professor Dave here, I want to tell you the history of atomic theory. these days 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 9th hydrogen and 8 9th 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 combined 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 units of matter that Democritus envisioned. J.J. Thompson showed that the 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 that almost all of them passed right through to the detector on the other side, but around one in 8,000 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 bounced 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% 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 arrived 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 professordaveexplains at gmail.com