1. Section 1- pg 124 Introduction to Atoms
Chapter 4
Section 1- pg 124
Introduction to Atoms

2. Development of Atomic Theory
Pg 125
Development of Atomic Theory
Democritus (a smart Greek dude)->
proposed that matter is formed of
small pieces that could not be cut into
smaller parts – atomos “uncuttable”
Atom: the smallest particle of an
element
Atomic theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised

3. Dalton’s Atomic Theory
Pg 125
Dalton’s Atomic Theory
Formed by English chemist John Dalton
All elements are composed of atoms that cannot be divided
All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses
An atom of one element cannot be changed into a atom of a different element. Atoms cannot be created or destroyed in any chemical change, only rearranged
Every compound is composed of atoms of different elements, combined in a specific ratio

4. Thomson’s Model Pg 126 1897: English Scientist – J.J. Thompson
Found that atoms contain negatively charged particles
But atoms themselves have no electrical charge therefore there must also be a positively charged particle
Described an atom that consisted of negative charges scattered throughout a ball of positive charge (like chocolate chips in a cookie)
Later the negatively charged particles became known as electrons

5. Rutherford’s Gold Foil Experiment
Pg 126
Rutherford’s Gold Foil Experiment
Gold Foil Experiment proved Thomson was mistaken

6. Pg 127
Based on experiment Rutherford inferred that an atom’s positive charge must be clustered in a tiny region in its center – nucleus
This is where all of the atoms mass came from
Positively charged particles in an atom’s nucleus- Protons
Except Rutherford was not aware of neutrons

7. Bohr’s Model Pg 127 1913 – Neils Bohr – Danish Scientist
Revised atomic model to show that electrons had specific orbits

9. Cloud Model Pg 127 1920s – Change again!
Electrons do not orbit, instead are like a cloud around nucleus
An electron’s movement is related to its energy level, or the specific amount of energy it has
The energy of each electron keeps it in motion around the positive nucleus to which it is attracted

10. The Modern Atomic Model
Pg 128
The Modern Atomic Model
1932 – James Chadwick
New particle in the nucleus called neutron
Has no electrical charge and same mass as a proton
Modern Atom Model: at the center of the atom is a tiny, massive nucleus containing protons and neutrons. Surrounding the nucleus is a cloudlike region of moving electrons

12. Comparing Particle Masses
Pg 129
Comparing Particle Masses
Atomic Mass Units – amu
Proton and Neutron = 1 amu
Electron = 1/1836 amu
Proton (p+) – Positive Charge
Neutron (n) – No charge
Electron (e-) – Negative Charge

13. Pg 129
Scale and Size of Atoms
To picture the scale of an atom, imagine that the nucleus were the size of an eraser on a pencil. If you put this “nucleus” on the pitcher’s mound of a baseball stadium, the electrons could be as far away as the top row of seats!

14. Pg 129
Atomic Number
Atomic number = how many protons it has

15. Pg 130
Isotopes
Atoms of the same element that have different numbers of neutrons = Isotopes
An isotope is identified by its mass number, which is the sum of the protons and neutrons in the nucleus of an atom

16. Chapter 4 Section 1 Homework Pg 130

17. 1A. Why did the Atomic Theory change with time?

18. 1B. Describe Bohr’s model of the atom
1B. Describe Bohr’s model of the atom. What specific information did Bohr contribute to scientists’ understanding of the atom?

19. 1C. How is the modern model different from Bohr’s model?

20. 2A. What are the three main particles in the modern model of an atom?

21. 2B. What is the atomic number
2B. What is the atomic number? How is it used to distinguish one element from another?

22. 2C. The atomic number of nitrogen is 7
2C. The atomic number of nitrogen is 7. How many protons, neutrons, and electrons make up an atom of nitrogen-15?