Download History of the Atom

September 30, 2014
History of the Atom
• We know today that all matter is composed of atoms.
• Atoms: basic unit of matter. Atoms are the smallest
particle of an element that retains its identity.
How did scientists figure out the structure
of the atom? Can you just "look" at it?
September 30, 2014
(Democritus)
• 460-370 B.C.
• He hypothesized that all matter is composed of tiny
indestructible units called atoms.
• Atoms and voids
• Atoms are solid.
• Different shapes = different properties.
• Not based on scientific method.
Atomos = indestructible
http://home.wlu.edu/~mahonj/Ancient_Philosophers/Atomists.htm
September 30, 2014
John Dalton
• 1766-1844
• English chemist and a schoolteacher
• Used the scientific method to study Democritus's
ideas
> Proposed his Atomic Theory
http://www.biography.com/people/johndalton-9265201
September 30, 2014
Dalton's Atomic Theory (1808)
1. All matter is made of indestructible particles called
atoms.
September 30, 2014
Dalton's Atomic Theory (1808)
2. All atoms of the same element are identical.
September 30, 2014
Dalton's Atomic Theory (1808)
3. Atoms cannot be created or destroyed in chemical
reactions: They just rearrange.
4. Law of definite proportions: In a chemical
reaction, atoms of one element combine with atoms
of other elements to form compounds in whole ratios.
+
September 30, 2014
J. J. Thomson
• 1856-1940
• English physicist
• Cathode ray tube experiment
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_03.JPG
September 30, 2014
J. J. Thomson
• Cathode rays are rays emitted when high voltage is
applied between two electrodes in an evacuated glass
tube.
• Observations:
> The cathode ray is deflected by a magnetic field.
> The cathode ray is repelled by a negative electric
field.
> The cathode ray is the same kind of negative
particle no matter what metal is used.
September 30, 2014
J. J. Thomson
• Conclusions:
> Cathode rays are streams of negatively charged
particles.
– Discovered first subatomic particle: electrons.
> Determined the charge to mass ratio (e/me)
JJ Thomson knew that an atom is neither
positively or negatively charged (neutral)
and contains electrons.
What model of the atom explains J. J.
Thomson's findings? How do you think
he conceptualized the atom?
September 30, 2014
William Thomson (Lord Kelvin)
• Plum pudding model (or chocolate chip ice cream)
> Uniform "pudding" of positive charge with
electrons scattered throughout.
> Total charge is neutral.
http://en.wikipedia.org/wiki/Christmas_pudding
http://en.wikipedia.org/wiki/Plum_pudding_model
September 30, 2014
Ernest Rutherford
• 1871-1937
• New Zealand physicist
• Gold foil experiment
http://staff.norman.k12.ok.us/~cyohn/index_files/atom1notes.htm
September 30, 2014
Ernest Rutherford
• Observations:
> Most of the alpha ( α) particles (positively charged
particles) passed straight through the foil.
> Some were deflected at slight angles, a few even
backwards.
Can this happen with the plum pudding model?
What is a better model for the atom?
September 30, 2014
Ernest Rutherford
• Conclusions:
> Atom is mostly empty space.
> Atom's mass is concentrated at the center
(nucleus)
> Nucleus has a positive charge-made of protons
http://myweb.usf.edu/~mhight/goldfoil.html
September 30, 2014
James Chadwick
Discovered neutrons--a particle in the nucleus with no
charge (neutral)
http://www.tvguide.com/tvshows/the-adventures-of-jimmy-neutron-boy-genius/photos/195915
September 30, 2014
Robert Milikan
• 1868-1953
• Oil drop experiment
> Suspended negatively charged oil droplets
between two charged plates.
> Calculated how strong an applied electric field has
to be to stop the drop from falling due to gravity.
September 30, 2014
Robert Milikan
• He noticed that the charge was always a multiple of
-1.60 x 10-19 Coulombs.
• Conclusions: The charge of an electron is -1.60 x 10-19
Coulombs.
September 30, 2014
We know much more now than Dalton did
when he first described his "Atomic theory".
Some of the laws still hold true:
• Law of Conservation of Mass-In a chemical
reaction, atoms (and therefore mass) are never lost or
gained, only rearranged.
• Law of Definite Proportions (constant
composition)-in a pure compound, the proportions of
elements by mass are always the same.
September 30, 2014
1 Which experiment showed that the atom has a dense,
positive region called the nucleus?
A Rutherford's Gold Foil Experiment
B Milikan's Oil Drop Experiment
C JJ Thomson's Cathode Ray Experiment
September 30, 2014
2 Milikan's Oil Drop Experiment allowed the calculation
of the charge of the electron
True
False
September 30, 2014
Parts of the Atom
http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle
September 30, 2014
Parts of the Atom
• Proton: positively charged particle found in the
nucleus (charge opposite of electron)
> Mass of 1.0073 amu (2000x bigger than electron)
> Atomic number: (Z) # of protons in nucleus,
identifies elements
• Neutron: neutral particle found in the nucleus
> Mass of 1.0087 amu (about the same as a proton)
> Number of neutrons determines isotopes
http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html
September 30, 2014
Parts of the Atom
• Electron: negatively charged particle that occupies a
3D region around the nucleus called orbitals
> Mass of 5.5 x 10-4 amu (1/2000 the mass of a
proton)
> Electrons are involved in chemical reactions.
http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html
September 30, 2014
Parts of the Atom
• The nucleus makes up a very small part of the atom's
volume.
• The electrons occupy a 3D regions of space called
orbitals that surround the nucleus.
http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle
September 30, 2014
Parts of the Atom
September 30, 2014
3 Which subatomic particle has a neutral charge and is
found in the nucleus?
A Electron
B Proton
C Neutron
September 30, 2014
4 Which subatomic particle is represented by the symbol p+
A Electron
B Proton
C Neutron
September 30, 2014
5 Which subatomic particle has the smallest mass?
A Electron
B Proton
C Neutron
September 30, 2014
Elements
• Atomic number (Z): Number of protons in their
nucleus. Defines elements.
http://chemistry.about.com/od/workedchemistryproblems/a/isotopes-nuclear-symbols-1.htm
Which element has 35 protons?
Which element is Z=12?
September 30, 2014
Ions
• Ions are atoms that have gained or lost electrons.
• # electrons ≠ # protons
> e- > p+ anion (negative charge) Xn> e- < p+ cation (positive charge) Xn+
http://www.gcsescience.com/a4-sodium-ion.htm
September 30, 2014
Example 1:
• Write the symbol for a sodium that has lost 1 electron.
• Write the symbol for a magnesium that has lost 2
electrons.
• Write the symbol for a oxygen that has gained 2
electrons.
• Write the symbol for a fluorine that has gained 1
electron.
September 30, 2014
6 Which symbol represents a chlorine that has gained
one electron?
A Cl
B ClC Cl+
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7 Which element has Z=47?
A Au
B Ag
C Ti
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Isotopes
• Isotopes: Atoms of the same element with different
number of neutrons
> Isotopes have the same # of protons, different #
of neutrons
• Example: Hydrogen had 3 naturally occurring isotopes.
http://wikis.lawrence.edu/display/CHEM/5.+Isotopes+and+Ions+%28Brittany+Oleson%29
September 30, 2014
Isotopes
• Atomic mass: total mass of an atom.
• Atomic mass unit (amu): unit to measure atomic
mass.
> 1 amu = 1.66 x 10-24g
> 1 amu = 1/12 the mass of a C-12 atom.
• Average atomic mass: weighted average mass of all
isotopes of an element that occurs in nature.
massavg = %1m1 + %2m2 + ...
Atomic number (Z)
http://intro.chem.okstate.edu/1215F99/Lecture/Lec101599.html
Average atomic mass
September 30, 2014
Example 2: Silicone
massavg = %1m1 + %2m2 + ...
mass number
exact mass % abundance
28
27.976927
92.23
29
28.976495
4.67
30
29.973770
3.10
September 30, 2014
Example 3: Carbon
massavg = %1m1 + %2m2 + ...
mass number
exact mass
% abundance
12
12.000000
98.90
13
13.003355
1.10
September 30, 2014
Example 4: Copper
massavg = %1m1 + %2m2 + ...
Copper has two naturally occuring isotopes.
Cu-63 has an atomic mass of 62.9296 amu and
an abundance of 69.15%. What is the atomic
mass of the second isotope?
September 30, 2014
Standard Nuclear Notation
• Atomic number (Z) = # of protons
• Mass number (A) = # of protons + # of neutrons
> Identifies isotopes
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.html
September 30, 2014
Example 5
Write the standard nuclear notation for the following:
• An atom with 8 protons and 9 neutrons.
• An atom with 15 protons and 15 neutrons.
• An atom with 34 protons and 38 neutrons.
September 30, 2014