hi in this video we'll cover 2.2 evolution of atomic theory if you wish pause now to read each learning objective as we saw in the previous video dalton's postulates were very important to the understanding of what matter is composed of he believed atoms were very small indivisible particles you're probably familiar with a representation of an atom that looks like this while this is not a perfect model it shows that atoms are actually composed of even smaller particles the subatomic particles what are these particles how do we know they exist in the late 1800s j.j thomson experimented with cathode ray tubes crt an early cathode ray tube invented in 1897 is shown here you've probably seen one of this before as crt tvs and computer monitors were very popular not too long ago a crt consists of a sealed glass tube from which almost all the air had been removed it contains two methyl electrodes cathode and an anode when a high voltage is applied across them a visible beam called a cathode ray appeared between them you can see the yellow line here this beam originates from the cathode and is accelerated past the anode towards the fluorescent scale at the end of the tube thompson used a variety of metals and in all cases the cathode ray deflected towards the positive charge and away from the negative charge with this thompson could calculate the charge of mass ratio of the cathode ray particles which is 1.759 times 10 to the 11th power coulombs per kilogram thompson was able to draw three very important conclusions the particles in this experience in his experiment were much lighter than any atoms they had negative charge since the deflection was always towards the positive side of the tube opposite charges attract each other remember and they were the same for all elements he tested cathode ray particles are none nowadays as electrons a negatively charged subatomic particle with a mass more than one thousand times less than that of an atom in 1909 milligan created microscopic oil droplets that could be electrically charged with x-rays so he would spray this very fine droplets and they would naturally fall due to gravity however malignant could slow them down or even reverse the direction by applying an electric field with this he was able to determine the charge of an individual drop actually he did that multiple times and he calculated that these charges were always multiples of 1.6 times 10 to the negative 19th coulomb which he concluded to be the charge of a single electron with the charge of an electron calculated by milligan right here and the charge mass ratio calculated by thomson before we can now calculate the mass of a single electron as being 9.107 times 10 to the negative 31st kilograms which is so light that we often consider it negligible in the beginning of the 1900s science had arrived at a very good knowledge about the negative subatomic particles the electrons and scientists knew that positive particles must exist as well since atoms are electrically neutral thomson suggested that atoms resemble plum pudding with electrons being the plums or raisins embedded in a positive mass that would be the mass of the cake an alternative model by japanese physicist nagaoka compared atoms with the planet setter with a ring of electrons surrounding a positive planet ernest rudeford and his colleagues performed an experiment with gold foil that helped elucidate the nucleus of the atom a beam of alpha particles from a radium source right here on this end which are positive in nature were fired in the direction of a very thin piece of gold foil coat a luminescent screen which glows when hit was used to detect where those alpha particles went the results of these experiments were the most that most of the particles passed straight through the the foil a few were slightly deflected and a very small number of particles were significantly deflected like this why was that happening the alpha particles that were significantly deflected were those that collided directly with the positively charged gold nucleus the ones that slightly deflected passed very close and were repelled by the positive nucleus remember the alpha particles are also positive since very few alpha particles deflected it was deduced that the nucleus is actually very small compared to the size of an atom most particles pass through the relatively large empty region occupied by electrons which have very little mass as we saw based on these observations root 4 concluded that most of the atom is empty space where the electrons are located most of the mass of the atom is located in the nucleus which has a positive charge electrons are located around the nucleus and finally root of four eventually concluded that there is a positively charged subatomic particle that he called proton which is part of the nucleus although some additions were made later we still use this atomic model nowadays in the early 1900s scientists obtained what they thought were new elements by isolating substances from radioactive wars for example they named mesothorium1 an element produced from the radioactive decay of thorium however further analysis proved that this was chemically identical to the non-element radium except for having a different atomic mass nowadays we know 33 isotopes of radium with the most common having an atomic mass of 226 isotopes are therefore defined as atoms of the same element with different mass frederick sodi of england won the nobel prize in 1921 for this work at this point in time scientists knew that the nucleus concentrated most of the mass of the atom however only about half of that mass was accounted for by the protons there were proposals to explain the existence of neutral particles in the nucleus as well however it wasn't easy to detect uncharged species only in 1932 less than a century ago james chadwick was able to prove that neutrons are thus uncharged subatomic particles with a mass very similar to that of protons and that are also located in the nucleus this helps us understand the existence of isotopes while the number of protons in the nucleus gives an atoms its chemical identity different atoms of the same element can differ in the number of neutrons it contains