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The Atom
Dalton’s Atomic Theory
1. Elements are made of tiny particles called atoms.
2. All atoms of a given element are identical.
3. The atoms of a given element are different from
those of any other element.
4. Atoms of one element can combine with atoms of
other elements to form compounds. A given
compound always has the same relative numbers
and types of atoms.
5. Atoms are indivisible in chemical process. Atoms
are not created nor destroyed.
Which numbers in Dalton’s Atomic
theory are no longer valid?
2. All atoms of a given element are identical.
Why not? Isotopes
5. Atoms are indivisible in chemical process.
Atoms are not created nor destroyed.
Why not?
We can split an atom
Subatomic Particles
Thomson’s experiment
Discovered electrons are negative (cathode ray
tube and a magnet)
Plum pudding model of atom
Had negatives and positive all mixed together
Rutherford’s experiment
Shot alpha particles at thin sheet of gold foil.
All of the particles should have passed through,
but some bounced back. (like shooting a
cannon ball at a sheet of paper and having it
bounce back)
Discovered the Nucleus Small dense core of an atom
Contains: Protons (+) and neutron (neutral)
http://micro.magnet.fsu.edu/electromag/java/r
utherford/
Modern Atomic Model
Nucleus: in the middle
Electron cloud
surrounds nucleus
Electrons:
Mass = 0
(-) charge
Protons:
Mass = 1 a.m.u.
(+) charge
Neutrons:
Mass = 1 a.m.u.
neutral
Symbols of Atoms
Atomic mass
at. mass =(#p+) + (#n)
Symbol of
element
Atomic number
at. # = number of protons
Isotopes
Atoms of the same element with different
number of neutrons.
carbon - 12
p+ = 6
n=6
e- = 6
carbon - 14
p+ = 6
n= 8
e- = 6
Isotope Symbol Examples
Symbol at. number at. Mass
# p+ # n
# e-
26
57
26
31
26
79
198
79
119
79
17
35
17
18
17
35
81
35
46
36