1. Atomic Structure
What is an atom?

2. Atom
 An atom is the smallest
physical particle of an
element that still retains
the properties of that
element.

3. Atomic Theory
 First scientist to suggest
atoms: Democritus
 “Everything that is must
be eternal.”
 Democritus believed that
there were basic things
that always exist, but
can be rearranged; he
called these “atomos”.

4. Atomic Theory
 John Dalton
 exceptionally bright Quaker
 put in charge of the local school at
age 12
 Color blind (was once called
Daltonism)
 First scientist to create a modern
definition of “atom”

5. Atomic Theory
 “We might as well attempt to
annihilate [a planet], as to
create or destroy a particle of
hydrogen.”
 Determined relative weights of
known elements by breaking
apart compounds consisting of
them.

6. Atomic Theory
1. All elements are composed
of atoms.
2. Atoms of the same element
are identical. Atoms of
different elements are
always different.
3. Atoms of different elements
can chemically combine in
fixed ratios to form
compounds.

7. Atomic Theory
4. Chemical reactions occur when atoms are joined,
separated, or rearranged. Atoms of one element
are NEVER changed into another during a
chemical reaction.

8. Atomic Size
 At sea level, one cubic
centimeter of air (size of
a sugar cube, or marble)
will have 45 billion billion
atoms within it.
 45,000,000,000,000,000,000
 How many atoms would it
take to fill a universe?

9. Count to 45,000,000,000,000,000,000
 Enough popcorn kernels to
cover the US 3 feet deep
 If divided up in dollars, every
person in America would be
twice as rich as Bill Gates

10. Atomic Size
 To see the atoms in a
drop of water, you
would need to enlarge
the drop until…
 Think of a line one
millimeter long. If this
line were blown up to
the size of the empire
state building, an atom
would be…

11. Review of Atoms?
 All atoms are identical.
 Chemical reactions occur when atoms of one
element are changed into atoms of another
element.

12. Atomic Structure
What does an atom look like…
And how do we know?

13. Proof of Atoms
 One century after Dalton,
Viennese scientist Ernst Mach
wrote that “atoms cannot be
perceived by the senses; they
are things of thought…”
 …Four scientists that made
contributions to atomic
structure – J.J. Thompson,
Ernest Rutherford, James
Chadwick and Neils Bohr.

14. John Dalton’s Atom
 Atoms are solid and indivisible.

15. J. J. Thompson’s Atom
 Thompson passed electric current through gases in
a glass tube. The result was a glowing beam
flowing from the negative side to the positive side.

16. J. J. Thompson’s Atom
 Glowing ray (cathode ray) was attracted to a positive
magnet on the side of the tube.
 Beam must be negatively charged.

17. J. J. Thompson’s Atom
 Thompson
hypothesized that all
atoms had such
negative charges
within. (plum pudding
model)

18. Electrons and Protons
 Electrons – negatively
charged portion of an
atom
 Protons – positively
charged portion of an
atom

19. James Chadwick’s Atom
 Chadwick found that the mass
of an atom is greater than the
mass of protons plus
electrons.
 Neutrons – portion of an atom
with no charge
 Protons, neutrons, and
electrons are called subatomic
particles.

20. Ernest Rutherford’s Atom
 Born in New Zealand.
 Terrible at
mathematics.
 Diligent problem solver.
 First to discover the
power of the atom,
power to make bombs
that could “make this
old world vanish in
smoke.”

21. Ernest Rutherford’s Atom
 Shot radioactive particles
through a sheet of gold foil.
 Most went straight through,
but others were knocked off
course or bounced back.
 “Like shooting a fifteen-inch
shell at a sheet of paper
and having it bounce off.”

22. Ernest Rutherford’s Atom

23. Ernest Rutherford’s Atom
 Nucleus – compact
center of the atom
where protons and
neutrons are located
 Nucleus has very little
of the atom’s volume,
but contains most of
the atom’s mass.

24. Niels Bohr’s Atom
 Electrons orbit the nucleus in orbits, like a solar
system.
Planetary Model

25. Atomic Properties
How do atoms differ?

26. Atomic Number
 Atoms of different elements
have different structures.
 Elements differ because
their atoms contain different
numbers of protons!
 Atomic Number – number of
protons in the atom

27. Mass Number
 Electrons have negligible
mass compared to protons
and neutrons.
 Mass of a neutron is
approximately equal to
mass of a proton.
 Mass Number - number of
protons plus number of
neutrons in an atom.

28. Names and Symbols
 Gold – 118 neutrons
 Mass Number = ?
 Beryllium – 5 neutrons
 Mass Number – ?
 Sodium – 12 neutrons
 Mass Number – ?

29. Isotopes
 Atoms of the same
element with different
number of neutrons
 Chemically equivalent
 Most elements occur as
a mixture of isotopes

30. Isotopes of Hydrogen

31. Calculating Atomic Mass
 Mass of a proton is more
than 1800 times the mass
of an electron
 Spectrometer used to find
masses of atoms
 Masses of subatomic
particles too small to work
with, so a reference isotope
is used

32. Calculating Atomic Mass
 One amu (atomic mass
unit) is defined as 1/12
the mass of a Carbon-
12 atom
 1 amu is approx. equal
to the mass of a proton
or neutron

33. Calculating Atomic Mass
 Atomic masses found on periodic table
are averages of all isotopes of that
element
 Based on abundance (weighted mass)
To calculate the atomic
mass of an element, multiply
the mass of each isotope by
its natural abundance
(expressed as a decimal),
and then add the products.

34. Calculating Atomic Mass
 Chlorine is a mixture of Cl-35 and Cl-37.
 75.77% Cl-35 (35)
 24.23% Cl-37 (37)
 0.7577 x 35 amu = 26.50 amu
 0.2423 x 37 amu = 8.957 amu
Added together… 35.45 amu

35. Example #1
 Carbon-12 98.89%
 Carbon-13 1.11%
 Atomic Mass?

36. Example #2
 Silicon-28 92.23%
 Silicon-29 4.67%
 Silicon-30 3.10%
 Atomic Mass?

37. Self Quiz, Part 1
 What is an atom?
 What is the symbol for…
 Calcium?
 Phosphorus?
 Potassium?
 Copper?
 Silver?
 Tin?
 Mercury?

38. Self Quiz, Part 2
 What is the atomic number of…
 Lithium?
 Lead?
 Gold?
 Neon?
 Describe Dalton’s atomic model.
 Name the four premises of Dalton’s atomic theory.

39. Self Quiz, Part 3
 Explain the difference between atoms and
molecules.
 Describe Thompson’s experiments and his atomic
model.
 Describe Rutherford’s experiments and his atomic
model.
 What is an atomic number?

40. Self Quiz, Part 4
 Compare and contrast protons, neutrons, and
electrons.
 Chlorine-35?
 Mass Number?
 Number of Protons?
 Number of Electrons?