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Early Models of the Atom
ATOMOS = INDIVISIBLE
Atom – The smallest particle of an element that still retains the properties of that element
or its identity in a chemical reaction.
Democritus (460 to 370 BC)
Lavoisier (1743 to1794)
Proust (1871 to 1922)
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First to suggest idea of atoms, they are invisible and indestructible.
Law of conservation of matter.
Law of constant composition, compounds contain the same
elements in the same proportions by mass.
– FORM A BASIC UNDERSTANDING…
 All elements are composed of tiny indivisible particles called
atoms
 Atoms of the same element are identical.
 Atoms of different elements mix.
 Atoms of different elements chemically combine in simple whole
number ratios.
 Chemical reactions occur when atoms are separated from or joined
to other atoms.
– Atoms contain particles that have electric charge.
– Discovered that atoms have a substructure using a cathode ray tube.
– 1897 Discovered the electron and said the atom was like plum
pudding.
Dalton (1766 to 1844)
Faraday (1791 to 1867)
Thomson (1856 to 1940)
Millikan (1868 to 1953)
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Bacquerel (1852 to 1908)
M. Curie (1867 to 1934)
Rutherford (1871 to 1937)
Schrodinger (1887 to 1961)
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1916 used oil drop experiment to calculate the charge and mass of the
electron.
1896 discovered uranium exhibits radioactivity.
Discovered that Polonium and Radium were radioactive.
1909 discovered the nucleus of the atom with his gold foil experiment.
Current quantum mechanical model is based on his equations.
Summary – Atoms exist, different elements have different atoms, atoms are made of particles w/ charge,
atoms have electrons, atoms have a nucleus (protons and neutrons)!
Subatomic
Particle
Proton (p+)
Neutron (no)
Electron (e-)
Mass
Charge
1
1
0
+1
0
-1
Atomic Number = number of p+
Mass Number = number of p+ + no
Atomic Weight* = calculated
value see isotopes.
ATOMS
An atom is an atom is an atom because of the number of protons!!! In ordinary chemical
processes never change the number of protons!!! (Exception – nuclear processes.)
1.
Use the periodic table to determine the number of protons (ATOMIC NUMBER,
number of protons).
In atoms the number of electrons equals the number of protons because an atom does
not have a charge.
To determine the number of neutrons in an atom you need the MASS NUMBER,
number of protons and neutrons (NOT ON THE PERIODIC TABLE!!!)
2.
3.
ISOTOPES
Atoms of the same element with different numbers of neutrons OR different mass
numbers.
Determine mass number from symbols below to determine neutrons.
Carbon –14 (14 is the mass number). 6 protons, 8 neutrons.
238
92
U
The top number is the mass number
The bottom number is the atomic number
92 protons, 146 neutrons (Remember subtract protons from mass
number to get neutrons).
*Atomic weight = a calculated value that takes into account the mass and % abundance of all
isotopes of a particular element (to be revisited later).
IONS
An atom with a charge. Atoms acquire a charge when they gain or lose electrons.
A superscript number will symbolize charge. Cu2+ or Cl-
+
Copper (2+) = 29 p+, 27 e-.
lose electrons
Chlorine (1-) = 17 p+, 18 e-.
gain electrons