Chapter 4
The Structure of the Atom
Democritus (C4:1)
• First philosopher to propose the idea that
matter was not infinitely divisible
• Believed matter was made up of particles called
atomos (atoms)
• Believed that atoms could not be created,
destroyed, or further divided
• His ideas were met with criticism
Aristotle
• Rejected the notion of atoms because the idea
did not line up with his idea of nature
• Did not believe that empty space could exist
• Believed that matter is made up of air, water,
fire, and earth
• Aristotle was so influential that his rejection of
Democritus’ ideas caused the whole world to
debunk Democritus
Dalton’s Atomic Theory
• Matter is composed of extremely small particles called
atoms
• Atoms are invisible and indestructible
• Atoms of a given element are identical size, mass, and
chemical property
• Atoms of a specific element are different than those of
another element
• Different atoms combine in a simple whole-number
ratios to form compounds
• In chemical reactions, atoms are separated, combined,
or rearranged
Conservation of Mass
• Mass is conserved in any process
• Dalton’s theory explains that in any
chemical reaction mass is conserved
through the separation, rearrangement or
combination of atoms
• Dalton was wrong about atoms being
indivisible, however
• Atoms can be divided into subatomic particles
• Dalton was wrong about atoms of the
same element having identical properties,
these can have slightly different masses.
The Atom (C4:2)
• An atom is the smallest particle of an element that still retains all of
the properties of an element
• Because atoms are so small, you need special equipment to see them
• Nanotechnology is used to manufacture machines the size of atoms
The Electron
• Scientists began to make connections between atoms and electric
change – like what occurs when your hair clings to a comb
• To test their theory, scientists used cathode-ray tubes
• A cathode-ray tube is a glass tube from which air has been removed
• The scientists inserted a positively charged electrode on one side of
the tube and a negatively charged electrode on the other side of the
tube
• Electricity passed from one side of the tube to the other
The Electron
• Sir William Crookes – an English physicist was working with a
cathode-ray tube and a green flash was produced when radiation
struck the coating of the tube – the coating was made out of zincsulfide
• This was called a cathode ray
• The discovery of the cathode ray led to the invention of television
• Scientists at the end of the 1800s were convinced that 1) cathode
rays were a stream of charged particles and 2) that some particles
carry a negative charge
The Electron
• Scientists concluded that the ray’s
negatively charged particles were found
in all forms of matter and are known as
electrons
• J. J. Thomson determined the charge-tomass ratio of the charged particle in a
cathode ray
• J. J. Thomson concluded that the mass of
an electron is less than that of a
hydrogen atom (lightest atom)
The Electron
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Robert Millikan: used the oil-drop apparatus to
determine that electrons have a negative charge oil droplets were given a negative charge after
colliding with electrons in the apparatus
Millikan also determined that 1.602 x 10-19 is the
charge of an electron - later equated to a charge of
-1 (what we use today)
The Atom
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Matter is neutral - you are not shocked when
you touch an object, unless under certain
conditions
Plum-Pudding Model: proposed by J. J.
Thomson as a way to determine how matter
could be neutral if electrons are negatively
charged
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•
Spherically-shaped atom composed of uniformly
distributed positive charges, in which the
negative charges reside
this model did not last long
The Nucleus
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•
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Ernest Rutherford: conducted an experiment to determine if alpha
particles (radioactive particles) would be deflected as they passed
through a thin gold foil
When the alpha particles made contact with the zinc-sulfide screen
coating of the foil, a flash of light was produced
A few of the particles were deflected at various angles and several
were deflected back to the source
Rutherford determined that the Plum-Pudding model was incorrect
because it could not explain his results
The Nucleus
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•
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Rutherford determined that most of the atom
is made up of empty space through which
electrons move
Determined that almost all of the atom’s mass
and positive charges were contained in a tiny
dense region at the center of the atom called
the nucleus.
The negatively charged particles are held
within the atom by their attraction to the
positively charged nucleus.
The Proton and The Neutron
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•
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Rutherford determined that the positively
charged particles in an atom are protons
Protons carry a charge that is equal to but
opposite of electrons: +1
James Chadwick showed that the nucleus
also contains a subatomic particle called a
neutron that has no electric charge
The Atom
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•
•
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All atoms are made up of 3 subatomic particles: neutrons, protons,
and electrons
Atoms are spherical, with a small, dense nucleus of positive charge,
and are surrounded by one or more electrons
The composition of the atom is mostly made up of fast-moving
electrons that move through the empty space surrounding the
nucleus
The electrons are held to the atom by their attraction to the protons
in the nucleus
The Atom
•
•
The nucleus of the atom
contains most of its mass protons and neutrons are in
the nucleus
Atoms are electrically neutral the number of protons in the
nucleus equals the number of
electrons surrounding the
nucleus