Professor Dave here, I wanna tell you the
history of atomic theory. Most people are somewhat aware of what
an atom is, and accept that they make up the things around us. But for a long time atomic theory was
controversial, and to understand chemistry is to understand the process by which
these particles and their components came to be accepted knowledge. Democritus, a Greek philosopher, was the first to
propose that matter is comprised of fundamental indivisible particles, which he called "atomos". Though this marks the first
time matter was viewed in this way, it was more of a philosophy than real
science, and no supporting evidence was gathered for a long time until John Dalton came along and provided the first empirical evidence that atoms exist. He proposed that compounds consist of different types of atoms in whole number ratios, and that chemical
reactions involve the rearrangement of those atoms to give new combinations. This idea explained certain observations. For example, when a sample of water was split into
hydrogen and oxygen the result was always 1/9 hydrogen and 8/9 oxygen by mass. With atomic theory this makes perfect sense, if we
understand that water is made up of tiny identical water molecules that all have
the same exact number of hydrogen and oxygen atoms. Then no matter how much water there is it will always have the same ratio of hydrogen to oxygen by number of atoms, and therefore also by mass. It also explained the way certain elements
combine to form different compounds. If carbon reacts completely with either a certain mass of oxygen or precisely double that mass of oxygen, that probably means every single carbon
atom is bonding to either exactly one or exactly two oxygen atoms. This was the
first solid evidence that atoms exist. However, we soon discovered
that Dalton's atoms were not the fundamental, indivisible unit of matter that
Democritus envisioned. JJ Thomson showed that an atom must consist of smaller particles when he discovered the electron. In his cathode ray experiment, particles
ejected from a neutral material were shown to have a negative electrical
charge since they were attracted to a positively charged metal plate. This means atoms must have within them
positively charged and negatively charged particles, but he
did not know how they were distributed so he proposed his plum pudding model.
Just a bit later Ernest Rutherford took it further with
his gold foil experiment. He fired positively charged alpha particles which are very tiny, at a thin foil of gold. He was shocked to find almost all of
them passed right through to the detector on the other side, but around one in eight thousand scattered wildly and with high energy in
random directions. He himself said that it was as though
you fired a shotgun at a piece of tissue paper and the shell bounce back and hit you.
This suggested that the positive charges in the atom are highly condensed in the
center of the atom which was given the name "nucleus", and
that the negatively charged electrons were far away, making the atom 99.95 percent empty space. That's why alpha particles
typically pass right through, but if they are just perfectly aligned
to strike a nucleus the huge repulsion between the positive
charges makes them fly violently in a random direction. So we arrive at a model that looks like this which should be a familiar image. We aren't
done revising our view of atomic structure quite yet, but the next chapter will have to wait
for a little later in your understanding. However certainly we've shown that atoms exist and have proven a thing or two about
their structure, which allowed for the field of chemistry to truly begin to flourish. Thanks for watching, guys. Subscribe to my channel for more
tutorials, and as always feel free to email me: