Ch. 4 Sec. 1 Introduction to Atoms
Development of Atomic Models
*In 430 B.C. a Greek philosopher named Democritus had an idea that matter could
be broken down into smaller pieces.
*In the 1600s, scientists proved Democritus right by discovering the atom.
-An atom is the smallest particle of an element
*The Atomic Theory was born.
Dalton's Atomic Theory
1. All elements are composed of atoms that cannot be divided.
2. All atoms of the same element are exactly alike and have the same mass. Atoms
of different elements are different and have different masses.
3. An atom of one element cannot be changed into an atom of a different element.
Atoms cannot be created or destroyed in any chemical change, only rearranged.
4. Every compound is composed of atoms of different elements, combined in a
specific ratio.
Thomson and Smaller Parts of Atoms
*In the early 20th century, J. J. Thomson discovered that atoms could be broken
down into even smaller parts.
*He discovered the electron.
-Electrons are negatively charged particles
Rutherford and the Nucleus
*In 1911, Ernest Rutherford found protons in the nucleus of an atom using gold foil
-Protons are positively charged particles found in the nucleus.
*Rutherford created an atomic model
Bohr's Model
*In 1913, Niels Bohr revised Rutherford's atomic model from random orbits to
specific orbits.
Ex. Looks like the rings of an onion.
*In the 1920s, the atomic model changed again.
-Scientists discovered that electrons do not orbit the nucleus like planets
-Electrons form an 'electron cloud'
-Move rapidly in every direction around the nucleus
*An electron's movement is related to its energy level, or specific amount of
energy it has.
The Modern Atomic Model
*In 1932, James Chadwick discovered the neutron.
-The neutron is a neutrally charged particle in the nucleus of atoms
*Chadwick's discovery led to another change to the atomic model
The modern model describes an atom as consisting of a nucleus that contains
protons and neutrons, surrounded by a cloudlike region of moving electrons.
Particle Charges
-Protons have the symbol p+
-Positively charged
-Electrons have the symbol e-Negatively charged
-Neutrons have the symbol n0
-Neutrally charged (no charge)
Comparing Particle Mass
2,000 electrons = 1 proton
1 proton = 1 neutron
*These particles are measured in AMU's (atomic mass unit)
Scale and Size of Atoms
Pencil eraser = nucleus = pitcher's mound
Nose-bleed seats = closest electrons
Atomic Number
*Atomic number = the number of protons that are in an element
Ex: Carbon is element #6. It has 6 protons in it.
Oxygen is element #8. It has 8 protons in it.
Lead is element #82. It has 82 protons in it.