Atomic Theory Notes

1. Atomic Theory
• Democritus (440 BCE)- realized that if you
continued to cut something, eventually you
would end up with something that couldn’t
be cut anymore, atomos – meaning not
able to divide
• Atoms are the smallest particle that an
element can be divided and still be the
same substance
• All matter is made of atoms

3. John Dalton (1803)
• realized that atoms combine in very specific
ratios based on mass (specific mass ratios)
• all substances are made of atoms and they can
not be created, divided or destroyed because
they were made of a single substance
• All atoms of the same element are exactly alike
and different from other elements, they are
unique
• Atoms join with other atoms to form new
substance
• Solid ball

4. Billiard Ball Model

5. J. J. Thomson (1897)
• discovered that there were small particles
inside the atom, meaning that atoms can
be divided into smaller substances
• Electrons – negatively charged particles
attracted to positively charged particles
• Plum pudding model – electrons are
mixed throughout the atom, soft blobs of
matter
• Cathode ray tube experiment

7. Plum Pudding Model

8. Ernest Rutherford (1909-1911)
• Discovered that an atom contains a
nucleus with positively charged particles
and that the electrons must be “floating”
around the nucleus
• Most of an atom is empty space
• Gold foil experiment

11. Niels Bohr (1913)
• Proposed that electron moved around the
nucleus in energy levels (shells), but no
electrons between the energy level (think
ladder)
• Electrons can jump from one level to
another
• Travel in a definite path – solar system
model
• Solar System Model

12. Solar System Model

13. 20th
Century
Modern Atomic Theory
• Erwin Shrodinger & Werner Heisenberg
• Electrons have no predictable pattern and
move in a region where electrons are
likely to be found called the electron
cloud

14. Electron Cloud Model

16. Atoms
• All atoms have a nucleus
– protons (+),
– neutrons (no chg)
– electrons (-)
• Same number of protons and electrons an atom
has no charge
• More protons (+) than electrons (-) the atom has
a positive ion is formed (more positives than negatives)
• More electrons (-) than protons (+) a negative
ion is formed (more negatives than positives)

17. • Simplest atom is made of one proton, and
1 electron – hydrogen (has no neutrons)
• All additional element will have protons,
neutrons and electrons
• The atomic number of an element is
determined by the number of protons,
– you can not change the number of protons
• To find neutrons take the mass number
(rounded) and subtract the protons.

18. Isotopes
• Isotopes have the same number of protons
but additional neutrons which causes the atomic
mass to be different
• Isotopes can be stable (maintain there structure)
and unstable (fall apart over time)
• Unstable isotopes are radioactive and will decay
over time giving off particles and energy
(radioactive)
• Mass number determines how many isotope an
element has
• Most elements have isotopes
• All isotopes of an element have the exact
properties of the element

19. Forces in atoms
• Gravitational force – pulls objects toward
each other—depends on mass and
distances between the objects—very small
force in atoms
• Electromagnetic force –– proton (+) and
electrons (-) have strong attraction which
keeps the electrons in motion around the
nucleus of atoms

20. • Strong force – force which keeps protons
from flying apart due to close distance
between protons and neutrons
• Weak force – relevant to radioactive
atoms- allows neutrons to change into
proton and electron