Transcript Structure of the Atom

Early Atomic Theory and the
Structure of the Atom
Chemistry
Structure of an Atom Discovery
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Compare and contrast the different atomic models.
Distinguish between the subatomic particles.
Describe the structure of the nuclear atom.
Define atom
Explain the role of atomic number in determining the
identity of an atom.
Define isotope and explain why atomic masses are
not whole numbers.
Calculate the number of electrons, proton, neutrons.
Explain the relationship between unstable nuclei and
radioactive decay.
Characterize the different types of radiation.
Atomic Structure Outline
Make a timeline for the discovery of the structure of an
Part 1: Find all 8 scientists involved with discovering the
atom (Democritus, Aristotle, Dalton, Crookes,
Thomson, Millikin, Rutherford, Chadwick)
-give their name, approximate date, and discovery
-explain the discovery in 1-2 sentences
-show to teacher before doing Part 2
Part 2: In chronological order, put the scientists, with
their date, and one sentence explaining what they
discovered
-put this on 8 ½ x 14 paper (with title) - Be Neat!!
Early Theories of Matter
Democritus (460-370 BC)- first person to propose the
idea matter was not infinitely divisible and that matter
was made of tiny individual particles called the atom
Aristotle (384-322 BC)- rejected Democritus’ view and
that atoms existed
-since Democritus could not prove his theory,
Aristotle’s view become widely known
John Dalton (1803)-revised Democritus’ view based on
scientific research
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Dalton’s Atomic Theory
All matter is composed of extremely small particles
called atoms.
Atoms of a given element are identical in size, mass,
and other properties; atoms of different elements
vary.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
In chemical reactions, atoms are combined,
separated, or rearranged.
Changes to Dalton’s Atomic Theory
Because of Dalton’s atomic theory, most scientists in the
1800s believed that the atom was like a tiny solid ball
that could not be broken up into parts.
Eventually, Dalton’s theory had to be revised as
technology and new information became available:
-atoms are divisible (made up of smaller particles)
-all atoms of a given element do not have
identical properties (isotopes)
William Crookes (1880’s)-discovered the cathode ray, a
ray of radiation that travels down a cathode ray tube.
-accidental discovery
JJ Thomson (1890’s)-continued
cathode tube work and
concluded the electron
was smaller than even H
-cathode ray was negative (because it
traveled away from the negative plate
and attracted to the positive plate)
-renamed the electron
Robert Millikan (1909)-confirmed the charge of an
electron and determined its mass
Matter is not negatively charged, so atoms can’t be
negatively charged either.
If atoms contained extremely light, negatively charged
particles, then they must also contain positively
charged particles—probably with a much greater
mass than electrons.
Ernest Rutherford (1911)- most
of the mass of an atom is in
the nucleus, which is
positively charged; most of
the space comes from the
electrons
-by 1920 he concluded the
positive charge was actually
a proton (+1 charge)
-“gold foil experiment”: a
beam of positively charged
subatomic particles toward a
thin piece of gold foil
~Because most particles
passed through the foil
they concluded the atom
is nearly all empty space.
~Because so few particles
were deflected, they
proposed that the atom
has a small, dense,
positively charged central
core
James Chadwick (1932)- nucleus contains another
subatomic particle, the neutron, but does not carry a
charge (neutral)
Notes:
-the proton and neutron are about the same size; each
is about 1836 times larger than the electron
-mass of an atom comes from the nucleus, but the
volume (size) comes from the electron cloud
What is an Atom? Activity
Objectives
1. Define atom, atomic number, and mass number.
2. Determine the number of electrons, protons, and
neutrons in an atom.
3. Differentiate between atoms and isotopes.
4. Determine the charge on the atom based on if it
gains or loses electrons.