1. AP Chem Today: Atoms Review
You’re welcome to start coming in during help session to do test corrections
Next test in ~ 2 weeks

2. Evolution of the Atomic Model
Since atoms are too small to see even with a very powerful microscope, scientists rely upon indirect evidence and models to help them understand and predict the structure of an atom.

3. Democritus & Leucippus (~400 BC)
Greek philosophers
first to propose that matter made up of tiny, indivisible particles called atomos, the Greek word for atoms

4. John Dalton (1803) Billiard Ball Model:
Dalton theorized that the basic unit of matter is a tiny particle called an atom
Dalton’s Theory:
All elements are composed of indivisible (can’t be broken down) atoms
All atoms of a given element are identical
Atoms of different elements are different;
Compounds are formed by the combination of atoms of different elements
Billiard Ball Model:
An atom is represented by a hard sphere

5. JJ Thomson (1897)
Used a cathode ray tube to show one of the smaller units that make up an atom
Because the cathode ray deflected towards the + charged plate when an electric field was applied, Thomson concluded that the ray was formed by particles and the particles were negatively charged

6. JJ Thomson (1897)
Thomson discovered that the atom is made up of small, negatively charged particles which he called electrons
Developed the Plum Pudding Model of the atom

7. Robert Millikan (1909)
Millikan’s Oil drop experiment allowed him to determine the mass and charge of an electron

8. Rutherford: Gold Foil Experiment

9. Ernest Rutherford (1911) Conducted the gold-foil experiment:
Directed alpha particles (small, positively charged particles), at a thin piece of gold foil
Most of the alpha particles passed straight through the foil and a few were slightly deflected
Some of the alpha particles were greatly deflected and bounced back  

10. Ernest Rutherford (1911)
Rutherford concluded that atoms have a dense, central core called the nucleus, while the remainder of the atom is essentially empty space
Positively charged particles known as the protons are found in the nucleus

11. James Chadwick First to prove the existence of the neutron
Provided explanation as to why the positively charged protons in the nucleus stayed intact and did not repel each other.

12. Neils Bohr (1913) Bohr Model
The nucleus contained the protons and neutrons
The electrons orbited around the nucleus (like planets orbiting the sun)  
Electrons are shown in concentric circles or shells around the nucleus
The 1st shell can hold a maximum of 2 electrons
The 2nd shell can hold a maximum of 8 electrons
The 3rd shell can hold a maximum of 18 electrons
Electrons in the outermost shell are called the valence electrons

13. Wave-Mechanical/Cloud Model (modern, present-day model)
Developed after the famous discovery that electrons can be viewed as both a wave and particle
Like planetary model, atom is pictured as having dense, positively charged nucleus
The difference in this model is how the electrons are pictured

14. Wave-Mechanical Model
Electrons have distinct amounts of energy and move in areas called orbitals (clouds)
An orbital is a region in which an electron is most likely (most probable) to be located
Value of l
Orbital (subshell)
Orbital Shape
Name* s
sharp 1 p
principal 2 d
diffuse 3 f
fine

15. What is important about the atomic number?
Unique for each element
= # protons in nucleus

16. How do you figure out the number of:
Protons in an atom?
# protons = the atomic number
Electrons in an atom?
# electrons = # protons
FOR NEUTRAL ATOMS, only!
Neutrons in an atom?
# neutrons = mass – # protons

17. What are Isotopes?
Isotopes are different forms of the same element that have a different mass
Isotopes have the same # protons but different # neutrons

18. What are Ions? Ions are atoms that have a positive or negative charge
In an Ion, the number of protons and electrons are not equal

19. Standard Nuclear Notation
Note: for ions, the charge (+ or -) is indicated in the upper right corner

20. What is the ONE THING that determines the identity of an atom?
# of protons (atomic #)

21. Practice

22. Average atomic mass on the periodic table is a weighted average of the masses of all naturally occurring isotopes

23. Average Atomic Mass 28.085 amu , 14 28 𝑆𝑖 151.9641 amu, 63 152 𝐸𝑢
75.91 % Cl-35, 24.08% Cl-37