of an element.
= 43.47
100
= 20.15

14. Finally we add both masses.
= 63.6 amu

15. Chlorine-35 makes up 76% of all chlorine in nature, and chlorine-37 makes up the other 24%. What is the atomic mass of chlorine.

16. Page 325 questions 1, 2, 3, 4, 6, 8
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