1 00:00:00,269 --> 00:00:02,580 here's a very simplified model of an 2 00:00:02,580 --> 00:00:04,620 atom the nucleus at the center of the 3 00:00:04,620 --> 00:00:06,660 atom is where the protons and neutrons 4 00:00:06,660 --> 00:00:09,929 live but they're kind of boring because 5 00:00:09,929 --> 00:00:11,820 for the most part they just sit there 6 00:00:11,820 --> 00:00:13,710 the real star of the show is the 7 00:00:13,710 --> 00:00:16,410 electron the electron gets to do all the 8 00:00:16,410 --> 00:00:18,570 interesting stuff like move around jump 9 00:00:18,570 --> 00:00:20,880 around bind with other atoms these 10 00:00:20,880 --> 00:00:23,070 dashed lines represent the different 11 00:00:23,070 --> 00:00:24,900 energy levels the electron can have 12 00:00:24,900 --> 00:00:27,840 while in the atom we like representing 13 00:00:27,840 --> 00:00:29,789 these energy levels with an energy level 14 00:00:29,789 --> 00:00:32,969 diagram the energy level diagram gives 15 00:00:32,969 --> 00:00:34,829 us a way to show what energy the 16 00:00:34,829 --> 00:00:37,290 electron has without having to draw an 17 00:00:37,290 --> 00:00:39,360 atom with a bunch of circles all the 18 00:00:39,360 --> 00:00:42,270 time let's say our pretend Adam has 19 00:00:42,270 --> 00:00:45,690 electron energy levels of zero Evie for 20 00:00:45,690 --> 00:00:49,800 Evie six Evie and seven Evie note that 21 00:00:49,800 --> 00:00:51,809 moving left or right on an energy level 22 00:00:51,809 --> 00:00:53,579 diagram doesn't actually represent 23 00:00:53,579 --> 00:00:55,379 anything meaningful 24 00:00:55,379 --> 00:00:58,800 so technically there is no x-axis on an 25 00:00:58,800 --> 00:01:00,930 energy level diagram but we draw it 26 00:01:00,930 --> 00:01:02,640 there anyway because it makes it look 27 00:01:02,640 --> 00:01:05,280 nice all that matters is what energy 28 00:01:05,280 --> 00:01:07,350 level or wrong on the ladder the 29 00:01:07,350 --> 00:01:10,200 electron is f note that the electron for 30 00:01:10,200 --> 00:01:12,270 our hypothetical atom here can only 31 00:01:12,270 --> 00:01:15,240 exist with zero evey for six or seven 32 00:01:15,240 --> 00:01:18,090 Evie the electron just cannot exist 33 00:01:18,090 --> 00:01:21,000 between energy levels it's always got to 34 00:01:21,000 --> 00:01:23,570 be right on one of the energy levels 35 00:01:23,570 --> 00:01:26,220 okay so let's say our electron starts 36 00:01:26,220 --> 00:01:29,220 off on the zero evey energy level it's 37 00:01:29,220 --> 00:01:30,659 good to note that the lowest energy 38 00:01:30,659 --> 00:01:33,000 level an electron can have in an atom is 39 00:01:33,000 --> 00:01:35,340 called the ground state so how could our 40 00:01:35,340 --> 00:01:37,200 electron get from the ground state to 41 00:01:37,200 --> 00:01:39,750 any of the higher energy levels well for 42 00:01:39,750 --> 00:01:41,579 the electron to get to a higher energy 43 00:01:41,579 --> 00:01:43,740 level we've got to give the electron 44 00:01:43,740 --> 00:01:46,170 more energy and we know how to give an 45 00:01:46,170 --> 00:01:48,090 electron more energy you just shoot 46 00:01:48,090 --> 00:01:50,579 light at it if a photon of the right 47 00:01:50,579 --> 00:01:52,979 energy can strike an electron the 48 00:01:52,979 --> 00:01:54,960 electron will absorb all the photons 49 00:01:54,960 --> 00:01:57,210 energy and jump to a higher energy level 50 00:01:57,210 --> 00:01:59,729 the electron in this ground state needs 51 00:01:59,729 --> 00:02:02,340 four Evie to jump to the next energy 52 00:02:02,340 --> 00:02:04,979 level that means if a photon that had an 53 00:02:04,979 --> 00:02:07,619 energy of four Evie came in and struck 54 00:02:07,619 --> 00:02:10,080 the electron the electron would absorb 55 00:02:10,080 --> 00:02:12,360 the energy of the photon causing the 56 00:02:12,360 --> 00:02:12,960 photon 57 00:02:12,960 --> 00:02:15,300 to disappear and that electron would 58 00:02:15,300 --> 00:02:18,150 jump up to the next energy level we call 59 00:02:18,150 --> 00:02:20,220 the first energy level after the ground 60 00:02:20,220 --> 00:02:23,670 state the first excited state once the 61 00:02:23,670 --> 00:02:25,680 electrons at the higher energy level it 62 00:02:25,680 --> 00:02:28,590 won't stay there long electrons if given 63 00:02:28,590 --> 00:02:30,690 the chance will fall towards the lowest 64 00:02:30,690 --> 00:02:33,240 energy level they can so our electron 65 00:02:33,240 --> 00:02:35,250 will fall back down to the ground state 66 00:02:35,250 --> 00:02:38,400 and give up for evey of energy the way 67 00:02:38,400 --> 00:02:40,590 an electron can give up energy is by 68 00:02:40,590 --> 00:02:43,350 emitting a photon so after falling back 69 00:02:43,350 --> 00:02:45,330 down to the ground state this electron 70 00:02:45,330 --> 00:02:48,810 would emit a four evey photon electrons 71 00:02:48,810 --> 00:02:51,030 don't have to just jump one energy level 72 00:02:51,030 --> 00:02:53,760 at a time though if the electron in our 73 00:02:53,760 --> 00:02:56,190 ground state were to absorb a six evey 74 00:02:56,190 --> 00:02:58,800 Photon the electron can jump all the way 75 00:02:58,800 --> 00:03:01,560 up to the six evey energy level now that 76 00:03:01,560 --> 00:03:03,150 the electrons at a higher energy level 77 00:03:03,150 --> 00:03:05,790 it's gonna try to fall back down but 78 00:03:05,790 --> 00:03:07,500 there's a couple ways it could fall back 79 00:03:07,500 --> 00:03:10,080 down in this case the electron could 80 00:03:10,080 --> 00:03:12,540 fall down to the ground state all in one 81 00:03:12,540 --> 00:03:15,330 shot giving up a six evey photon in the 82 00:03:15,330 --> 00:03:17,970 process but since the electron started 83 00:03:17,970 --> 00:03:20,400 at the six evey energy level it could 84 00:03:20,400 --> 00:03:23,280 have also fallen first to the four evey 85 00:03:23,280 --> 00:03:26,640 energy level emitting a to evey photon 86 00:03:26,640 --> 00:03:29,730 in the process it's a to evey photon 87 00:03:29,730 --> 00:03:31,680 because the electron dropped two 88 00:03:31,680 --> 00:03:34,170 electron volts in energy and now that 89 00:03:34,170 --> 00:03:36,240 the electrons at the four evey energy 90 00:03:36,240 --> 00:03:38,250 level it'll fall back down to the ground 91 00:03:38,250 --> 00:03:40,980 state emitting a four evey photon in the 92 00:03:40,980 --> 00:03:43,710 process so electrons will sometimes drop 93 00:03:43,710 --> 00:03:45,900 multiple energy levels at a time and 94 00:03:45,900 --> 00:03:47,700 sometimes they'll choose to take 95 00:03:47,700 --> 00:03:50,730 individual steps but regardless the 96 00:03:50,730 --> 00:03:52,860 energy of the photon is always equal to 97 00:03:52,860 --> 00:03:55,320 the difference in electron energy levels 98 00:03:55,320 --> 00:03:57,150 what if our electrons in the ground 99 00:03:57,150 --> 00:03:59,820 state and we send a five evey photon at 100 00:03:59,820 --> 00:04:01,980 it if the electron were to absorb all of 101 00:04:01,980 --> 00:04:04,590 the energy of the five evey photon it 102 00:04:04,590 --> 00:04:07,290 would now have five electron volts but 103 00:04:07,290 --> 00:04:09,210 that's not an allowed energy level so 104 00:04:09,210 --> 00:04:11,490 the electron can't absorb this photon 105 00:04:11,490 --> 00:04:13,140 and the photon will pass straight 106 00:04:13,140 --> 00:04:14,940 through the atom keep in mind the 107 00:04:14,940 --> 00:04:16,798 electron in the atom has to absorb all 108 00:04:16,798 --> 00:04:19,350 of the photons energy or none of it it 109 00:04:19,350 --> 00:04:21,600 can't just absorb part of it all right 110 00:04:21,600 --> 00:04:23,220 so now we could figure out every 111 00:04:23,220 --> 00:04:24,780 possible photon this 112 00:04:24,780 --> 00:04:27,060 Adam could absorb if the electrons in 113 00:04:27,060 --> 00:04:29,040 the ground state it could absorb a for 114 00:04:29,040 --> 00:04:32,250 evey photon or a six evey photon or a 115 00:04:32,250 --> 00:04:35,070 seven evey photon if the electrons at 116 00:04:35,070 --> 00:04:37,230 the second energy level also called the 117 00:04:37,230 --> 00:04:39,510 first excited state the electron could 118 00:04:39,510 --> 00:04:42,510 absorb a to evey photon or a three evey 119 00:04:42,510 --> 00:04:44,850 photon and if the electron were at the 120 00:04:44,850 --> 00:04:47,370 third energy level or the second excited 121 00:04:47,370 --> 00:04:49,830 state the electron could absorb a one 122 00:04:49,830 --> 00:04:52,710 evey Photon those are the only photons 123 00:04:52,710 --> 00:04:55,880 that this atom will be seen to absorb 124 00:04:55,880 --> 00:04:58,890 2.5 evey photons will pass straight 125 00:04:58,890 --> 00:05:01,500 through five evey photons will pass 126 00:05:01,500 --> 00:05:04,560 straight through 6.3 evey photons will 127 00:05:04,560 --> 00:05:07,260 pass straight through what this means is 128 00:05:07,260 --> 00:05:09,090 that if you were to shine light that 129 00:05:09,090 --> 00:05:11,370 consisted of all possible wavelengths 130 00:05:11,370 --> 00:05:13,620 through a gas that was composed of our 131 00:05:13,620 --> 00:05:16,260 pretend atoms all the wavelengths would 132 00:05:16,260 --> 00:05:18,060 not make it through some of the 133 00:05:18,060 --> 00:05:19,830 wavelengths would get absorbed then 134 00:05:19,830 --> 00:05:22,770 scattered away in random directions this 135 00:05:22,770 --> 00:05:25,620 would manifest itself as dark lines in 136 00:05:25,620 --> 00:05:28,080 the spectrum missing wavelengths or 137 00:05:28,080 --> 00:05:30,660 missing energy levels that correspond to 138 00:05:30,660 --> 00:05:32,640 the energies of photons that our 139 00:05:32,640 --> 00:05:34,680 electron can absorb this is like a 140 00:05:34,680 --> 00:05:37,260 fingerprint for an atom and it's called 141 00:05:37,260 --> 00:05:40,470 that atoms absorption spectrum if you 142 00:05:40,470 --> 00:05:42,120 were to ever see this progression of 143 00:05:42,120 --> 00:05:44,760 dark lines in these exact positions you 144 00:05:44,760 --> 00:05:46,590 would know that the gas you were looking 145 00:05:46,590 --> 00:05:49,229 at was composed at least partly of our 146 00:05:49,229 --> 00:05:51,419 hypothetical atom this also allows 147 00:05:51,419 --> 00:05:53,610 astronomers to determine what stuff in 148 00:05:53,610 --> 00:05:55,470 our universe is made out of even though 149 00:05:55,470 --> 00:05:57,330 we can't get close enough to collect a 150 00:05:57,330 --> 00:05:59,280 sample all we have to do is collect 151 00:05:59,280 --> 00:06:02,160 light from a distant star or quasar that 152 00:06:02,160 --> 00:06:03,690 shines through the stuff we're 153 00:06:03,690 --> 00:06:05,850 interested in then just determine which 154 00:06:05,850 --> 00:06:08,160 wavelengths or energies got taken out 155 00:06:08,160 --> 00:06:10,350 the details are a little messier than 156 00:06:10,350 --> 00:06:12,450 that but this provides astronomers with 157 00:06:12,450 --> 00:06:14,580 maybe the most important tool at their 158 00:06:14,580 --> 00:06:17,370 disposal now the absorption spectrum are 159 00:06:17,370 --> 00:06:19,710 all of the wavelengths or energies that 160 00:06:19,710 --> 00:06:21,720 an atom will absorb from light that 161 00:06:21,720 --> 00:06:24,000 passes through it you could also ask 162 00:06:24,000 --> 00:06:27,180 about the emission spectrum the emission 163 00:06:27,180 --> 00:06:29,100 spectrum are all of the wavelengths or 164 00:06:29,100 --> 00:06:31,680 energies that an atom will emit due to 165 00:06:31,680 --> 00:06:34,169 electrons falling down an energy level 166 00:06:34,169 --> 00:06:35,610 you could go through all the 167 00:06:35,610 --> 00:06:37,620 possibilities of an electron falling 168 00:06:37,620 --> 00:06:39,330 down again but you'd realize you're 169 00:06:39,330 --> 00:06:41,219 going to get the exact same energies for 170 00:06:41,219 --> 00:06:43,259 the emission spectrum that you got for 171 00:06:43,259 --> 00:06:45,719 the absorption spectrum so instead of 172 00:06:45,719 --> 00:06:47,430 letting light pass through a gas 173 00:06:47,430 --> 00:06:49,499 composed of your hypothetical atoms 174 00:06:49,499 --> 00:06:51,870 let's say you made a container that had 175 00:06:51,870 --> 00:06:53,789 the gas of your hypothetical atoms and 176 00:06:53,789 --> 00:06:55,439 you ran an electric current through it 177 00:06:55,439 --> 00:06:57,479 exciting those electrons to higher 178 00:06:57,479 --> 00:06:59,310 energy levels and letting them fall back 179 00:06:59,310 --> 00:07:02,129 down to lower energy levels this is what 180 00:07:02,129 --> 00:07:04,650 happens in neon lights or if you're in 181 00:07:04,650 --> 00:07:06,719 science class is what happens in gas 182 00:07:06,719 --> 00:07:09,449 discharge tubes so for the emission 183 00:07:09,449 --> 00:07:11,099 spectrum instead of seeing the whole 184 00:07:11,099 --> 00:07:13,169 electromagnetic spectrum with a few 185 00:07:13,169 --> 00:07:15,300 lines missing you're going to only see a 186 00:07:15,300 --> 00:07:17,639 handful of lines that correspond to the 187 00:07:17,639 --> 00:07:20,189 energies of those photons that that atom 188 00:07:20,189 --> 00:07:22,349 will emit okay I've got to be honest 189 00:07:22,349 --> 00:07:24,360 about something if any physicists are 190 00:07:24,360 --> 00:07:26,129 watching this video they're cringing 191 00:07:26,129 --> 00:07:28,289 because the energies that electrons will 192 00:07:28,289 --> 00:07:30,750 have in an atom are not positive the 193 00:07:30,750 --> 00:07:32,729 energies an electron can have in an atom 194 00:07:32,729 --> 00:07:35,279 are actually all negative values this is 195 00:07:35,279 --> 00:07:37,830 because the electrons bound to the atom 196 00:07:37,830 --> 00:07:40,020 anything that's bound to something else 197 00:07:40,020 --> 00:07:41,669 will have total energies that are 198 00:07:41,669 --> 00:07:44,250 negative this is analogous to a ball 199 00:07:44,250 --> 00:07:46,469 stuck at the bottom of a ditch if the 200 00:07:46,469 --> 00:07:48,180 ball is not moving it has no kinetic 201 00:07:48,180 --> 00:07:50,580 energy and if we assume that ground 202 00:07:50,580 --> 00:07:53,879 level is the h equals zero position then 203 00:07:53,879 --> 00:07:55,229 this ball is going to have a negative 204 00:07:55,229 --> 00:07:57,449 gravitational potential energy 205 00:07:57,449 --> 00:07:59,370 since this ball has a negative total 206 00:07:59,370 --> 00:08:01,680 energy it's stuck and bound to the ditch 207 00:08:01,680 --> 00:08:03,839 if someone could give this ball enough 208 00:08:03,839 --> 00:08:05,460 energy so that it would have positive 209 00:08:05,460 --> 00:08:07,740 total energy the ball could leave the 210 00:08:07,740 --> 00:08:10,289 ditch it would not be found any more so 211 00:08:10,289 --> 00:08:12,240 to make our hypothetical atom a little 212 00:08:12,240 --> 00:08:15,389 more realistic let's subtract 10 Evie 213 00:08:15,389 --> 00:08:17,909 from each energy level this doesn't 214 00:08:17,909 --> 00:08:20,310 really change anything in order for the 215 00:08:20,310 --> 00:08:22,139 electron to get from the negative 10 216 00:08:22,139 --> 00:08:25,199 Evie ground state to the negative 6 Evie 217 00:08:25,199 --> 00:08:27,449 first excited state it's still going to 218 00:08:27,449 --> 00:08:30,509 take a 4 Evie photon people do get 219 00:08:30,509 --> 00:08:32,370 confused with the negative signs though 220 00:08:32,370 --> 00:08:34,589 so be careful in order to find the 221 00:08:34,589 --> 00:08:36,809 energy of the photon that was absorbed 222 00:08:36,809 --> 00:08:39,269 or emitted you always take the higher 223 00:08:39,269 --> 00:08:40,130 energy 224 00:08:40,130 --> 00:08:42,860 and subtract from it the lower energy 225 00:08:42,860 --> 00:08:44,960 level so in this case we would take 226 00:08:44,960 --> 00:08:47,750 negative 6 evie and subtract from it 227 00:08:47,750 --> 00:08:50,630 negative 10 evie which tells us that it 228 00:08:50,630 --> 00:08:53,240 would take a for evey photon to bump an 229 00:08:53,240 --> 00:08:55,340 electron up to that energy level and the 230 00:08:55,340 --> 00:08:58,550 electron would emit a 4 evey photon if 231 00:08:58,550 --> 00:09:00,820 it dropped it back down from that level 232 00:09:00,820 --> 00:09:03,290 something else that's unrealistic about 233 00:09:03,290 --> 00:09:06,440 our hypothetical atom is that real atoms 234 00:09:06,440 --> 00:09:09,140 wouldn't just stop at negative 3 evey 235 00:09:09,140 --> 00:09:12,050 for the highest energy level real atoms 236 00:09:12,050 --> 00:09:14,300 have energy levels that get closer and 237 00:09:14,300 --> 00:09:16,670 closer together as you approaches zero 238 00:09:16,670 --> 00:09:19,340 evey what happens when an electron gets 239 00:09:19,340 --> 00:09:22,730 more than zero evey energy well if an 240 00:09:22,730 --> 00:09:25,010 electron has more than zero energy that 241 00:09:25,010 --> 00:09:27,500 means it's got positive energy and if 242 00:09:27,500 --> 00:09:29,600 it's got positive energy it's not bound 243 00:09:29,600 --> 00:09:32,600 to the atom anymore it'll be free to 244 00:09:32,600 --> 00:09:34,460 leave it'll be gone and we'll say that 245 00:09:34,460 --> 00:09:37,040 we've ionized the atom by removing an 246 00:09:37,040 --> 00:09:39,440 electron so for example say the 247 00:09:39,440 --> 00:09:41,990 electrons started at the negative 4 evie 248 00:09:41,990 --> 00:09:45,080 energy level and it absorbed a 7 evey 249 00:09:45,080 --> 00:09:47,540 photon that electron would have a total 250 00:09:47,540 --> 00:09:50,270 energy of positive 3 evey and so it 251 00:09:50,270 --> 00:00:00,000 would be gone from the atom