Download Chapter 4 Section 4.1 & 4.2

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Do Now:
1. On the blank side of an index card, draw a
picture of an atom.
2. On the other side of the index card, write
down things that you know about atoms.
• 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.
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
• Greek philosopher (460– 370BC)
• Among the first to suggest the existence of
atomos – Greek word for “not to be cut”
• 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?
• In 1897, physicist J.J. Thomson discovered the
• Electrons are negatively charged subatomic
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.
The Bohr Model
• Niels Bohr (1913)
• Incorporated discoveries about how the
energy of atoms changes when the atom
absorbs or emits light.
• Stated that the electrons orbit the nucleus like
planets orbit the Earth.
Quantum Mechanical Model
• The probability of finding an electron is
represented as a fuzzy cloudlike region.
• The cloud is more dense where the probability
of finding the electron is high.
Atomic Orbital – region of space
where there is a high probability
of finding an electron