Which experiment and subsequent observation led to the discovery that atoms contain negatively charged particles, now known as electrons? (LO 2.10–2.12) (a) Oil is sprayed into a chamber and the speed at which the oil droplets fall is measured with and without an applied voltage. X rays in the chamber knock electrons out of air molecules. The electrons stick to the oil pro-ducing an overall negative charge on the drops. Adjusting the voltage changes the speed at which the negatively charged oil droplets fall. (b) When a high voltage is applied across metal electrodes at opposite ends of a sealed glass tube, a cathode ray is produced. The cathode ray is repelled by a negatively charged plate. (c) A radioactive substance emits alpha particles, which are directed at a thin gold foil. Most of the alpha par-ticles pass through the foil, but a few alpha particles are slightly deflected and some even bounce back toward the radioactive source. (d) The mass of different elements in a pure chemical com-pound are measured. Different samples of the compound always contains the same proportion of elements by mass.

Video transcript

Welcome back everyone. What experiment revealed that Adams possess electrons which we recall are negatively charged particles in their structure. So, looking at statement A. It says that a negatively charged plate repels a cathode ray, which is generated by placing a high voltage across metal electrodes at opposite ends of a sealed glass tube. We're going to recognize that statement A. Is describing Thompson's cathode ray tube. And recall that Thompson's cathode ray tube led to the discovery of electrons, which is why it makes sense. That statement A. Says that the negatively charged plate repels the cathode ray. We want to recall that our cathode ray has a positive charge. And since this negatively charged plate repelled the cathode ray, which is positively charged. This meant that this negatively charged plate consisted of atoms that were also negatively charged. Which we can relate to our prompt being negatively charged particles which are describing our electrons. So right now we can think of a as a good choice for our final answer. But let's go ahead and keep reading the next statement statement B says that the speed of falling oil droplets sprayed into a chamber was measured with and without voltage using X rays, the oil drops were given a negative charge and suspended between electrically charged plates. The speed of the oil droplets dropping changes. Or sorry, the speed of the oil droplets dropping changes as the voltage varies. And so what statement B is describing is what we would recall as our Malik UN's oil drop experiment. And we should recall that based on this description of this experiment. This did lead to the discovery of specifically the electron charge. So it allowed this scientist robert Millikan to discover the magnitude of charge of a single electron. Now our prompt does not go that into depth as far as are negatively charged particles. So it doesn't cover discovering the magnitude of how negative these charged particles are. So we would rule out choice B so far because it didn't specifically just lead to the discovery of simple electrons being negatively charged. It specifically led to discovering the magnitude of that negative charge. So moving onto statement, see we have alpha particles are emitted by a radioactive substance. They are directed at gold foil where the majority of them passed through and a few are significantly deflected and some bounced back toward the radioactive source. Now we want to recall that from this experiment, alpha particles have a positive charge. And according to statement, see if a majority of these alpha particles passed through the gold foil that they were aimed at where a few are being significantly deflected. So let's draw this out. We have our positively charged alpha particles. They're being aimed at our gold foil here where some of these alpha particles are passing through. Some are being deflected back to our radioactive source here Now because as the prompt states some of these alpha particles which we know are positively charged are deflected from the foil where a lot of them actually were able to pass through the foil. This allowed us to determine that an atom, particularly the gold atom of the foil in this case, so an atom is mostly empty space. Hence most of our alpha particles passing through surrounding a small nucleus. And if we recall our nucleus contains are positively charged protons of an atom but is surrounded by electrons on the outside, which is why these alpha particles were getting deflected back to the radioactive source. Now this is not describing are negatively charged particles. This is describing positively charged protons. So we're going to rule out also statement C. Meaning that the only correct experiment that revealed that Adams possess electrons which are negatively charged particles in their structure is going to be statement A which described our Thompson's cathode ray tube experiment. So this is going to be our final answer. To complete this example. I hope everything I reviewed was clear. If you have any questions, leave them down below and I'll see everyone in the next practice video.