Historical Development of the Atomic Theory
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Transcript of Historical Development of the Atomic Theory
Democritus
Ancient Greek philosopher
(circa 400 BC)
Matter is made of tiny particles that we can’t see
Atomos means indivisibleThought these spheres of matter
were solid and could not be broken down further
John Dalton (1766 – 1844)
English scientist and teacher
Performed experiments
4 points to his atomic theory:
Dalton’s atomic theory:
1. All elements are made of tiny indivisible atoms
2. Atoms of the same element are the same as each other and different from other elements
3. Atoms of different elements can mix together in whole-number ratios
4. Chemical reactions occur when atoms change how they are arranged.
J.J. Thompson (1856 – 1940)
English Physicist
Passes electric current through a cathode ray tube
The cathode ray is a stream of negatively charged electrons
Electrons are a sub-atomic particle
Must be a corresponding positive part to the atom
“discovers” the electron
What was learned from the gold foil experiment:
Small, massive center called the nucleus
Nucleus has a positive charge (+)
Electrons (-) are outside the nucleus
Most of atom is empty space
Neils Bohr (1885 – 1962)
Danish Physicist
Used Spectral Line Data from hydrogen
Organized electrons into discrete shells or orbitals.
The Bohr Model is what is used to explain electron configurations in Regents Chemistry
All matter is composed of atoms
An element is composed of only one type of atom
Compounds are different kinds of elements bonded together in definite proportions
• Atoms can be broken down into smaller units called subatomic particles:• Proton• Neutron• Electron
Electron
Negatively charged
Mass of 0 (zero) amu
Found outside the nucleus
“Shells” or energy levels
Usually = protons
Wave-Mechanical Model
AKA (Also Known As):
Quantum Mechanical Model
Electron Cloud
Deals with the probability of finding an electron in a certain area of the atom outside the nucleus.
Electron Shells(or orbitals or energy levels):
(crack out your Periodic Table)
Row # = # of electron shells.
Valence Electrons
electrons in the outermost energy level
Column # = # of valence electrons sort of…
• Column 1 = 1 valence electron• Column 2 = 2 Valence electrons• Columns 3 – 12 there is no
obvious pattern