October 24, 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?
October 24, 2014
Democritus and John Dalton
• Democritus: Greek philosopher, believed matter was
made of indestructible particles.
• John Dalton: Used the scientific method to test
Democritus's ideas and came up with the atomic
theory.
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October 24, 2014
J. J. Thomson
• Cathode ray tube experiment
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_03.JPG
October 24, 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.
October 24, 2014
J. J. Thomson
• Conclusions:
> Cathode rays are streams of negatively charged
particles.
– Discovered first subatomic particle: electrons.
JJ Thomson knew that an atom is
neutrally charged and contains electrons.
What model of the atom explains J. J.
Thomson's findings?
October 24, 2014
Plum pudding model
• 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
October 24, 2014
Ernest Rutherford
• Gold foil experiment
http://staff.norman.k12.ok.us/~cyohn/index_files/atom1notes.htm
October 24, 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?
October 24, 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
October 24, 2014
Parts of the Atom
http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle
October 24, 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
October 24, 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
October 24, 2014
Parts of the Atom
• The nucleus makes up a very small part of the atom's
volume and contains the protons and neutrons.
• 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
October 24, 2014
Electron Orbitals and Energy Level
• Quantum Mechanical Model of the Atom
> Describes electrons as waves
> Electrons are found in 3-D atomic orbitals.
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October 24, 2014
Electron Orbitals and Energy Levels
• Principle energy level (shell) (n) indicates the
relative size and energy of the various atomic orbitals.
> n= integers: n=1, 2, 3...
> As n gets bigger
– the orbitals (shell) gets larger
– the electrons spends more time away from the
nucleus
– the electrons are less tightly bound to nucleus
http://www.chem.ox.ac.uk/vrchemistry/Machinery/html/page02.htm
October 24, 2014
Electron Orbitals and Energy Level
• Each shell can hold a maximum of 2n2 electrons.
> n=1 can hold 2 electrons
> n=2 can hold 8 electrons
> n=3 can hold 18 electrons
> n=4 can hold 32 electrons
• Valence electrons: electrons in outermost shell,
involved in forming bonds.
2
middle school?
32
18
8
October 24, 2014
Parts of the Atom
October 24, 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?
October 24, 2014
Isotopes
• Isotopes: Atoms of the same element with different
number of neutrons
> Isotopes have the same # of protons, different #
of neutrons
• Example: Hydrogen has 3 naturally occurring isotopes.
http://wikis.lawrence.edu/display/CHEM/5.+Isotopes+and+Ions+%28Brittany+Oleson%29
October 24, 2014
Isotopes
• Isotopes have different masses
• 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.
October 24, 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
October 24, 2014
Example
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.
October 24, 2014