Download History of the Atom

History of the Atom
Imagine if you were a scientist in the time of the
Greek philosophers. How do you see the world? What
are we made out of? If we could look closely, what
would we see?
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.
(Democritus)
http://home.wlu.edu/~mahonj/Ancient_Philosophers/Atomists.htm
Atomos = indestructible
• 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.
John Dalton
• 1766-1844
• English chemist and a
schoolteacher
• Used the scientific method to
study Democritus's ideas
> Studied the ratios in
which elements combine
in chemical reactions.
> A compound always
contains the same ratio
of elements (by mass).
http://www.biography.com/people/johndalton-9265201
Dalton's Atomic Theory (1808)
1. All matter is made of indestructible particles called
atoms.
Dalton's Atomic Theory (1808)
1. All matter is made of indestructible particles called
atoms.
2. All atoms of the same element are identical.
Dalton's Atomic Theory (1808)
1. All matter is made of indestructible particles called
atoms.
2. All atoms of the same element are identical.
3. Atoms cannot be created or destroyed in chemical
reactions: They just rearrange.
4. In a chemical reaction, atoms of one element
combine with atoms of other elements to form
compounds in whole ratios* Law of definite
proportions
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
J. J. Thomson
Cathode rays are rays emitted when high voltage is
applied between two electrodes in an evacuated glass
tube.
Observations:
1. The cathode ray is deflected by a magnetic field.
2. The cathode ray is repelled by a negative electric
field.
3. The cathode ray is the same kind of negative particle
no matter what metal is used.
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.
J. J. Thomson
J. J. Thomson discovered the electron, which has a
negative charge.
He knew that an atom is neither positively or
negatively charged.
Please draw a diagram of an atom that explains J. J.
Thomson's findings.
William Thomson (Lord Kelvin)
Plum pudding model:
Uniform "pudding" of positive charge with electrons
scattered throughout.
Total charge is neutral.
http://en.wikipedia.org/wiki/Plum_pudding_model
http://en.wikipedia.org/wiki/Christmas_pudding
Ernest Rutherford
• 1871-1937
• New Zealand physicist
• Gold foil experiment
http://staff.norman.k12.ok.us/~cyohn/index_files/atom1notes.htm
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?
*Please draw a model for the atom that incorporates Rutherford
Ernest Rutherford
Conclusions:
1. Atom is mostly empty
space.
2. Atom's mass is
concentrated at the center
(nucleus)
3. Nucleus has a positive
charge-made of protons
http://myweb.usf.edu/~mhight/goldfoil.html
James Chadwick
Discovered neutrons--a particle in the nucleus with no
charge (neutral)
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.
Robert Milikan
He noticed that the charge was always a multiple of
-1.60 x 10-19 Coulombs.
Conclusions:
1. The charge of an electron is -1.60 x 10-19 Coulombs.
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.
Parts of the Atom
http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle
Parts of the Atom
Particle
Location
Relative Charge
Mass (amu’s)
Symbol
Proton
Nucleus
+ 1
1.0073
p+ or 11H
Neutron
Nucleus
0
1.0087
-­‐1
.00055
Orbital
Electron
n or n0
1
0
0
e or e-
-­‐1
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
Elements
Elements are defined by the number of protons in
their nucleus = atomic number (Z)
Each element has a symbol (look at periodic
table). For Example, carbon is C and Neon is
Ne.
Isotopes = Atoms of the same element with different
number of neutrons
Example: Hydrogen had 3 naturally occurring isotopes.
http://wikis.lawrence.edu/display/CHEM/5.+Isotopes+and+Ions+%28Brittany+Oleson%29
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
Standard Nuclear Notation:
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.html
atomic number (Z) = # of protons
mass number (A) = # of protons + # of neutrons
-Identifies isotopes
Practice: How do you write a Zinc isotope with 35
neutrons in standard nuclear notation?
Atomic number (Z)
http://intro.chem.okstate.edu/1215F99/Lecture/Lec101599.html
Average atomic mass
Atomic mass = total mass of an atom.
Average atomic mass = weighted average mass of all
isotopes of an element that occurs in nature.
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.