ATOMIC THEORY TIMELINE
As a science, the Atomic Theory is all relatively new. If you think about the laws of
motion, that goes back to the BCE times, and then it really got rolling with Galileo and
then Newton in the 1600s. While Democritus was around in Ancient Greece, Dalton
proposed the Atomic Theory in 1803, and the later research is all, relatively speaking,
somewhat “recent.”
After Dalton’s proposal that all matter was made out of atoms and that individual
atoms were different, the focus shifted from the big picture to the really, really small –
that is, what is an atom like? There were three important players involved with figuring
this out. The first is:
J.J. (Joseph John) Thomson – 1897. Throughout all the time after Dalton, the
idea of what the atom looked like was that it was a ball – small, and relatively solid, and
with a positive charge. Thomson’s experiments challenged that idea, and he found that
atoms had smaller “bits” of negatively charged material in them. Thomson kept the idea
that atoms were small solid “chunks,” but said that they had these “electrons” in them –
he compared them to plum pudding or raisin bread, or, as we can compare them –
chocolate chip cookie dough. The dough is the positively charged material, Thomson
reasoned, and the “bits” – in this case the chocolate chips – are the negative “bits,” that is,
the electrons. The science was moving quickly, and this idea only held for a little while,
in fact up to:
Ernest Rutherford – 1908 – A former student of Thomson’s performed an
experiment known as the GOLD FOIL EXPERIEMENT that proved Thomson’s ideas
wrong. Rutherford used thin gold foil, which is made up of gold atoms. He fired alpha
particles (very small particles) at the gold foil. He saw that some hit “something,” some
bounced off at an angle, and some bounced directly back.
This experiment suggested that the Gold Foil had “holes” in it – “holes” big
enough for an alpha particle to get through, while a small number ricocheted.
In 1911, Rutherford suggested a new idea about atoms, one that said atoms have
a small dense nucleus, but Thomson’s electrons were far away from it. Therefore, an
atom is mostly empty space. The positively charged particles were called protons, and
they were in the middle, and the electrons orbited it, or circled it like planets around the
sun.
Again, the science was moving REALLY fast, so Rutherford’s ideas only stood
for two years before being tweaked by…
Niels Bohr – 1913 - Danish physicist who through experiments with light and
light given off by heating matter, proposed that the electrons that Thomson discovered,
and Rutherford put outside the nucleus OCCUPY SPECIFIC ENERGY LEVELS.
That is, they circle the nucleus, but only in specific spots, not all over. Each of these
shells is an exclusive club, and only permits a select number of electrons in. The
electrons can visit other levels, but have to remain, most of the time, in their “home”
levels.
Bohr says that the first orbit, or SHELL of electrons can only hold 2 electrons. In
fact, it follows a pretty strict rule: 2n2, where “n” is the level number. Bohr also realized
that electrons were responsible for the chemical properties that elements showed.
Like Rutherford before him, Bohr’s ideas held for a while before they were
challenged and modified. This modification had to do with the speed of the electrons and
PROBABILITY.
Electron Cloud Model – the most current view of the atom. Says that it’s
impossible to predict where an electron will be at any specific time, and the best we can
do is show a “cloud” of possible locations it will be at any point in time – because it’s
moving so fast. The electrons, if they could be visualized, make a “haze” around the
nucleus.
The Atomic Theory through time.
Theory – collection of data and data that all points in one specific direction, and
leads to a certain conclusion. NOT A GUESS. In science a “theory” means that all the
evidence to date supports the idea, but there hasn’t been, or can’t be direct observation of
the thing, the phenomena.
Why hasn’t there been direct observation to change this from the Atomic Theory to “THE
LAW OF THE ATOM?”
Practical Science – research of atoms
Applied Science – application of what was being learned. Was happening all along with
the development of the Atomic Theory.