CHEM 251 Week 1
CHEM 251 Week 1 Chem 251
University of Louisiana at Lafayette
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This 10 page Class Notes was uploaded by Lance Notetaker on Friday August 26, 2016. The Class Notes belongs to Chem 251 at University of Louisiana at Lafayette taught by Dr. Radhey Srivastava in Fall 2016. Since its upload, it has received 12 views. For similar materials see Descriptive Inorganic Chemistry in Chemistry at University of Louisiana at Lafayette.
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Date Created: 08/26/16
1 1 2 TEACHER: Basic concepts, atoms. We 3 are going to start the subject from today 4 onward. The basic concept we are going 5 to talk about before I go to that I 6 will tell you, this is the book I was 7 talking about the other day. You are not 8 going to study all the pages. There are 9 only a few chapters and some part of that 10 chapters we are going to study. The 11 basic concept you are going to start with 12 is the atom. 13 STUDENT: Is the book required or is 14 it good enough to be studying the 15 PowerPoint slides? 16 TEACHER: Hard to say. 17 STUDENT: I went to the bookstore and 18 they told me that they didn't sell it. 19 It was totally empty. Well, they had the 20 section number and everything but the 21 books were totally gone. 22 TEACHER: I will find out from the 23 secretary. 24 25 So I am going to talk about the TEACHER: We are talking about the atoms. And we are talking about how the atoms and the function that is called Schrodingers wave equation. I told you he was a Dutch chemist and he gave a equation and he describes the square. It means that the probability of finding electrons in an atom is rare. That is the topic of the Schrodingers wave. It means that suppose we draw a diagram like this, this is the atom and here is the nucleus, so where are the electrons? Are the electrons are here, here or here? So that is the concept of Schrodingers wave, how to find out the probability of the electrons in an atom. So the wave function is four numbers and they are called quantum numbers. The quantum numbers are called principal quantum numbers and L, angular momentum quantum number, and M and MS. You visualize like you are sending a letter from someone. This is just like a postal address. When you send an address by post, not by email, you have to write the name first and then the street or house address. So the house number and then the street name and then city and then country and so forth. Exactly the same way if you write it in this way you can find these four quantum numbers. They are going to tell you the position of electrons in an atom. So that is the meaning of the four quantum numbers. Let's talk about them one by one. The first is the principal quantum number and that is the N. So principal quantum number is called the distance from the nucleus where the electron is. Suppose we think that this is the center of the nucleus and the electrons are here, so this is the principal quantum number. That is going to tell you the distinction from the nucleus to the electron. And the principal quantum number value could be one to seven. It never will be zero. Please remember N value can never ever be zero. It always has minimum one and to the maximum seven. It may go further than eight, but until now we have the elements that can be described within the seven quantum numbers. And N is also the major energy level. This is the electrons energy levels and this is also called the major energy levels. This is the smallest one, the smallest ball. The N1 means the principal quantum number is one here. And if you go to the next ball, the higher one, N becomes 2. So if you go further away from the nucleus the N value will increase. For example, this is the nucleus and you can complete the cell, 2, 3, 4. So this is N is one, 2, 3, 4, 5, six, seven and so. So as you go further away from the nucleus the N value will increase. Similarly in this ball, so this is the N equal to 3. If you compare, this is the smallest ball, meaning that the radius is small and this is larger than one principal quantum number. This is bigger than the N value one and this is much bigger than 2. So as you go further away, the size of the atom will increase. The second quantum -- and, also, before I go to that, the principal quantum number also can be called shells. K shell, L shell, M shell, and N shell. If you want to calculate how many electrons there are, you can calculate by 2N squared. 2 times N is the principal quantum number and then square it. That will give you the total number of electrons. Suppose you want to calculate N value one. Then it's going to be 2. Suppose you want the principal quantum number 2, so 2 times 2 squared and this is going to be eight and so forth. You can easily calculate by the total number of electrons by using 2 and the square number. You can see that in principal quantum N total electron is 2. Quantum 2 number is 8, 3 is 18, 4 is 32. Shell N1 is known at the K shell and M shell and L shell and N4 is N shell. So KMLN. The second quantum number called azimuthal quantum number or angular momentum quantum number. And this is designated by a small l. This quantum number explains about the shape of an orbital. It may be elliptical. This quantum number may tell you the shape of the orbital and it is within the major energy levels, within the N value means that if you write the value like this, the value of the azimuthal quantum number is N minus one. It means that when you have N equal to one, then L equals zero. And when you have N equal to the 2, then L equals to the zero and N1. Let me write here including zero. And when you have the N equal to the 3, then L equal to the 0, 1 and 2, and when you have N equal to 4, L is 1, 2, and 3, and these values tell you the shape. This zero means you are talking about the S orbital. S atomic orbital, P orbital, D and F orbital. This is SPDF notation of atomic orbitals. Maximum number of electrons. You can also calculate 2 times 2L plus one. That is the formula. 2 times 2L is the quantum number and add 1 and that will give you the maximum number of electrons. If you calculate it you will find the maximum number in S orbital is 2, P orbital is 6, D orbital is 10 and F is 14 electrons. Just by calculation S cannot have more than 2, P cannot have more than 6, D cannot have more than 10, and F cannot have more than 14. They may contain less than their numbers, less than 2, less than 6, less than 10, less than 14, but they cannot exceed the number between 2, 6, 10 and 14. Magnetic quantum number is the spinning of electrons and the orientation means. Which if you visualize the 3 dimension, so you have the 3 dimension, you may have the one axis X, Z, and Y. Which direction electron is X, Y or Z? That is the orientation of the electron and that can be explained by the magnetic quantum number. The value of the ML arranged from L minus one including zero. It means you write it this way. Let me wry the ML here. ML is equal to minus L0 and plus L. It means that all the value of L should be in minus and plus and also include 10. So if I write M value for here, L is zero so I write minus zero. Then an additional zero then plus zero. This is where you have ML. But minus zero plus zero doesn't mean anything, so eventually you write zero. So if you have the N1, then L equal to zero and ML equal to zero, it means that S orbital has only one direction of electrons because the ML value is zero. But if you come to here, ML value is going to be minus one, zero and plus one because you have one. So this is minus; this should be plus and an additional zero. So minus 10 plus one. I write in the increasing order. There is no rule to write any number. Zero plus one and then minus one. You can write it anyway, but I prefer to write it increasing order. So minus is lowest, then zero and then plus one. It means that you are taking the P orbital and P orbital has 3 orientations, X, Y and Z. And if you come here, so ML value here is minus 2, minus one, zero plus one and plus 2. And this is the D orbital. D orbital is five orientations. Similarly, if you write for F orbital, that will have seven orientations. So that is the way to figure it out the orientation of electrons with the help of the magnetic quantum number. The last quantum number is called spin quantum number. And spin quantum number means that spinning electron is already spinning. And that is spinning clockwise or anticlockwise. So spin quantum number MS. So is the spinning electron clockwise or counterclockwise? This direction or this direction? This is why MS value is always plus, minus, one by 2 for each value of ML. I will tell you how. So it means that N equals one, L equal to 0, ML equal to zero and MS is going to have plus one by 2, and minus one by 2. And when you write the electron and the progression, normally we write it is spinning in the arrow. This one may be clockwise and then counterclockwise is like this. That is the meaning of the arrow writing on the top. That means maybe clockwise. You have to assume that one is clockwise and the other is counterclockwise. There is no mathematical formula to decide which is which. So these are the quantum numbers and that is going to tell you the value of the atoms. Let's come to the N equal to 3 and then L equal to zero, 1, and 2. And then M is equal to minus 2, minus one, and zero. And then you have plus one and plus 2 and then MS is going to equal, for this is going to be plus and minus 1/2. This value is also going to have plus/minus 1/2, and this value will have plus/minus 1/2. This also will have plus/minus 1/2, and this also is going to have plus/minus 1/2. So two electrons are here, two electrons are here, two electrons are here and two electrons are here and total number is 10 and this is the D orbital. Does it make sense? Questions? STUDENT: So I'm trying to see the difference between the first row and then the other side, because on the first side it looks separate where you have only a plus and only a negative and on the second side you have both. TEACHER: I don't understand. STUDENT: On the first column that you wrote, the other one. The plus and the minus are separate there. TEACHER: You can write it this way. You can also write plus 1/2 and minus 1/2. Either way. STUDENT: Okay. So either way is fine? TEACHER: Either you can write. Any other questions? Let's move further. Paulis exclusion principal. Paulis got 2 Nobel Prizes. He explained the principal states that no two electrons in an atom can have the same set of both quantum numbers. Two electrons is not going to have identical value of N, L, ML and MS. One electron is spinning clockwise and the other is spinning anticlockwise. If one electron is going clockwise, the other will be this. So if one electron is this, second electron is this, they are both different. One is clockwise and that is anticlockwise. It is true that two electrons cannot have the same quantum number for N, L, ML and MS. That is what we explained about that. So he said the same thing with the plus half and minus one half. One thing you should know that if electrons are paired, if it's odd number of electrons, they are called paramagnetic and unpaired electrons are normally called paramagnetic. And if an even number of electrons which can be divided by 2, they are called diamagnetic. Let's look at the square. It tells us about what information you got, the probability of finding the quantum mechanical entity in a certain state. So finding electrons. The probability is this psi square. So psi square is the probability of finding a square. So the wave function of atomic orbital can be divided into two types of wave orbitals, one is radial and that depends on the distance from the nucleus, and the other is angular and that depends on the angled of the side and theta angle. STUDENT: Is it 2 parts or 2 kinds? TEACHER: 2 kinds. So to the chemistry people, the second is more important. They normally talk about the molecule and what is the angle of the molecule, what is the angle between X and Y and so forth. So that is more important and; therefore, that is talking about the molecular shape and bonding of the molecule. STUDENT: So the theta is more important or the psi? TEACHER: Both. So there is one thing that is called nodes. I will tell you they are places where the probability of finding electron is zero. That is called nodes. If there is no electron, that is a node. So that is the square becomes zero if. Psi square is zero, that means nodes, if it is 1, 2, or 3, that means it's the probability of finding an electron. I will go over it a little bit more. These are the atomic orbitals, as I told you, that the atomic orbital depends on the quantum number and S is spherical shape and the number of orbitals is only one. P orbital is dumbbell. Anybody has a muscle man doing the exercise in the gym? No? Weight lifting? The dumbbell is just like that. If I draw the P orbital, suppose this is the X orbital. I can draw P just like this. This is a heavy portion and this is the part that holds it. It's very similar to the dumbbell, and D orbital is also dumbbell, but it is complex, and F orbital is difficult to draw. S is spherical and P, D and F are complex and S is one orbital, P is 3, D has 5, and F has 7. It means it has the 2 electrons, it has 6 electrons, each orbital will have 2 electrons, so 3 times 2 is 6. 5 times 2 is 10 and 7 times 2 is 14. So maximum number of electrons here. So this is the atom and this is the probability of finding of where the electron is. Electron is here, so maximum electron. That is defined here that radial distribution is the probability of density of finding electron in the radius. Suppose from nucleus to here is R. Radius is R. This portion the colored one, is the DR, for example. So this stems from a rare nucleus. You can see just like the onion, peel the onion. One layer you will remove it, and the second layer. So suppose, for example, one layer is the principal quantum number one and the second layer is principal quantum number 2 and third layer is principal quantum 4, and so forth. You can imagine this is what will happen. The probability of finding the electron is included between the sphere of a radius R and the DR. So here the electron density is the maximum. That is the radial distribution of the electron. I'm not going to spend more time here. This is the radial distribution of the S orbital, 2S orbital, 3S orbital, as you go further away from the nucleus this is the one nearest to the nucleus, 2 is the next and 3 is after the 2. As you go further away, then you can see the probability of finding electrons will be less. So for the S orbital, this probability maximum electron density and 2S orbital is here and 3 is here. So 1S and if you go further away that is 2S orbital and further away is 3S orbital, and you can also draw the 2P and 3P and so forth. I'm not going to draw that one. I want to tell you how we are moving is the distribution of the electron in S orbital. STUDENT: So we don't need to draw that one? TEACHER: No. So some examples on here. The maximum probability of S orbital mean it occurs at one bore. The maximum probability for the one orbital is here. 2S orbital is 5.3 bohr. This is 1 bohr and this is 5.3 bohr. It is far away from the . 529A. And the 3S orbital is still far away, 13 bohr so 1 bohr, 5 point 3 bohr and 13 bohr, this is close to the nucleus and this is farther away. This is the distribution of the electron. The S orbital is similar. So that is the S orbital and P orbital it is not symmetric. This is the radial distribution of 1S orbital here as I showed earlier. This is the contour. This is the maximum dense color is the electron density. White color there, so no electron here. So that is contour, so that where the electrons are the maximum density is. So angular part, we are going to talk about the S orbital and the same angular sign and that is why S orbital is symmetrical. If the angular value is different or sign is different, then you cannot have the circular orbital. P has different signs and it's not very symmetrical. That is why I will tell you in a minute what the S, P and D orbital shapes are. This is the P orbital. This is the Z axis and this is Y axis. So the electron density on the Z axis and this is 2P orbital. Electron density maximum and the PZ axis. Electron density maximum in the PX axis and electron density maximum in the PY axis. Electrons can be anywhere in the X direction or the Y or the Z direction. And when the electrons are here, it means that this is electron density is here, maximum, and here, there is no electron. Suppose, for example, S orbital wants to combine to P orbital so S orbital will overlap here or over here. Not going to overlap here because there is no electron at all. Electron overlapping between the S orbital and P orbital or between the P and D orbital should be where the electron density is the maximum. These are the D orbitals. You can see DX squared minus Y squared. So these are the 5 atomic orbitals. So it depends upon the direction of the orbitals. I don't have the structure I guess. It is more complicated. Even you can see that this is also dumbbell. So all of these are dumbbell sets. You can see the contour here. All of the dumbbells are P orbital or D or the F orbital. So what is the energy of the orbital? You can see that the shape of electron distribution is going to control the energy of the orbital. So probability of being closer to the nucleus based on the orbital shape, S orbital is the closest to the nucleus. N is the principal quantum number. And if you go from NS to the NP, NP is away from the NS and ND is further away from the NP and NF is further away from the ND. As you move further, the energy is going to be away from the nucleus. One thing I should explain is that the shielding effect. What is happening? What is the shielding effect? Suppose you have a nucleus here and here is the electrons. I am just counting two electrons, here is one electron. This is hypothetical. What is happening here? The nucleus is positively charged. And electron is negatively charged. So two forces acting at the same time. One is the attraction between all the electrons plus and minus and the others are negatively charged, so they repel. So two forces acting together and that is creating a problem. What is going to happen that these two electrons repelling each other, these two electrons repelling each other and this electron is also repelling each other. At the same time the nucleus is trying to go to the electron. The two electrons can repel this electron further away and this electron repel these two towards the nucleus. Nucleus is trying to attract the outer electron, but these two electrons create a seal between the outer electron and the nucleus. That is called shielding effect or screening effect. A curtain is here between outer electron and the nucleus. That is called shield or the screening effect. If these two electrons are not there, this outer electron may be closer to the nucleus, not away from the nucleus. So the Z effect -- "Z" means nuclear effect -- is equal to nuclear charges minus sigma. Sigma is the shielding effect. So if you subtract the sigma then you can figure out what is the Z and what is the nuclear charge of the electron, and that is the controlling electron position. So as you go farther away, you can see that they are shielding. Increased orbital is less than the 3D orbital. 3D orbital has the most shielding effect. And also shielding effect because there is 2 forces acting, place where the electrons are, where the nucleus is and also the shape of the orbital. These are the shielding effect. STUDENT: The electrons and the D energy experience the most shielding because they are being pushed away from the nucleus? TEACHER: Yeah, further away and the shape as well. I mentioned many electron atoms for a given principal. Quantum number N and Z decreases with increasing L. L is the quantum number value. So that is what you can understand from the energy of the orbitals. STUDENT: That's because the shielding effect increases? TEACHER: Yes. How to write electron configuration. That is called the Aufbau principal. How do you pronounce this? Aufbau. So I am right. So Aufbau principal. That's not the name of anybody. Is anybody from Germany? This is a German word. Aufbau means building up just like building a house. So you start from the ground level and go up. Building the building up. This is the way to write electrons. This one is the principal quantum number on the left hand side. One principal quantum number, and "S" is the angular value and this one is the number of electrons. When you write electron configuration, this is the way to represent the electron, and this will tell you where the electron is and if it near the nucleus away from the nucleus and what is the value of the quantum number. And I told you in the orbital diagram that there are three ways to write electron configuration. One is full or complete electronic configuration, second is the shorthand and third is the orbital picture or orbital diagram. So three ways to write electronic configuration. So orbital diagram means that suppose you are writing hydrogen. S orbital is 11 electron. Write in the box and write electron. If you have 2 electrons they are spinning opposite to each other. The other electron is going downward. That is the way to write it. I will show you. This is the energy level. You have to write the electrons on the increasing energy orbital. Electrons will prefer to reside in the lowest energy level. So 1S orbital or the N1 value will go first followed by N2, N3 and so forth. Energy increasing, the lowest is 1S, after that 2S, then 2P, 3S, 3P, 4S and then 3D and so forth. This is the other way, which I will start this way. This is the same thing. This is the lowest energy from top and highest at the bottom. You have to write 1S then 2S, then 2P, 3S, 3P, 4S and so forth. I'm going to write some of them. Let us write the hydrogen, the simplest atom. Hydrogen atomic number one and 1S. Unless this has 2 electrons, you can't write the next electron. So the helium this is going to be 1S2. The 1S orbital, 1S is complete. You cannot have more than w2. Then the next electron will go to the 2S orbital. So look at Lithium atomic number 3, this is 1S2 then you are going to the next energy level and this is going to be 3S1. The next example, BE, is 1S2 and 2S2. 1S is complete, 2S is complete and now the electron will go to the 2P orbital. I will write here boron, atomic number 5, 1S2, 2S2 and 2P1. P orbital can have maximum six electrons. So unless you have six electrons, you cannot add electrons to any other orbital, so carbon atomic number six is 1S2, 2S2 and 2P2. Then nitrogen seven is the same with 2P3. And then oxygen eight is 2P4. And atomic number nine is 2P5. And then neon is atomic number 10, this will be 2P6. And this way P orbital is filling one by one electron at each step. Now after filling the electron, 2P will go to the 3P orbital and that starts from the D block elements. Let's write sodium 11. These are the number of electrons, how many electrons you have. You are 11 electrons. So 2, 4, 6, 10, and make sure you write electronic configuration or not. And then magnesium, L is the same thing and here is 3S2. And then aluminum you can write the same way just like this. Let's talk about the D block or D orbital and that has some abnormal behavior that I will point out. Let's write electronic configuration of the element of the D block. SC. Atomic number 21. It is going to be 1S2, 2S2, 2P6, 3S2, 3P6, and 3D. Count the electrons. And then that is going to be 4S orbital squared. Look here. Lowest energy 1S2 electrons orbital and then 2S and then 2P, then 3S, then 3P, and then 4S after 3P. Please remember that energy level of 4S is lower than the 3D. So this is the different quantum numbers. This is the N equal to 3 and this is N equal to 4. Higher quantum number in N value is lower energy compared to the 3D orbital. When you write electronic configuration, we normally write here 4S2 and then write here 3D one. Now, titanium, everything is the same, no change except here is D2. D has maximum 10 electrons. V23 is going to be 3D to the 3. Chromium is going to be 3D4 and then atomic number 5MN is 3D 5 and 4S1. That was for element MG. STUDENT: So MG does not have a S orbital? TEACHER: I will come up with this one. So let's compare the chromium and MG. Chromium and MG. Chromium is going to equal 34 electrons. So count the electrons. 2, 4, 10, 12, 6, 18, 20, 24. Now, stability rule. This rule explains that which orbital is more stable. So three conditions: Orbital is vacant, no electron, second rule is exactly half filled, and third electron is completely filled. Okay? Now, consider the outer orbital, 3D. Is it vacant? Yes or no? No. Are they exactly half filled? No. Completely filled? No. It means that it is very unstable. So what is happening here to make this orbital, this outer orbital, to stabilize it. What is happening is, this is lower energy, 4S, this is high energy, 3D. What is happening, one electron from the lower energy level moves to the higher energy level. And I will write these two here, 4S becomes one electron and follow the second rule exactly half filled. Because the S orbital can have maximum 2 and one is exactly half. And D orbital, 3D orbital is going to be 5 and this is also part of the second rule is exactly half. There, the electronic configuration of chromium is going to be 3D5 and 4S1. There is one more in the D block. There is a lot of them but I am talking about the first row of the D block element. Copper, atomic number 29, if you write electronic configuration it will be 1S2, 2S2, 2P6, 3S2 and 3P6 and 4S2 and 3D9. STUDENT: Per MG? TEACHER: MG is going to be 2 and 5. They did not add. I was noticing that. TEACHER: He had the same situation. One electron will move from here to this. This becomes 1 and this becomes 10. Remember these two elements. Chromium and MG, they behave differently because they follow the rules of the stability, not only the energy level. This is two electronic configurations. Short hand, if you write to here, sodium, it's a very easy one. Sodium 11. One electron here. If you want to write the short hand of the sodium, find out the nearest atomic number from the noble gases. Look at the periodic chart and find out which one is the nearest to 11. 10. So write here sodium atomic number 11 in parentheses and here, neon, it means that you have neon up to this. You have to write only this, that's it. So it's short hand. You have to pick only from the noble gases, not from other elements. Noble gas is close to the element you are talking about. That's all for today. We will discuss this further.
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