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The atomic theory was originally proposed by the Greek philosopher Democritus in the
5th century B.C. He speculated that everything in the world is ultimately composed of combinations
of small, hard, and indivisible particles. He called the particles atoms (which in Greek
translates to "indivisible") and suggested that they came in various shapes and sizes,
but were all made of the same basic material.
The modern scientific theory of matter states that the great variety of substances we see
in the universe are made from combinations of different chemical elements; and these
elements do indeed consist of trillions of atoms, which themselves are made up of even
smaller particles.
To be clear, today the atomic theory is much more than just a theory; it's an undeniable
fact. Not only can we see individual atoms using electron microscopes, we can even trap
them and move them around using lasers. So when we talk about atomic theory, we don't
mean the theory that states that everything is made of atoms, but rather the theory that
describes how atoms behave and interact. (Quantum mechanics)
The internal structure of an atom is specific to each element and gives that element its
particular properties and characteristics. So for example, a hydrogen atom is constructed
differently than an atom of gold.
Modern atomic theory kicked off in the early 1800's with the work of English chemist, John
Dalton. The idea that matter is composed of tiny, indivisible particles was not generally
accepted until after his 1808 publication of "A New System of Chemical Philosophy."
In his paper he proposed that each chemical element is composed of atoms of a single,
unique type, and though they can't be altered or destroyed by chemical means, they can combine
to form more complex structures... aka chemical compounds... and this marked the first truly
scientific theory of the atom.
But it wasn't until 1905 that Albert Einstein came along and proved the existence of atoms
mathematically in his famous paper on Brownian motion. Up until that point, scientists were
unaware of the existence of an atom's nucleus, and thought of the atom as a web of positively
charged material with negatively charged electrons peppered throughout, like cherries in a cake.
This model of the atom was just used as a metaphor to describe what was thought of at
the time as a uniformly dense distribution of "stuff" within the atom.
However, a few years later this model was destroyed by Ernest Rutherford who was the
first person to actually look inside an atom. He was able to do this using an experiment
where he bombarded a thin sheet of gold foil with alpha particles (side note: an alpha
particle is just the nucleus of a helium atom). Most of the alpha particles went right on
through the foil without any kind of significant deflection, but a few bounced straight back.
Rutherford and his colleagues originally thought they would observe the alpha particles slowing
down as they passed through the foil, kind of like a bullet being fired through water,
but instead, what they saw was evidence that most of the mass of the nucleus was concentrated
in a very small part of the atom. On top of that, the nucleus must be positively charged,
since particles with a negative or neutral charge wouldn't deflect the positively charged
alpha particles. This observation is what led Rutherford to propose the atomic planetary
model in which a cloud of negatively charged electrons surrounded a small, compact nucleus with a
positive charge, which is why in most science classrooms you'll see a picture of an atom
that looks like this.
However, this kind of diagram is misleading because the size of the electrons, protons,
and neutrons, as well as the distance between them is severely disproportional. (Ok get
ready to have your mind blown) In reality, the nucleus has about one hundred-thousandth
the diameter of the whole atom. To put this into perspective, imagine that a hydrogen
atom is as large as a professional football stadium. The size of its nucleus would be
about as big as a grain of sand that's sitting at the middle of the 50 yard line. So as you
can see, the vast majority of an atom is made up of empty space; and since all matter, including
you, are made of atoms... you are mostly... nothing. I know right?
(Insert clip of Peter Griffin's head exploding)
Now in reality, empty space is mostly not empty. This is because electrons and photons
don't interact with quarks and gluon field fluctuations and other stuff that goes way
over my head, but if you're interested in learning more about this, Derek from the channel
Veritasium recently made an excellent video explaining this. So if you want to watch that
video, click right here.