[Models of the Atom]

1.  Ancient Greeks

-- Democritus -- all matter could not be subdivided forever
-- called the smallest piece of matter atomos (where the word atom comes from)

2.  John Dalton (early 1800’s)  -- devised first model of atomic theory

a.)  All elements are made of atoms which are indivisible and indestructible
b.)  The atoms of the same element are exactly alike
c.)   Atoms of different elements are different
d.)  Compounds are formed by joining the atoms of two or more atoms

3.)  J.J. Thomson  (Plum Pudding Model)

-- proposed plum pudding model of atomic structure
-- stated negatively charged electrons were scattered like plums in positively charged
    pudding-like material in the atom

4.)  Rutherford  (Gold Foil Experiment)

-- discovered positively charged particles “shot” at an atom passed mostly straight
through it
-- concluded Thomson was wrong, that most of the atom was empty space with positive
particles packed in the center and negative electrons on the outside of the atom

5.)  Bohr Model   (1913)  (Solar System Atomic Model)

-- stated that most of the mass of an atom (its protons and electrons) are in the center
-- stated electrons orbit the center (nucleus) of the atom in definite orbits

6.)  Wave Model

-- our current model of atom structure
-- state that electrons do NOT move in definite orbits around the nucleus of an atom
-- electrons have both wave and particle properties

[Subatomic Particles of an Atom]

1.)  protons

-- positively charged
-- mass of 1 A.M.U. (atomic mass unit)
-- found in the nucleus

2.)  neutrons

-- no charge or neutral
-- also a mass of 1 A.M.U.
-- found in the nucleus

3.)  electrons

-- negatively charged
-- mass of nearly 0 A.M.U.
-- found outside the nucleus

atomic number -- indicates the number of protons in an element
                           (usually the smaller number with an elemental symbol)

atomic mass -- the number of protons and neutrons in an element
                        (usually the bigger number with an elemental symbol)

isotopes -- atoms with the same atomic number (number of protons) but a
                  different atomic mass (or number of neutrons)

**  The atomic number defines what an element is!

electron cloud -- the space around an atom in which an electron is likely to be found

**  Electrons are arranged around the nucleus of the atom in energy levels.

ground state -- the lowest possible energy state of an electron

excited state -- electron is raised to an energy level above its normal energy level

-- The greater the distance away from the nucleus, the higher the energy of the electron.

Energy Level one -- holds a maximum of two electrons

Energy level two -- holds a maximum of eight electrons

Energy level three -- holds a maximum of 18 electrons

[Forces Within The Atom]

1.  Electromagnetic force
2.  Strong force (holds protons in nucleus together)
3.  Weak force
4.   Gravity

Other Facts About Atoms

1.  Atoms are tiny.   They are far too small to be seen with a light microscope.

2.   Atoms and molecules are perpetually (forever) in motion.   The greater the temperature, the greater their motion.

3.   The atoms of one element are always different from the atoms of another element.

4.   Atoms may join together in well defined molecules or be arranged in regular geometric patterns.   

5.   There are well over 100 different elements.   Elements combine in a multitude of ways to produce compounds that account for all living and nonliving substances.   Few elements are found in their pure form.