Download Atomic structure

Integrated Science/ Mr. Forrest
Name: ____________________________
Notes on the History of the Atom / Atomic Structure
a good video on this topic is at: http://www.youtube.com/watch?v=h6LPAwAmnCQ
Models: Remember that ALL models fall short of reality, but that models are used to help
us understand what reality is.
Idea of the atom can be traced back to Greece (~ 400 B.C.)
Greek philosopher Democritus thought that atoms existed.
experimental evidence for his theories.
However, he had no
Aristotle disagreed with Democritus. Since Aristotle was much more respected and better
known, people agreed with him until the experimental evidence was too much to ignore.
However, this resulted in our ideas about the atom being wrong for almost ________
years!
Law of Conservation of Matter (1770’s)
Discovery of this idea is credited to _______________________
Matter cannot be created or destroyed in a chemical reaction.
Mass of starting materials (reactants) equals the mass of the products.
EXAMPLE :
C
Carbon atom
(12 mass units)
+
O
Oxygen molecule
(32 mass units)
--------- >
CO2
Carbon Dioxide molecule
(44 mass units)
Law of Definite Proportions (1797)
Discovered by Joseph Proust
A compound contains the same elements in exactly the same proportions (by mass)
no matter how much or how little of the compound you have.
These proportions are in _____________________; for example every water molecule has
two hydrogen atoms for each oxygen atom (H2O). You would not say that there is
1 hydrogen for each 1/2 oxygen.
So when elements combine during chemical reactions, they do so in small, whole number
ratios.
Dalton’s Atomic Theory (1803)
1. All matter is composed of atoms.
2. Atoms are indestructible and indivisible (cannot be divided into smaller particles).
3. All atoms of the same element are identical, but atoms of one element are different
from atoms of another element.
In other words, all oxygen atoms look exactly the same. Also, atoms of sodium do not look the same as atoms of hydrogen.
Dalton also built on Proust’s work and said that compounds are composed of two or more
different elements in a specific fixed ratio.
Dalton’s model of the atom was the basis for all future models. Although his ideas have
been modified and refined, they were the basis for all modern atomic theory. In particular,
only one of Dalton’s ideas is still accepted as completely true today, the idea that ______
__________________________________ .
Today we know for certain that atoms are made up of smaller particles. We also
know that atoms of the same element can be slightly different.
Model of an atom (Dalton’s Theory): Draw this in the space below
In the 1800’s there was evidence that atoms were made up of smaller particles. Cathode
Ray Tubes, like older TV’s (not flat screens) showed that there was a flow of particles that
went from the cathode (negatively charged electrode) to the anode (positively charged
electrode. Therefore, based on our modern day understanding of electricity, you should be
able to guess that these ‘Cathode Rays’ must be ______________ charged.
Discovery of the Electron (1897)
J.J. Thomson’s work with cathode rays led to the discovery of the electron. Thomson
found that particles in these cathode rays could be deflected by a magnetic field.
Cathode rays were found to be streams of negatively charged electrons. The mass of an
electron was determined to be about 1/2000 the mass of the smallest atom. Since all atoms
are neutral, these two facts should lead you to an idea, which you should record below:
_____________________________________________________________________
J.J. Thomson’s “Plum Pudding” Model of the Atom: Draw this in the space below
Rutherford’s Gold Foil Experiment (1909)
Experimental Apparatus :
Stream of positively
charged particles
Gold
foil
Rutherford fired positively charged (alpha) particles at very thin gold foil.
Rutherford’s Experimental Results:
1/18,000 trials showed a particle that bounced back
8/18,000 trials showed particles that were deflected
Since positively charged particles ______________ other positively charged particles,
there would not have been any particles bounce back if the “plum pudding” model were
true. Therefore, all the positive charge had to be together at the center of the atom. Since
so few particles bounced back, the nucleus of the atom (as Rutherford called it) must be
_____________ compared to the size of the entire atom. {In fact the radius of the atoms is
about ____________ times bigger than the nucleus. Rutherford’s team called the
positively charged particles protons.
The mass of a proton was found to be just slightly less than the mass of a hydrogen atom
(the simplest atom).
Rutherford’s Model of the atom : Draw this in the space below
PROBLEM: The mass of all atoms (except hydrogen) was quite a bit less than the
combined mass of protons and electrons. Therefore there had to be a ___________
subatomic particle. Since protons and electrons balanced out the charge of atoms, this
third subatomic particle had to be electrically _______________. It wasn’t until the
1930’s that the existence of the neutron was confirmed. Neutrons exist in the nucleus
along with protons. The neutron has about the same mass as a proton.
Note that it took much longer to discover neutrons than electrons …. Why???
_____________________________________________________________________
_____________________________________________________________________
In 1913 Neils Bohr realized that Rutherford’s model of the atom would have electrons
collapse to the nucleus. Therefore he proposed a ‘planetary model’ that allowed electrons
to only exist in certain stable energy levels. This (combined with the discovery of the
neutron) allowed us to have a much better model of the atom.
Bohr’s Model of the atom (revised to show neutrons): Draw this in the space below
Realize that the Bohr model, like all models still falls short of reality. Although it was
accepted for some time, the Bohr model of the atom has been revised to incorporate the
ideas of Quantum Mechanics, which were developed in the 1930’s and 1940’s. For right
now, we will think of Bohr’s planetary model as being adequate for our understanding, but
you'll learn a lot more if you take chemistry!
FINAL THOUGHTS TO REMEMBER!
All atoms are electrically neutral. This means that neutral atoms have the same number of
electrons as _________________.
Atomic Number – This is equal to the number of protons in an atom. All atoms of each
element always have atoms with the same number of protons.
Mass number – The sum of the number of protons and neutrons in a nucleus for a
particular atom.
Isotopes - Many elements have atoms with different atomic masses, yet they must have
the same number of protons. Atoms of an element that are chemically alike, but have
different masses must have different numbers of neutrons. These different versions of the
same atom are called isotopes. In fact, the existence of isotopes is one of the main things
that led to the discovery of the neutron!
Ions – These are individual atoms (or groups of combined atoms) that have a positive or
negative charge. We’ll discuss these in greater detail later, but based on the Bohr model of
the atom, you should be able to recognize that ions form when atoms gain or lose which
kind of subatomic particle? ____________________.
Why should this make sense? ____________________________________________
Good video reference for history of atomic structure: http://www.youtube.com/watch?v=NSAgLvKOPLQ