# Class notes on atomic theory

```ATOMIC STRUCTURE
THE HISTORY OF THE ATOM
Originally all matter was considered to be made up of 4 substances.
•
•
•
•
Earth
Wind
Fire
Water
Democritus developed the idea of atoms.
• He pounded up materials in his pestle and mortar until he had
reduced them to smaller and smaller particles, which he called
ATOMA – Greek for indivisible
John Dalton
• suggested that all matter was made up of tiny spheres that were
able to bounce around with perfect elasticity.
• he called these spheres ATOMS
J.J. Thompson
• found that atoms could sometimes eject a far smaller negative
particle.
• He called this particle the ELECTRON.
• His idea was that an atom was made up of electrons scattered
unevenly within an elastic sphere surrounded by a soup of
positive charge to balance the electron’s charge.
• This became known as the Plum Pudding Model
Ernest Rutherford
• Famous for the gold foil experiment
• While most nuclei passed through the foil, a small number were
deflected and some even bounced straight back.
• This new evidence allowed him to propose a more detailed model
with a central nucleus.
• He suggested that the positive charge was all in a central
nucleus.
Niels Bohr
• Refined Rutherford’s idea by adding that the electrons were in
orbits.
• Each orbit was only able to contain a set number of electrons.
Atomic Structure
Particle
Charge
Mass
Proton
Positive
1 amu
Neutron
No Charge
1 amu
Electron
Negative
0 amu
Terms to know:
Atomic Number
• the number of protons in the nucleus of an atom
• Atoms of the same element all have the same atomic number
Isotope
• atoms of the same elements with different masses
• number of neutrons changes
• changes mass, but atom remains neutral (has no charge)
• Writing a symbol for an isotope includes
• the element’s symbol
• the mass number = # of protons + # of neutrons
• the atomic number = # of protons
H
1 (mass #)
H
1 (atomic #)
Hydrogen 1
2
1
Hydrogen 2
(Deuterium)
H
3
1
Hydrogen 3
(Tritium)
Ion
• atoms of the same elements that have a charge
• number of electrons changes
• positive charge means an atom has lost electrons
• negative charge means an atom has gained electrons
• adding or taking away electrons does not affect an atom’s mass
```
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