in 183 the brilliant scientist John Dalton was able to prove in the lab what the ancient Greeks thousands of years earlier thought might be true that matter was made of of incredibly tiny building blocks called atoms Dalton thought that atoms were solid spheres like billiard balls and that they were the smallest particles in the universe but about 100 years later scientists discovered that atoms were made of even smaller particles this video shows how they were able to do that in 1897 a clever Englishman called JJ Thompson designed a glass tube filled with low pressure gas and containing two metal plates inside when he put a big electric charge on the plates he found that the positively charged plate called the anode was able to pull a beam of tiny particles out of the negatively charged plate called the Cath cathode because the beam came from the cathode he called them cathode rays and he called the apparatus a cathod ray tube you can see the beam as a blue line in the diagram where the cathode rays arrived at the anode there was a little hole that allowed them to travel through and continue and pass between a pair of parallel plates that he could put varying charges on sometimes the top one was positive with the bottom one negative and sometimes the other way round the beam always bent away from whichever plate was negative and towards whichever plate was positive this meant that the particles must have carried a negative charge because we know that like charges repel and unlike charges attract but here's the thing because the beams bent so easily more than other particles going through the plates he could work out that they must have been very light thousands of times lighter than a hydrogen atom Thompson realized that atoms had even smaller particles inside them and that They Carried a negative charge he had discovered the electron he won a Nobel Prize for this JJ Thompson was not only a brilliant scientist he was a really good bloke as well and he helped many of his students become famous scientists too seven of them went on to win the Nobel Prize including his own son one of his students was nnest Rutherford an extroverted New Zealander with a booming voice and a big laugh Rutherford was playing around with beams of positively charged particles which he called alpha particles that were Ed from uranium atoms he knew they were positively charged because they bent in the opposite direction to the electrons of a cathode ray when passed through charge parallel plates they bent more slowly than electrons so he knew they were heavier than electrons too Rutherford used these alpha particles as atomic bullets and fired them at a very thin sheet of gold that was only about 1,000 atoms thick on the opposite side to the uranium Source he placed a scintillation screen that lit up when particles hit it so he could then measure where the alpha particles landed if the gold atoms were like billiard balls then the alpha particles would be blocked and none would arrive at the screen instead almost all the alpha particles passed straight through the gold foil as though it wasn't there Rutherford realized that the gold atoms were mostly empty space JJ Thompson thought that the mass of atoms was pretty evenly smeared out within the substance and he also thought that most of the alpha particles would pass through but something very surprising happened some of the particles were bent a few degrees off of course and about one alpha particle in 20,000 bounced right back in the direction from where it came Rutherford said at the time that it was almost as incredible as if you fired a 15in shell at a piece of tissue paper and it came back and hit you what this meant was that the mass of the gold was not evenly spread out at all but concentrated in very small lumps that were positively charged he had discovered the atom's nucleus because the nucleus was so small most alpha particles didn't even come close and completely miss them they went straight through if some went close to the nucleus its positive charge would cause repulsion and bend them off course if an alpha particle made an occasional Direct Hit It would bounce right back from where it came because the nucleus was much much heavier than the alpha particle itself the nucleus is incredibly heavy over 99% of the atom's weight is concentrated there and Incredibly small if we magnified an atom to be the size of a football stadium the nucleus would be the size of a p Rutherford later discovered a particle inside the nucleus that was responsible for its positive charge and he called it the proton an atom's overall nuclear charge was equal to the number of protons it had in its nucleus and this also turned out to be the same as the atom's atomic number each element had its own number of protons in its nucleus how's that for a coincidence but there was puzzling evidence that the nucleus was heavier than could be accounted for by protons alone scientists could measure the weight of protons and they knew how many protons were in the nucleus of each kind of atom but when they measured the overall mass of these nuclei they were usually about double the mass of all the proton put together was there another particle in the nucleus in 1932 James Chadwick a student of rutherfords discovered a beam of new particles with about the same mass as protons he did this by shooting alpha particles into burum atoms and knocking these particles out of the burum nuclei the burum target was much thicker than Rutherford's gold foil so there were many more nuclei for the alpha particles to make direct hits with Bam's nuclei has an atomic number of four so he knew that they had four protons each but he was experimenting to find a new particle in the nucleus when these particles were struck by the alpha particle bullets they were knocked out of the nucleus and then continued in a beam that Chadwick could study this beam did not Bend when passed through charge parallel plates like the electrons of Thompson's experiment did remember it was the electron negative charge that made them bend when going through the plates this meant that the newly discovered particles had no charge because these particles were electrically neutral he named them neutrons Chadwick had an eventful life during World War I he was a prisoner of war in Europe and he even did a lot of his experiments while he was a prisoner then during World War II he helped invent the atomic bomb in America by now scientists had a fuller picture of how atoms were made they had a nucleus consisting of positively charged protons and neutral neutrons at their Center and this was responsible for most of their Mass light negatively charged electrons whizzed around the outside of the nuclei in facted about 3/4 of the speed of light and gave atoms most of their size the question of how atoms could stick together was also answered having an opposite charge to the nucleus electrons are attracted to it so the electric forces can hold electrons traveling near the speed of light and bend them into an orbit around the nucleus electric forces act as a kind of atomic glue holding atoms together they are quadrillions of quadrillions of times stronger than gravity for those of you who have leared about the powers of 10 way of writing numbers electric forces are 10 to the^ of 33 times more powerful than gravity that's one with 33 Zer in the next video called atomic structure we will look at how electrons are wrapped around the nucleus in shells and how this dictat the properties of atoms for example whether they will be a gas liquid or solid or their color or brittleness or whether they will be a metal or a non-metal pretty much everything down