1. Chapter 4
The Structure of an Atom

2. Early Theories of Matter - Democritus
Thought that matter is composed of the empty space through which atoms move
Atoms are solid, homogenous, indestructible and indivisible
Different kinds of atoms have different shapes
The properties of matter are due to the different sizes, shapes and movements of atoms
Changes in matter result from the different groupings of atoms – NOT from changing atomic properties

3. Early Theories of Matter - Aristotle
One of the influential philosophers
Denied the existence of atoms

4. Early Theories of Matter – John Dalton
All matter is composed of extremely small particles called atoms
All atoms of a given element are identical (same size, mass and chemical properties)
Atoms cannot be created, divided and/or destroyed
Different atoms combine in whole-number ratios to form compounds
In a chemical reaction, atoms are separated, combined and/or rearranged.

5. What is an atom?
An atom is the smallest particle of an element that retains its physical properties

6. Discovering the Electron
Sir William Crookes (English Physicist) discovered the electron while working in a cathode ray lab
Cathode Rays were streams of charged particles that carried a negative charge
He changed both the electrode and type of gas in the cathode ray, but neither affected the cathode ray produced.
Therefore, conclusion was negatively charged particles were found in all forms of matter
These are called electrons

7. Discovering the Electron, Cont.
Robert Millikan, an American Physicist, determined the charge of an electron (-1)
Mass of an electrons:
9.1 x = 1/1840 Mass of an hydrogen atom

8. The Nuclear Atom
Rutherford conducted an experiment with gold foil that created the Rutherford Model of an Atom. He concluded that there was a tiny centrally located dense region of an atom called a nucleus, which contains most of the atoms mass.

9. Atoms
The Structure of an Atom

10. Space surrounding the nucleus
Completing the Atom
The Atom is made up of three things:
Proton: Found in the nucleus and is positively charged
Electron: Found in the space surrounding the nucleus and negatively charged
Neutron: Found in the nucleus and is neutrally charged
Particle
Symbol
Location
Relative Electrical Charge
Relative Mass
Actual Mass
Electron e-
Space surrounding the nucleus 1-
1/1840
9.11 x 10-28
Proton p+
Nucleus 1+ 1
1.673 x 10-24
Neutron No
1.675 x 10-24

11. How Atoms Differ
Atomic Number = Number of Protons = Number of Electrons
Chemical Name
Chemical Symbol
Atomic Number
Average Atomic Mass

12. Isotopes and Mass Number
Atoms with the same number of protons but differing amounts of neutrons are isotopes
Different isotopes have different mass numbers, which are added after the element, to denote the different amount of neutrons
For example, Carbon-15 has 6 Protons and 9 Neutrons

13. How Do You Find the Number of Neutrons?
Number of Neutrons = Mass Number – Atomic Number

14. Mass of Individual Atoms
Atomic Mass Unit is defined as 1/12 of a carbon atom
Atomic Mass of an element is the weighted average mass of the isotopes
Particle
Mass (AMU)
Electron
Proton
Neutron

15. Atoms and Their Interactions
Elements
A substance that can’t be broken down into simpler chemical substances
Everything is made up of elements

16. Half-Life
Half-Life
The amount of time it takes for half the amount of a substance to dissipate or transform
For example, Carbon-15 has a half-life of seconds. That means that every seconds, half the amount of substance dissipates and/or transforms.

17. Half-Life, Continued Answer: 3.75 grams.
Carbon-15 has a half-life of seconds. That means that every seconds, half the amount of substance dissipates and/or transforms.
If you start with 30 grams of Carbon-15, how much will you have left after seconds?
Determine the number of half-lives. (7.347 seconds / seconds) = 3 Half-Lives
Complete the math for each of the half-lives. The “0” half-life is NOT the first half-life, but instead, the initializing moment – the amount of substance at “0” seconds, before the clock starts.
Label the half-lives and determine the amount of substance in each.
0 – 30 grams
1 – 15 grams (30 grams / 2)
2 – 7.5 grams (15 grams / 2)
3 – 3.75 grams (7.5 grams / 2)
Answer: grams.