Video: Development of the atomic model

﻿Hi, my name's Dr. Patel, and welcome to the seventh lesson in this topic of atomic structure and periodic table. In today's lesson, we're going to be learning all about the history of the atomic model, because believe it or not, it wasn't always the same as the one used today. Before we start, I want you to do a few things for me. Firstly, make sure you're sitting in a nice, quiet space free of any distractions. So you might need to move your mobile phone away from you if you're not using it to access today's lesson, or you might need to close down any extra computer tabs. Great. Second thing is, can you write today's date and title at the top of your page? Today's title is the development of the atomic model. So in today's lesson, you're going to need a few pieces of equipment. The equipment is listed on the screen. You may find it useful to have a copy of the periodic table, although we're not going to be using it massively in today's lesson. This can be found in the downloadable resource section of today's lesson, if you want to download it. So in today's lesson, we're going to start off by looking at the history of the atomic model. Hopefully by the end of today's lesson, you'll have a better understanding of the correct order that each subatomic particle was discovered, and you should be able to compare a couple of different models. So we're going to start off today's lesson with a warmup quiz. Pause the video now and answer these three questions. They're linked to knowledge that you should have covered previously in this topic of atomic structure. Pause the video now. So here's what you should have got. So hopefully you remembered some of these key facts. Subatomic particles are the particles that are found inside an atom. And those are protons, neutrons and electrons. We're going to be talking a lot about each individual particle today, so it's really important that you know some of these key facts about them. Electrons are negative and protons are positive. Electrons are found on energy shells and protons and neutrons are found in the very centre of the atom, inside the nucleus. Throughout today's lesson, we're going to be referring a lot to this slide. These images can be found in the downloadable resource section of this lesson. Pause the video now if you need to download it and you haven't already done so. So believe it or not, the structure of the atom has not always looked the same as it does today. There's been lots of scientists that have contributed in developing the model that we use today. It started off with a really, really simple model. And as we've learned more about subatomic particles and the basic structure, more and more detail has been added and the models have been refined. I'm going to talk you through each individual model. So let's start off with the first model. So there were two scientists that were mainly responsible for hypothesising the existence of atoms. Back in 400 BC, Democritus suggested that everything was composed of atoms and that these atoms were indivisible. So atomos just means that they were indivisible. What does atomos mean? Good. Say it again. Excellent. So Democritus said that everything was made up of atoms and they couldn't be divided into anything smaller, and he also suggested that they were tiny spheres. in the 1800s, John Dalton also suggested that atoms were tiny spheres. He also agreed with Democritus and said that atoms couldn't be divided into anything smaller. However, he also developed this model a little bit further and he suggested that each sphere was a different size for different elements. So for example, hydrogen would have a different size sphere or different sized atom compared to chlorine. If you need to pause the video now and make notes on these two scientists' contribution to the very first ideas of an atom, do so now. Okay, so let's go into what happened next, because the next model that came along was the plum pudding model. And you can already see that there's been more detail added to it. So why exactly is the plum pudding model known as the plum pudding model? Well, let's look at the diagram. So as you can see from the diagram, J. J. Thomson suggested that the atom was a ball of positive charge and that it had negative charge scattered all around that ball of positive charge, and those negative charges were randomly distributed. What do you think the negative charge in the atom represents? Good. Electrons. So the electrons, the negatively charged electrons were scattered randomly around the ball of positive charge. So how does that link to the plum pudding model? Well, the raisins in the plum pudding represent the electrons that are randomly distributed around the ball of positive charge, and the cake part of the plum pudding model represented the ball of positive charge. If you need to take a note of this diagram, please do so and label it. Okay, so let's see how much you remember about the first two models. So do you remember what the word atomos means? Good if you said indivisible. It just means it can't be divided into anything smaller. So even if you didn't remember the key word and you just said it can't be divided into anything smaller, that's fine, but it means indivisible. So which two scientists said that atoms were tiny spheres? Well done if you remembered Democritus and John Dalton. So they're the two scientists that very first came up with the idea of an atom. Who suggested the plum pudding model, do you remember? Good if you said J. J. Thomson. He was responsible for developing the plum pudding model. Okay, so in the plum pudding model, which subatomic particle do the plums represent? Which subatomic particle do the plums represent? Good. It's the electrons. Remember, the electrons were randomly distributed around the atom. Okay, true or false, in the plum pudding model, the positive charge could be found in the nucleus. So true or false, in the plum pudding model, the positive charge could be found in the nucleus. False. Good. The plum pudding model doesn't have a nucleus, so the positive charges could not possibly be in the nucleus. Where are the positive charges found? Good. Okay. They're found in a ball of positive charge that is scattered all around the atom. So true or false, in the plum pudding model, the electrons are scattered randomly. True or false? Good. It's true. So well done for remembering that key word. They are scattered randomly. Okay, true or false, Dalton said that all elements are made up of tiny spheres which are the same size. False, okay? It wasn't Dalton that said that they were all the same size. Who was it? Good. It was Democritus. So Democritus said that they were all the same size. Dalton said that they were different sizes. Okay, so just to reflect on what you've learned so far, please pause the video now and have a go at answering these four questions. Remember to write your answers using full sentences. So here's what you should have got. Pause the video now and make any corrections that you need to. Pay special attention to the names of scientists and make sure that you spelled their names properly. Also make sure that you've been very specific with your descriptions and naming of models. Okay, so we've looked at the tiny sphere model and we've looked at the plum pudding model. We're now going to look at the nuclear model. Have a look at the diagram. Can you predict what changed between the plum pudding model and the nuclear model? Pause the video now and just have a think about it before we go into a discussion on it. So the nuclear model was developed by the scientist Ernest Rutherford. He proposed that all the positive charge was concentrated in the very centre of the atom in the form of a nucleus, hence the name nuclear model. He also suggested that the electrons, rather than being randomly distributed throughout the atom, that they were arranged in fixed positions around the edges of the atom. So the key things that were different was the fact that there was actually the presence of a positively charged nucleus in the centre of the atom and that the electrons were randomly arranged on the outside. How does this differ from the plum pudding model? Good. So in the plum posing model, the electrons were randomly distributed, whereas in this model, they are in fixed positions. And the second thing is that in the plum pudding model, there was no nucleus. Whereas in this model, there is a nucleus. Now you will notice that there is no mention of neutrons. The reality is that neutrons hadn't been discovered yet and they were only discovered much, much later. And we're going to come into that. So in this model, there was only positive and negative charges. So let's go into finding out just how Ernest Rutherford came to his conclusions about this structure of the atom. He used his famous gold foil experiment. Okay, so let's look at the evidence provided by the gold foil experiment. So in this experiment, alpha particles were fired at a really thin sheet of gold foil. Now, alpha particles are something that you may have heard about, but it might be a completely new word to you. All it means is it's the nucleus of a helium atom. So on the diagram, you can see the nucleus of a helium atom. What do you think the positive charges represent? So think of your knowledge about atomic structure. Good. They represent protons. How about the particles which have no charge? Good. They represent neutrons. And what do we know? We know that protons and neutrons are found in the nucleus of an atom. So at the time, scientists predicted that these alpha particles would go straight through the atom. And this was all based on J. J. Thomson's suggestion of the plum pudding model. However, when they actually did this experiment, what they found was yes, most of the particles did go straight through the gold atoms. However, there were some alpha particles that were actually deflected back. Now this suggested that there was something within the atom that was deflecting them back. So what they proposed was that at the centre of the atom, there was a very concentrated, positive charge. And that's called the nucleus. And when those positively charged alpha particles came into contact with those positive nuclei in the centre of the atom, they got deflected back or repelled backwards, 'cause positive and positive repel each other. Though this suggested that whilst most of the atom was considered to be empty space because the alpha particles went straight through, there was a concentrated positive charge in the centre of the atom which caused some of the alpha particles to be deflected back. So let me just summarise. Ernest Rutherford and his team fired alpha particles at gold atoms. Those atoms were found to be mostly empty space. And lastly, they concluded that the positive charge was concentrated in the nucleus of an atom. And that's how the nuclear model came to be. If you need to pause the video right now to make some notes and make sure that you've got your head around the gold foil experiment, please pause the video now. Okay, so let's see how much you remember. Which scientist developed the nuclear model? So which scientist was responsible for developing the nuclear model? Well done if you said Ernest Rutherford. Name the famous experiment that was performed by Ernest Rutherford. Good. It was the gold foil experiment. Well remembered if you said that. what is the charge of an alpha particle, do you remember? Think about the diagram. Excellent. It's positive. So alpha particles are the nucleus of a helium atom and all nuclei are positive. So true or false, the gold foil experiment revealed that the atom is largely made up of empty space. Excellent. It's true. So true or false, the gold foil experiment revealed that the atom contains a nucleus. Excellent. It's true. It was the first model to suggest the existence of a nucleus. So, so far we've looked at the tiny sphere model, we've looked at the plum pudding model and we've looked at the nuclear model. We're now going to look at Bohr's model of the atom. So as with the others, Bohr also just developed the ideas of the scientists that came before him. Bohr's model was largely based on the nuclear model. In fact, all he did was add an extra feature to the nuclear model. Can you spot what feature is on Bohr's model that was absent in the nuclear model? Well done if you spotted the fact that there are electron shells in Bohr's model. He developed the idea that electrons are in fixed positions and at fixed distances from the nucleus, and that energy shells were responsible for these distances. So similar to the nuclear model, his model also contained a positively charged nucleus in the centre. But what was different about his model was that the electrons were in fixed positions on energy shells, whereas in the nuclear model, they were all found on the edges of the atom in fixed positions. Obviously, this is still slightly different to the model that we use today. Can you spot what is missing in Bohr's model that we have in today's atomic structure model? Well done if you noticed that the neutrons are missing in the nucleus. In actual fact, the neutrons weren't discovered until after Bohr made his model in 1932. And so obviously he couldn't add these to his model because he didn't know they existed at that point. So that brings us to today's model. So in 1932, James Chadwick discovered the neutron, and that was the final subatomic particle that needed to be added to our structure of the atom. Now, this model is actually a very simple version of the model and has actually been developed into even greater detail. So let's see how much you remember. Which scientists developed the early nuclear model? Well done if you said it was Ernest Rutherford. What was present in the nuclear model that was absent in the plum pudding model? Use the diagrams to support you. So what's in the nuclear model that isn't in the plum pudding model? Well spotted. If you said there is a positive nucleus, you are spot on. Where were electrons found in the first nuclear model? So look at the diagram, where are the electrons? Excellent. They're found at the edges of the atom. How were the electrons arranged in Bohr's model of the atom? So look at Bohr's model, how were the electrons arranged? Where were they found? Excellent. They were in fixed positions. Well done if you said the word fixed, because that's really good specific terminology, in fixed positions on energy shelves. If you said energy levels, then that's fine too, well done. True or false, there were neutrons in the nucleus of the nuclear model. So have a look at the diagram. Are there neutrons in the nucleus of the nuclear model? Of course not. Neutrons weren't discovered until much later, so they wouldn't have been able to feature in the nuclear model if no one knew they existed. There were neutrons in the nucleus of Bohr's model, true or false? So neutrons in Bohr's model, true or false? Good. It's false. Again, neutrons hadn't been discovered yet, so there's no way that Bohr can include it in his model if he didn't know it existed. Okay, let's see how much you remember about the order in which the subatomic particles were discovered. Which subatomic particle was discovered first? Well done if you said it's an electron. They feature in even the earlier models of the atomic structure. Which atomic particle was the last to be discovered? Excellent. It was the neutron. Remember, it was the very last one, so it's actually not in any of the earlier models. Who discovered the neutron? Excellent. It's James Chadwick. J. J. Thomson, what did he discover? Good. It was the plum pudding model. And what about Ernest Rutherford? Good. It was the nuclear model. Okay, so let's try and pull some of that information together. Let's start off with some quick questions as an independent task and it will lead on to an exam-style question. So pause the video now and have a go at answering these five questions. You can use the diagrams to support your answers. Remember, try and be specific with your descriptions and your answers and always use full sentences. So here's the answers to question one and two. Pause the video now and compare what you've written down with the answers on the screen. Here's the answer to question three. Pause the video now and compare your answers to the ones provided. And here's the answers to question four and five. Again, pause the video and make sure you've corrected your answers if they need correcting. Okay, so let's take all of the information that you've learned in today's lesson and apply it to an exam-style question. So in this question, you need to compare the plum pudding model with the nuclear model. So when we're comparing two things, we need to use comparative language and we need to talk about the similarities and the differences. To support you in being able to write a really strong answer, please try and use the sentence structure that is provided in the support box. So you need to be using comparative language such as whereas or however, or in comparison to, so you need to be using comparative language. You also need to include all of the key words that are on the screen to make sure that your answers are really, really precise. Pause the video now and write your answer to this compare question and try and write down as many similarities and differences as you can think of. So here's what you should have got. You'll notice that the answer that's provided has got all of those key words that I mentioned written in bold. You'll also notice that each sentence uses comparative words, such as whereas. Pause the video now and compare this answer with the one that you've got written down. You may also have added in some other similarities and differences. You still need to have made sure that you included all of these, as these are the ones that commonly come up. Pause the video now and compare your answer with the one on the screen. Resume playing the video once you've done this. So hopefully you have a better understanding about the history of the atomic model. You'll realise that it's actually taken hundreds and hundreds of years to actually develop a model that we use today. It's also taken the collaborative work of many scientists to be able to produce these models. Before you finish today's lesson, please make sure that you go and complete the quiz on today's lesson. It will really test your understanding of today's lesson and help reinforce some of the ideas that you've learned. Thank you for all your hard work today, and I look forward to seeing you next lesson.

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In this lesson, we will describe the work that led to an increased understanding of what atoms are made of and how this led to changes to the atomic model.

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Development of the atomic model

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