Bohr Model and Atomic Structure

by Jared Rovny

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    00:02 Now that we've discussed the atomic nucleus and what's at the center of an atom.

    00:06 We're ready to discuss now the electronic structure of an atom.

    00:10 We'll start this what's call the Bohr model of the atom.

    00:13 And historically this is very significant development that how we think about what an atom is.

    00:18 What it structure is and therefore how it behaves.

    00:22 Historically, originally people knew that there are positive and negative charges.

    00:26 And they had seen this in experiments.

    00:28 But they didn't know how those really organize in actual atom.

    00:31 In the Plum pudding model as it's is called, we have sort of this distribution on the left here.

    00:36 Where we may be have a big positive charge so you could see a positive sign in the middle there.

    00:41 That was sort of distributed to the whole atom with may be negative charges and dispersed between sort of like plums in plum pudding.

    00:48 But then some experiments were conducted where different kinds of particles were shot at an atom to see how they bounced off of it.

    00:56 And to everybody's great surprise they found that they would bounce off a hardcore in the center of the atom.

    01:01 But would otherwise go right through.

    01:03 And this is the Bohr model of the item which you can see on the right.

    01:07 In the center we have the nucleus which we've already discussed.

    01:10 It has positive charges which are protons as well as neutral charges the neutrons.

    01:15 And then on the outside you have the electrons that are very small, small particles that are orbiting at a very great distance relative to the size of the nucleus itself.

    01:24 So this Bohr atom has a few more properties.

    01:27 Namely and perhaps most importantly, the electronic structure.

    01:31 So this electronic structure has different energy levels.

    01:35 These are the places that the electrons are allowed to be orbiting around your nucleus.

    01:40 So unlike planets that are orbiting each other, you can't have the orbits be anywhere you would like.

    01:45 For electrons orbiting a nucleus, they have to be in very discrete, very distinct energy levels.

    01:51 So for example, this first energy level, one closest to the nucleus, is called the First Energy Level or the Ground State of this atom.

    01:59 And then the electrons could go to the second energy level and so on.

    02:03 But again these have to be discrete.

    02:05 They have to be very particular levels and you can never be halfway between a level.

    02:09 One other things about these orbitals and we'll discuss this a little bit more later as well, is that there can only be a certain number of electrons in this orbitals.

    02:18 So you see we already producing many restrictions on exactly how this Bohr model of atom can work.

    02:24 So keep track of the restrictions that we are imposing.

    02:26 1. We can only be in particular energy levels.

    02:30 And 2. We can only have a certain number of electrons in a given energy level.

    02:35 This number is given by 2 times the energy level square.

    02:39 So for example, in the first energy level, that's ground state, that's closest to the nucleus, we can have two times 1 squared or two electrons in that energy level.

    02:48 And we could calculate this for a number of others.

    02:51 So far example for these second energy level, we have two times 2 squared or two times 4 which is 8 electrons that could reside in the second energy level.

    03:00 The typical hydrogen atom which is something we'll be focusing on as we go forward.

    03:05 Looks something like this.

    03:06 It has a proton at the center, with no neutrons and no other protons.

    03:10 Just one positive charge sitting right in the center.

    03:13 And then electrons are orbiting it.

    03:15 And again no other neutrons anywhere in the nucleus as well.

    03:18 And again the lowest energy level, the n = 1 level is called the "ground state" for the electron to be in.

    03:25 So this phrase the ground state comes up over and over again.

    03:29 So certainly be familiar with the fact that if I say "ground state," I'm referring to the n = 1 energy level.

    03:34 If the electrons goes to a higher state like the n = 2 or even the n = 3 energy levels, we say that the electron or the atom itself is an excited state.

    03:45 It has more energy.

    03:46 Because it needs energy to get that electron to that higher energy level.

    03:51 To that higher state.

    03:53 One way to get an electron to a higher energy level from a lower one like the ground state, is to hit it with some light.

    04:00 So we can see here we have photon on it's way in.

    04:02 It hits the electron and the electron is raised to the higher energy level.

    04:06 And then we would say again that the atom is in the excited state when the electron is in a higher energy level than the ground state.

    04:15 When the electron comes back down from that excited state, we have a very important phenomenon which is the exact opposite of what I just mentioned.

    04:23 Instead of absorbing a photon and going into a higher energy level, it falls to a lower energy level and emits light.

    04:30 So you can this exactly happening in the picture here.

    04:33 We have an electron in excited state.

    04:35 It falls into the ground state.

    04:37 And it will emit a photon.

    04:39 And that photon has a particular amount of energy, when it gets emitted.

    04:43 We can calculate this energy just by using conservation of energy.

    04:47 So if the atom had 1 amount of energy and then change to a lower amount of energy, we can calculate the difference.

    04:53 We can calculate how much energy was lost.

    04:56 So we write that in the same terminology that we've been using, using a delta sign to represent change.

    05:01 So the change in the energy of the atom is the final energy that it had or rather the excited energy, the initial energy, the first energy, minus the ground state energy.

    05:13 And this is because we have the excited energy being a bigger number.

    05:17 And then the ground state energy being a smaller number.

    05:19 In using this equation we can find what energy the photon must have.

    05:24 Again just using conservation of energy.

    05:26 The photon energy is actually something we've already calculated if you remember from before.

    05:31 So we have the photon energy here is just Planck's constant h times the frequency of that photon.

    05:37 Which again determines what color the photon is.

    05:40 If it's a red photon, a lower energy light, it is lower frequency.

    05:44 And vice versa for the higher frequency.

    05:47 It's more blue at higher frequencies which are higher energies.

    05:51 And also don't forget that we can also calculate this from the wavelength if we don't know the frequency but instead we know the wavelength of the light.

    05:57 Since we can relate the two using our velocity equation that we've already discussed.

    06:03 This atomic structure can keep going.

    06:05 We've only showed so far the first and the second energy levels.

    06:08 But we could in principle go to higher and higher energy levels.

    06:11 And they would look something like this.

    06:13 They are not equally spaced.

    06:14 You can see that going from the atom, the center of the atom to the ground state we have one radius, one distance.

    06:21 And then as we go further and further up, these distances decrease from one energy level to the next.

    About the Lecture

    The lecture Bohr Model and Atomic Structure by Jared Rovny is from the course Electronic Structure.

    Included Quiz Questions

    1. Spread out throughout the atom
    2. Concentrated in the center
    3. Orbiting the center
    4. Forming concentric rings
    5. Spread randomly throughout
    1. n = 1, 2, 3,...
    2. n = 0, 1, 2,...
    3. n = -2, -1, 0, 1, 2,...
    4. n = 1/2, 2/2, 3/2,...
    5. n = 0, 1/2, 2/2, 3/2,...
    1. 18
    2. 16
    3. 20
    4. 14
    5. 10
    1. By dropping electrons to lower energy levels
    2. By raising electrons to higher energy levels
    3. By swapping two electrons in adjacent energy levels
    4. By losing an electron from the atom
    5. By colliding two electrons in the same energy level

    Author of lecture Bohr Model and Atomic Structure

     Jared Rovny

    Jared Rovny

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