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Atomic Models
Chapter 5
1. John Dalton – Billiard Ball Model
John Dalton – English Chemist – 1803
Dalton’s postulates
1. All elements are composed of tiny indivisible
particles called atoms.
2. Atoms of the same element are identical. The
atoms of any one element are different from
those of any other element.
3. Atoms of different elements can physically mix
together or can chemically combine in simple
whole-number ratios to form compounds.
2. Understand Conclusions Show that you understand the conclusions used in the
development of modern atomic theory by describing how Democritus’s idea of the atom was
different from Dalton’s.
Dalton’s postulates
• Dalton’s postulates are illustrated
below.
3. Interpret Diagrams How does a mixture of atoms of different elements differ
from a compound?
Self-check:
• To demonstrate that you understand the
conclusions used in the development of modern
atomic theory, identify the statement that is NOT
part of Dalton’s postulates.
• A All elements are composed of atoms.
• B Atoms of the same element are alike.
• C Atoms are always in motion.
• D Atoms that combine do so in simple wholenumber ratios.
More Atomic Models
2. Thomson model – (1897) - Plum-pudding
model -a ball of positive charge containing
a number of electrons (ex: chocolate chip
cookie dough)
Self - check
To show that you understand the
experimental design used in the
development of atomic theory, summarize
the evidence used by Thomson to argue
that cathode rays consist of negatively
charged particles.
3. Rutherford Nuclear model – (1911) center is a positively - charged nucleus (ex:
peach)
Rutherford’s findings
Self-check
Compare Rutherford’s expected outcome of
the gold-foil experiment with the actual
outcome to show that you understand
experimental design used in the
development of modern atomic theory.
Self-check
Demonstrate that you understand the
conclusions used in the development of
modern atomic theory by describing the
evidence that led Rutherford to conclude
that an atom is mostly empty space.
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4. Bohr planetary model – (1913) electrons travel in definite orbits around
the nucleus (ex: planets around the sun)
Self-check
• Fill in the blanks to show that you understand
the experimental design used in the
development of modern atomic theory:
“According to Bohr’s atomic theory, the
atomic emission spectrum of hydrogen results
when atoms of hydrogen emit ____________ as
____________ move from a ____________
energy level to a ____________ energy level.”
Self-check
• To demonstrate that you understand the conclusions
used in the development of modern atomic theory,
identify how Bohr’s nuclear atom differs from
Rutherford’s nuclear atom.
• A Bohr’s model proposed that electrons occupy orbits
of different shapes.
• B Rutherford’s model explained the physical and
chemical properties of elements, whereas Bohr’s model
only explained the physical properties.
• C Bohr’s model states that electrons occupy orbits with
fixed energy levels.
• D In Bohr’s model, electrons occupy space within a
“cloud” surrounding the nucleus.
More atomic models
5. Quantum model – electrons can be
found in certain regions of space around
the nucleus called orbitals
Even though the quantum model is the most recent,
we still use the Bohr model to illustrate the atom in
high school chemistry!
Some specifics about the Bohr model:
The Octet Rule
Atoms are most stable when they have 8 electrons in
their outer shell
1st energy level holds 2 electrons
2nd energy level holds 8
3rd usually holds 8 (to a max of 18)
And so on…