First - Development of the Atomic Theory
• Democritus (440BC) developed the idea that all things were made of indivisible particles
(atoms).  They are constantly moving and join to form different substances.
• Aristotle (340BC) said that all things were made of the 4 elements earth, air, fire and water.
This concept was wrong, but was accepted for almost 2000 years, it did give us the idea that
elements join to make different substances.
Second - The Current Model of the Atom
1. An atom is small, really small, and I mean really, really small!  The average atom has a diameter of  0.00000003 cm.
2. The center of the atom is called the nucleus.  It is made of smaller particles call protons and neutrons
1. Protons have a positive charge and a mass of 1 AMU (Atomic Mass Unit)
2. Neutrons have a neutral charge and a mass of 1 AMU (Atomic Mass Unit)
1. Electrons orbit the atom.  They have a negative charge and a mass so small that we don't even count it.
2. Electrons orbit the nucleus in vibrating levels called valences.  When an atom absorbs energy the electron jumps
up to a higher level, when the electrons 'falls down' to a lower level it gives off colors of light.
3. When atoms gain or loose electrons they are called ions.
4. There are ~112 elements (different types of atoms) each has unique properties.  The number of protons in an atom
determines the element. The number of protons in an atom is called it atomic number.
5. The number of protons and neutrons in an atom is called the mass number of the atom.
6. If an atom has more neutrons than 'normal' (we'll discuss that in class) it is called an isotope and is radioactive.
This means it will break down into different elements using alpha, beta and gamma decay.
7. The atomic mass of an element is the average mass of all of the atoms of that element.
8. There are 4 forces that work within the atom.
1. Gravity - a very weak force that acts between all matter
2. Electromagnetic Force - the force between charged particles.
3. Weak force - A force that changes the neutron of an atom during radioactive decay.
4. Strong force - the force that holds together the protons of the nucleus. (it is obviously much stronger than the
electromagnetic force.
Chapter 6 - The Atom
• Dalton (1803) published his atomic theory that said:
• All substance were make of atoms
• Atoms are small particles that cannot be created, divided or destroyed.
• Atoms of the same element (Aristotle's word) are exactly alike and atoms of different elements are different.
• Atoms join with other atoms to make new substances
• J.J. Thomson (1897) Discovered a negative part of the atom called the electron and developed the 'plum pudding'
model of the atom
• Ernest Rutherford (1909) discovers that 99.9% of the atom is made of empty space.  His model has a positively
charged nucleus at the center and negative electrons in 'orbit' around it.
• Niels Bohr (1913) develops levels of orbits (valence levels) for the electrons.
• Modern Theory - Electron Cloud Model - Electrons orbit the nucleus in specific regions.  We can know the likely
position of the electron but never the exact position.