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학습트랜스크립트
00:04we are still working on electrochemistry and I am giving you some information about the
00:13bonding now here I start from the metal so I'm talking about I'm gonna talk about the
00:21bonding in metal but I'm going to extend this material to semiconductor you know metal and
00:32semiconductor those materials are very common material for the electrodes in in electrochemistry
00:41so if we know better much better about this metal and semiconductor that might be very helpful to
00:51understand the electrode and electrolyte contacts that's interface okay let's start from electron C
01:01model C electron C you know this electron C model is explained in even in high school textbook when I
01:12was
01:14middle school and high school students sometimes I heard about this even in the middle school
01:23science class but you know I didn't know much about the electron C model at that time but
01:35but still I don't know much about this electron C model let's go through this sentence a metal
01:44crystal is viewed as a three-dimensional array like this three-dimensional so this is two-dimensional
01:52one but that can be stacked over and over array of metal carions immersed in sea of the localized
02:01electrons so these are skeleton they are all the connected somehow but I I don't know how we can
02:12make array without many any frame or skeleton inside but anyway they are all connected somehow maybe through
02:24the charge or whatever and then they are immersed immersed in sea of the localized electrons it's like we are
02:35dipping
02:35this whole structure into the sea when the water is considered as a kind of electron
02:51so here we have to understand a few words like a balance electron the for example when we see sodium
03:01that
03:01will be considered as the atom in the next slide so atom and the sodium is 11th element so it
03:12has 11 11 plus
03:15as a central plus charge 11 plus this is nucleus and then 11 electrons are moving around right but in
03:27the inner side
03:29the most inside we have two electrons that's one s orbital and then there are eight electrons outside here that's
03:392s plus 2p orbital
03:41and then there is a one more electron here that's 11th electron that outermost electrons are called balance electron
03:57and then also here the balance electron balance they use this one again again again
04:05this balance electrons are used most commonly used for making bonds you know the the outer electrons the
04:16outer most the orbital the electrons in that those orbitals they are always the make some the chemical reaction
04:30because they are loosely the bounded to the nucleus see they are all shielded i'm sorry shielded by
04:41this electron and this electron so inside we have 11 plus and then this two electron may the substrate
04:50the two the charge of two the charge of two by the by this electrons so that's nine and then
04:59eight electron will
05:01shield all that nucleus charge by eight so only one plus will be filled by the outermost electron
05:12maybe but that's not exactly that way but i'm just roughly saying that if you go back to the general
05:20chemistry
05:22the textbook says the inside electron may shield it with a higher impact and then outside electron may
05:31shield it loosely so we have to consider that way exact calculation but i'm just saying roughly
05:39so uh uh this electron may feel one plus only
05:47because of those electron inside this balance electron one to one that's what what they can feel
05:56so this electron may use for the the bonding but here say it's free to move in case of metal
06:06so remember we are talking about metal not just a regular the the organic compound or
06:14any other kinds of situation just a metal case
06:21so um the cation is in an ordered array and then the the this valence electrons do not belong to
06:30any
06:30particular metal ion but to the crystal as a whole you understand what i'm talking about so um
06:39the in the in the case of the atom we consider this the inside the nucleus as a the football
06:49football
06:49then electron moving that orbital is considered as a some the stadium the soccer ball stadium
07:02or the the playground playground might be this way and then a little bit bigger for
07:10the stadium too but depending on the situation maybe the same ratio like uh the similar you know this this
07:24ratio might be almost the same as this ratio because of this the football is very small right
07:32and the most area they are empty that's what figured out from the the what is that the alpha particle
07:41scattering experiment by rutherford right so that's what we see and then let's move on to the molecular
07:51orbital theory so now the previous one's model so the model is not like a fully defined by a lot
08:01of
08:01scientists and also we report a lot of exceptions and that's that doesn't give us like exact the
08:12inside of something but now molecular orbital is a theory of metals for metals band theory
08:22this theory shouldn't have any kind of exception because it has been accepted as a theory but as you
08:32as you um the understand or study this kind of band theory it also has some sort of some what
08:43is that
08:43like a misunderstanding or some errors between each other so we will we will not go through for that but
09:04it's not exactly what really we need
09:10but sometimes we can explain a lot of things with this theory so uh although that exception is not our
09:20the our the studying category or inbound we uh we just
09:31understand um as long as we understand this like a molecular orbital theory okay i'm sorry i i i was
09:41um
09:42confused by myself too but we will see what we have to understand so let's go
09:49uh 3s one electron here so uh i mentioned that what is it this is a sodium 11 it has
10:031s2 electron 2s2 electron 2p6 electron and then 3s1 electron 11 2 2 6 that's 10 11
10:1711 so um this is valence electron this electrons are this this one electron is the only electron
10:30considered as a valence electron that can be used for making bonds so we we see here we do not
10:40consider
10:41this this inner electrons because they don't care much about the the bonding or chemical or physical bonding
10:49whatever so um uh 3s one orbital that's what we have this is called atomic orbital
11:02you know like orbital means kind of like a geometry where the electrons can look can locate
11:10at or just moving at or just moving around you know the it sounds like orbit the satellite is the
11:21moving
11:21along the orbit right and then orbital sounds like the same way like orbit electrons are moving somehow
11:32like s orbital s orbital electrons are moving along this s orbital or p orbital
11:40these are the orbital it sounds like orbit
11:45so uh we say like three s orbital three s orbital so it's one s orbital two s orbital three
11:53s orbital so
11:54it's a little bit bigger so it's a little bit bigger than but this is the same shape like the
12:00sphere sphere
12:02so uh we have one electron oh i'm sorry i forgot that uh it sounds like orbital orbit but it
12:11also represents
12:13energy level so atomic atomic atomic orbital is showing the it doesn't show actually what kind of the
12:24the geometry the electrons electrons are electron clouds are existing at but it also shows some energy level
12:34so that's what we see here in this diagram shows energy level
12:42but also like 3s orbital it should be like a spherical shape right so one electron exists in that orbital
12:51that's what we can obtain from this figure and then you know in certain area in small area we do
13:02not have
13:02just one sodium atom there might be a lot of sodium atom inside they are like affecting to each other
13:14so we may have another sodium right next to the original sodium we have one sodium and right next to
13:24that
13:24we have another sodium they are approaching to each other so atomic orbital this is also atomic orbital
13:33and then when they are very close to each other they try to make bonds and then we are making
13:42this
13:42kind of new orbital energy state that's molecular orbital we draw always like this
13:51so uh here's some rule if we have one uh atomic orbital we can make one molecular orbital but actually
14:01we cannot make any molecule
14:05from just one atom right so we need like two atom three atom or four atoms to make kind of
14:13molecular set
14:14so here we have two atoms are approaching to each other and then make one molecular orbital
14:24i'm sorry not one molecular orbital so two orbital approach it and two molecular orbital
14:33one of them has a lower energy state one the other one has a higher energy state because
14:41the the the energy should be conserved right energy cannot be generated or can energy cannot be
14:48disappeared so we have we have this energy level and this energy level let's say this energy level is
14:5610 and this energy level is also 10 then if this lowered energy is 8 then the other one should
15:04be 12
15:04that's how it makes the 12 or the 20 because the original atomic orbital makes the the sum of 22
15:16then um no when we make the molecule they are making bonds when do they make bonds when they can
15:27get
15:27that lowered by making bonds so actually they have a higher state as an atom and then when they make
15:37the molecular shape
15:39then they have lower energy otherwise they don't have any reason to make the connection
15:45so lowered part and then the uh the higher part upper part and lower part the energy conservation
16:00okay i mentioned that if we take a look at very small area there might be a lot of sodium
16:06not just one
16:06or two in the previous slide in the previous slide we saw one atom and two atoms and three atoms
16:13the same way
16:15see the energy the if we can make the more molecular molecular set that should have a lower energy
16:24right otherwise it doesn't have to go that way so it will go down but less down less down less
16:31down
16:32sometimes they are saturated and in certain area
16:41the textbook doesn't say what's the what's the minimum number to make this kind of a
16:50the the band then to make this like a band what's the the size of n and should be hundred
16:59or thousand
17:02but we don't know much about that but to be considered as a like a continuous energy level
17:09normally one of the dimensions should go over 100 nanometer so then we can
17:19calculate that the other way maybe this number should be like a thousand or something
17:24roughly i don't know i i haven't calculated that way but you can guess you can try and also let's
17:33see here
17:36the the total energy sum should be same as this one times n for example if we have a three
17:46then
17:46this and this and this this three the the all the sum of this three energy level divided by three
17:56should be the same as the original of the the energy level here because energy cannot be
18:04generated the same way all the time this way this way that it says go
18:10it goes on the same way and then in the case of metal this bonding area and the anti-bonding
18:20they make contact like this for some material for example semiconductor they sometimes have a like gap between
18:31this this band
18:35we will see that kind of experiment uh it's the the examples in the next slide somewhere
18:43okay let's take a look at this the band a little bit more detail
18:48so here i mentioned that like these electrons are what like a valence electron valence electrons are used for
18:56making bonds so they are all here used for making bonds but that's quite interesting right because they are making
19:09bonds
19:11that means that they are fully connected to each other from one atom to the other
19:17if they are used as a bonding how can they they be they how can they be considered as a
19:27like a as a free state
19:29like the electron c model they are free from this side to the other right so one day we can
19:38the we can find
19:40those electrons in korean peninsula and then the other day it can be not the other day just one second
19:48later
19:49it can exist in the the south africa somewhere right so the the sea water may not go that fast
20:00but in the case of electron that's what they are saying it's very fast inside the metal
20:07but we understand that because when we apply the voltage and the current it flows very quickly
20:17but now this according to this like a band theory it says a theory i mentioned that that there is
20:24no
20:24exception and then it says like bonding the electrons are used for bonding when they are used for bonding
20:34they cannot move they should make some bonds they're not supposed to move away because they are used for
20:42bonding otherwise the the metal may like a crash down or some they look like some what is that like
20:56the
20:56particles and flowers right i hope you understand what i'm talking about and then
21:03this is the the what is that bonding area and then also called like a balance
21:11band
21:14balance electrons are gathered balance electrons gathered to make this the the bonding part that's balance band
21:27but the other side i mentioned that so these are lowered energy to make a bond but the other one
21:34is
21:34this one uh i i'm sorry i forgot that this one is called like a sigma bond and this one
21:41is called the
21:41sigma star that's what previous slide says and this sigma star is anti-bonding see anti-bonding
21:52it sounds like it's the the opposite of bonding right it's not it's making some sort of
21:59uh it's breaking bonds so um anti-bonding that means it doesn't make any bond
22:09that's really free electron right because it doesn't make any bond they are free so they can move
22:17around at that from one side to the other so actual free electrons should exist in this
22:27anti-bonding area so we call this a balance band but this one is called
22:32conduction band
22:37that's quite interesting because uh we said this is the balance band this is conduction band
22:44but the the balance band means like all the electrons are bound to them to make some bonds
22:53and then if we see some electrons or any carriers in this conduction band that's conduction that means
23:02conduction that's how we can measure the electron flow so if we have some electron c model that c should
23:13be considered as this like the anti-bonding part not bonding part right so although we started the the very
23:24similar
23:26state saying the electron free area and c model but somehow we we came to a little bit different
23:37uh definition right but we will see what makes that kind of consideration or that changes
23:49okay that uh that's the uh the what what i explained like n increases at increases and then electrons are
23:59uh filled up to the uh filled up to the the any states so if we have this the band
24:06that means we have many states
24:08inside and this one is this one has like inside states too so that means any electron any electron here
24:18if we can stimulate them or gives give that electron to another kind of energy that will
24:27make jump make it jump make it jump to other states then if once this electron goes to this anti
24:36-bonding part
24:38that's c c model application they this electron can flow any place it can go from one side to the
24:47other very quickly
24:50okay okay that's what what just at the observed so here the the molecular theory for metals band theory we
24:59just
25:00observed that the electrical insulator materials that have only completely filled bands so uh we we just saw
25:08balance band and uh i'm sorry this balance band and conduction band and uh the complete filled
25:15bands if we have this bonds or this bonds if they have completely filled then uh we we may not
25:25see any
25:27the free electron because they do not make any bond it's it's totally completely the loosened
25:35and electrical conductor and electrical conductor materials that have partially filled bands
25:40so for example if we have a partially filled any any field can be partially filled then this
25:50conduction band the electron this electron may flow that's what can say it's a conductor
26:01and then in the in previous sodium case we we have this kind of thing and then we say if
26:09we do not have
26:09any anti-bonding electron then we cannot say we can see any electron flow that carrier flows but when we
26:20apply the the negative the negative or positive the voltage electric field then this the electric this
26:28negative means the push away push the electron away from this negative one so that that pushing electron
26:38will move that electron to the other side so that actually that that state was here then some of the
26:48electron may go over this anti-bonding part and this electron may play a very important role to
26:58move move from one side to the other and that considered as a carrier charge carrier so electron flows
27:09okay that's what it explained that and uh we we just observe that like we already know that like a
27:17balance band and conduction band so when we have a band this is conduction band and the balance band
27:24sometimes i mentioned that we might have some sort of gap so the the electron the bonding electron can
27:34jump from one side to the other by stimulating that by shining the light or the heating it up
27:42or any kind of the some other kind of the heating electron and band gap is considered as here this
27:54is band gap
27:57actually this like the band model does not explain why some the materials have this kind of gap
28:07when we start that the energy state for example this one some energy level should be located in the middle
28:21of this right
28:23but in some part it goes on the the if we go this one then it doesn't exist for this
28:32middle size
28:32so if we go a lot of things then this one the do not stay in the middle and they
28:42are approaching this
28:45some some energy level around here so we still might have the gap then how do we know about the
28:55which metal or which material can have this kind of situation actually um i don't see any any explanation about
29:07this kind of gap from the textbook but the any semiconductor they already experimentally measured
29:18the gap and then the then we just say oh this material has has this kind of band gap and
29:28also we measured
29:29the other material has a smaller band gap something like this so the window our glass glass window glass
29:40may have like a big band gap around like 8 as 10 electron volt and this small band gap for
29:49example like a silicon
29:50that's 1.1 electron volt so um if we give a very small like the outer simulation that's enough to
30:02jump
30:04or bring small amount of electron to upper conduction band but if we have a large band gap it's very
30:13difficult to move small amount of electron to upper conduction band but you know we may have a lot of
30:22different kind of energy source so i mean the glass is fully the insulator in our application but somehow if
30:36we
30:36give very high energy stimulation that x-ray light or some other kinds of light but still we can observe
30:45a
30:46small amount of current flow because that's also kind of the large band gap semiconductor that's called
30:56like insulator here are a few examples like a carbon diamond it has like a 520 kilojoule that's around
31:09like a 5 electron volt then it's called insulator silicon 1.1 electron volt semiconductor that's 0.6
31:22electron volt semiconductor 0.08 semiconductor and these are maybe tin this one is also maybe showing some
31:35metallic characteristic but here they categorized as a semiconductor but small amount of heat might
31:45enough might be enough to stimulate this semiconductor
31:55okay sometimes we do some doping work the addition of small amounts in ppm
32:03and then that impurity might enough to add extra electrons for example silicon has four arms and then
32:14they are all connected to silicon silicon because they are used for bonding you know they are all used for
32:22bonding then those electron may stay in the bonding area right so if they make only fully bonding that means
32:32that it doesn't have any other electron or the hole to move the charge from one side to the other
32:45so it's kind of a semiconductor
32:49but if we replace some of the silicon to boron for example silicon silicon they are all connected silicon silicon
33:00silicon silicon
33:01and then here b and silicon silicon silicon but this one does not have any electron so
33:09this silicon cannot make a bond here see the electrons are we need some electrons that's this one
33:23so uh what if we put uh the force spin here force spin is five electron so we have a
33:31four electron to make a bond and there is an extra electron
33:35and if we have p here then extra electron here too so those electrons are going to this conduction band
33:46so uh extra electrons are making negative type semiconductor that's n type and then lag of this electron is like
33:57a positive balance band the it doesn't fill up this balance band
34:04so that's a positive because it's lag of this electron right positive type semiconductor
34:13that's what we see here and then they can be used as a diode
34:19see like n type p type you know the the household the outer electric circuit provides like
34:28uh the a direct uh the current then that should be uh refined to use as a just a regular
34:38device in in
34:39household so we cannot use like both of them we use only this one uh the direct current
34:48if we can use this one too and by changing the direction that might be very useful to save the
34:56energy
34:58and then also let's say light emitting diode that we blow the electron and then n type and p type
35:08they
35:08are making some the energy recombination so actually this diagram that's what we are going to learn in
35:17the the next the uh class after that i will i might come back with this kind of application again
35:28and then the in the case of photovoltaic the other way light emitting device is electron flow this way
35:36to semiconductor but in this in this case electron is flowing the other way by accepting light
35:42light we will see okay and then also some diode laser transistor that's uh this the semiconductor work
35:55but you know uh i'm not going to give you very detailed information about this diode part that's not
36:04uh actually for the electrochemistry part that's not for the electrodes so
36:10um let's solve this kind of questions to see if we understood this the concept clearly
36:19the molecular diagram to the right are for the three different metals which one has the hardness and
36:27then the which one has the moderate and also soft so when we have this kind of the uh what
36:38is that like
36:39band the half half is for the half is the uh the what is that some uh baseline for
36:54determining this balance band and collection band and the balance band is for making bonds
37:01so if we have fully filled this balance band means they those electrons are fully used for making bonds
37:14if they make a bonds more electrons are used to make a bond then that bond this material should have
37:22a strong
37:24connection connection so is it okay if i say this one should have the hardest material among these three
37:34and then let's take a look at c they have a fully connected one fully connected the bonding
37:44but this one is not like anti-bonding right they are like one to one ratio cancelled so uh it's
37:53a feeling
37:54is uh the hardness feeling let's say that's like a 10 and then this one is negative 10 so that'll
38:01just
38:01cancel out that makes it zero so this is zero and this is 10 hardness 10
38:10and then in this case we see only half of 10 that's five so one which one is the hardest
38:19b b is the hardest
38:21and then a and then c b a c b a c this one is the the answer
38:33and then also the germanium molecular orbital doped with aluminum so the they gave some information
38:4414th and 13th so it's less less left left side it has a less electron so short of electron leg
38:54of electron
38:55electron so obviously there is only one thing which shows the leg of electron this is like a abundant
39:03electron and that's regular the semiconductor and this is the metal one metal case and then in this
39:11one electrons jump to the the conduction band by stimulating it by shining the light or heating the
39:21heating up the sample something like this and also this one is also doing this kind of thing so when
39:31regular room temperature might enough to jump the electron to the upper conduction band
39:40okay this is the last question you would like to solve let's see which picture corresponds to which
39:49metal explain i don't know much about yet like a silver molybdenum and uh what is that why
39:59is it itrium or something i don't even know but based on the information we uh observed in the previous
40:11class a previous question so let's say the bottom is 10 and with 10 upper one is negative 10
40:18and this is 10 and this is 10 and but this is 5 and here that's 10 and this is
40:27around like negative 7 correct
40:29so uh this is not positive 3 and this is positive 5 and this is 10 so uh hardest one
40:43is a one
40:46then two three one two three that way so which metal has the highest melting point
40:56if something is very hard then may melt uh at higher temperature correct because they have
41:04pretty strong bonding in the case of metal dissolving or melting means their connections are loosened
41:14right so maybe the highest melting point might be one and which one has the lowest three
41:23three i just explained it molybdenum is very hard oh that's one whereas the silver is relatively soft silver is
41:35relatively soft
41:39so this is silver and this is the molybdenum then the other one is y
41:48okay um i hope you uh understand the the electrode material as a conducting material or semiconductor material
42:00then on the next class we we are going to talk about the connection to as an elect as electrodes
42:09okay
42:10okay
42:14thank you we will see you again
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