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Do Now: Solve the following using the
correct number of significant figures.
1) 7.76 m + 2.1 m =
2) 5.750 cg - 1.1 cg =
3) 5555 kg + 444 kg =
4) 1.23 m x 3.2 m =
5) 6.9 mm / 3 mm =
• Explain how Democritus and John Dalton
described atoms
History of Atomic Theory
Do Now:
1. What models have you used?
2. What other models are you aware of?
3. Why do people use models?
• Explain how Democritus and John Dalton
described atoms
• Identify instruments used to observe atoms.
• Identify three types of subatomic particles.
• Describe the structure of atoms according to
the Rutherford atomic model.
• Greek philosopher (460– 370BC)
• Among the first to suggest the existence of
atomos – Greek word for indivisible
• Reasoned that atoms were indivisible and
• English chemist & school teacher
• 1766 – 1864
• Used experimental methods to transform
Democritus’ ideas into scientific theory
• Studied ratios in which elements combine in
chemical reactions.
Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible
particles called atoms.
Atoms of
element A
Dalton’s Atomic Theory
2. Atoms of the same element are identical. The
atoms of any one element are different from
those of any other element.
Atoms of
element A
Atoms of
element B
Dalton’s Atomic Theory
3. Atoms of different elements can physically
mix together or can chemically combine in
simple whole-number ratios to form
Mixture of atoms of
elements A and B
Dalton’s Atomic Theory
4. Chemical reactions occur when atoms are
separated from each other, joined, or
rearranged in different combinations. Atoms of
one element are never changed into atoms of
another element as a result of a chemical
Compound made by
chemically combining
atoms of elements A
and B
Dalton’s Atomic Theory
• Do you think all parts of Dalton’s Atomic
Theory are still believed to be true today?
Sizing Up The Atom
• A pure copper coin the size of a penny
contains about 2.4 x 1022 atoms
24,000,000,000,000,000,000,000 atoms
* Note – Earth’s population is ~ 7 x 109 people
Sizing Up The Atom
• Individual atoms are observable with a
Scanning Electron Microscope (SEM).
Pollen grains
Structure of the Atom
What is in the box?
Subatomic Particles
1. Protons
2. Neutrons
3. Electrons
• In 1897, physicist J.J. Thomson discovered the
• Electrons are negatively charged subatomic
• In 1886, Eugene Goldstein observed a cathode
ray tube and found rays traveling in the
direction opposite to that of the cathode ray
– He concluded they were composed of positive
– These positively charged subatomic particles are
called protons
• In 1932, James Chadwick confirmed the
existence of the neutron.
– Neutrons are subatomic particles with no charge
but with a mass nearly equal to that of a proton.
Subatomic Particles
How are these three subatomic particles
(protons, neutrons, and electrons) put
together in an atom?
Plum Pudding Model
• Thompson’s atomic model
• Electron’s stuck in a lump of positive
charge, similar to raisins stuck in dough.
Rutherford’s Gold Foil Experiment
• In 1911, Ernest Rutherford, a former
student of Thomson’s, tested the plumpudding model.
• A narrow beam of alpha particles was
directed at a very thin sheet of gold
Rutherford’s Gold Foil Experiment
• Most alpha particles went straight through, or
were slightly deflected.
• A small fraction of the alpha particles bounced
off the gold foil at very large angles.
Rutherford’s Atomic Model
The Nuclear Atom
• Rutherford suggested a new theory of the
atom based on the experimental results.
– The atom is mostly empty space.
– All the positive charge and amost all of the mass
are concentrated in a small positively charged
region (nucleus)
– Protons & neutrons are in the positively charged
– Electrons are distributed around the nucleus and
occupy almost all the volume of the atom.
Rutherford’s Atomic Model
The Nuclear Atom
If an atom were the size of a football stadium,
the nucleus would be the size of a marble.