1 00:00:07,027 --> 00:00:09,404 Most atoms don't ride solo, 2 00:00:09,404 --> 00:00:12,125 instead they bond with other atoms. 3 00:00:12,125 --> 00:00:13,743 And bonds can form between atoms 4 00:00:13,743 --> 00:00:14,943 of the same element 5 00:00:14,943 --> 00:00:17,065 or atoms of different elements. 6 00:00:17,065 --> 00:00:20,190 You've probably imagined bonding as a tug of war. 7 00:00:20,190 --> 00:00:21,966 If one atom is really strong, 8 00:00:21,966 --> 00:00:24,110 it can pull one or more electrons 9 00:00:24,110 --> 00:00:25,775 off another atom. 10 00:00:25,775 --> 00:00:28,801 Then you end up with one negatively charged ion 11 00:00:28,801 --> 00:00:31,042 and one positively charged ion. 12 00:00:31,042 --> 00:00:33,811 And the attraction between these opposite charges 13 00:00:33,811 --> 00:00:35,975 is called an ionic bond. 14 00:00:35,975 --> 00:00:37,233 This is the kind of sharing 15 00:00:37,233 --> 00:00:39,993 where you just give away your toy to someone else 16 00:00:39,993 --> 00:00:41,958 and then never get it back. 17 00:00:43,324 --> 00:00:45,005 Table salt, sodium chloride, 18 00:00:45,005 --> 00:00:47,757 is held together by ionic bonds. 19 00:00:47,757 --> 00:00:50,331 Every atom of sodium gives up one electron 20 00:00:50,331 --> 00:00:52,252 to every atom of chlorine, 21 00:00:52,252 --> 00:00:53,362 ions are formed, 22 00:00:53,362 --> 00:00:55,244 and those ions arrange themselves 23 00:00:55,244 --> 00:00:57,911 in a 3D grid called a lattice, 24 00:00:57,911 --> 00:00:59,441 in which every sodium ion 25 00:00:59,441 --> 00:01:01,667 is bonded to six chloride ions, 26 00:01:01,667 --> 00:01:03,498 and every chloride ion is bonded 27 00:01:03,498 --> 00:01:05,706 to six sodium ions. 28 00:01:05,706 --> 00:01:07,472 The chlorine atoms never give 29 00:01:07,472 --> 00:01:09,925 the sodium atoms their electrons back. 30 00:01:10,585 --> 00:01:13,684 Now, these transactions aren't always so cut-and-dried. 31 00:01:13,684 --> 00:01:16,622 If one atom doesn't completely overwhelm the other, 32 00:01:16,622 --> 00:01:19,316 they can actually share each other's electrons. 33 00:01:19,316 --> 00:01:20,819 This is like a pot luck 34 00:01:20,819 --> 00:01:22,919 where you and a friend each bring a dish 35 00:01:22,919 --> 00:01:25,920 and then both of you share both dishes. 36 00:01:25,920 --> 00:01:27,353 Each atom is attracted to the shared electrons 37 00:01:27,353 --> 00:01:28,652 in between them, 38 00:01:28,652 --> 00:01:31,726 and this attraction is called a covalent bond. 39 00:01:31,726 --> 00:01:33,757 The proteins and DNA in our bodies, 40 00:01:33,757 --> 00:01:34,584 for example, 41 00:01:34,584 --> 00:01:37,775 are held together largely by these covalent bonds. 42 00:01:37,775 --> 00:01:39,358 Some atoms can covalently bond 43 00:01:39,358 --> 00:01:41,250 with just one other atom, 44 00:01:41,250 --> 00:01:42,917 others with many more. 45 00:01:42,917 --> 00:01:44,367 The number of other atoms 46 00:01:44,367 --> 00:01:45,690 one atom can bond with 47 00:01:45,690 --> 00:01:48,577 depends on how its electrons are arranged. 48 00:01:48,577 --> 00:01:51,543 So, how are electrons arranged? 49 00:01:51,543 --> 00:01:53,972 Every atom of a pure, unbonded element 50 00:01:53,972 --> 00:01:55,393 is electrically neutral 51 00:01:55,393 --> 00:01:56,829 because it contains the same number 52 00:01:56,829 --> 00:01:58,458 of protons in the nucleus 53 00:01:58,458 --> 00:02:01,247 as it does electrons around the nucleus. 54 00:02:01,247 --> 00:02:04,422 And not all of those electrons are available for bonding. 55 00:02:04,422 --> 00:02:06,420 Only the outermost electrons, 56 00:02:06,420 --> 00:02:08,684 the ones in orbitals furthest from the nucleus, 57 00:02:08,684 --> 00:02:10,338 the ones with the most energy, 58 00:02:10,338 --> 00:02:12,731 only those participate in bonding. 59 00:02:12,731 --> 00:02:15,808 By the way, this applies to ionic bonding too. 60 00:02:15,808 --> 00:02:17,487 Remember sodium chloride? 61 00:02:17,487 --> 00:02:19,400 Well, the electron that sodium loses 62 00:02:19,400 --> 00:02:21,604 is the one furthest from its nucleus, 63 00:02:21,604 --> 00:02:23,410 and the orbital that electron occupies 64 00:02:23,410 --> 00:02:25,034 when it goes over to chlorine 65 00:02:25,034 --> 00:02:28,283 is also the one furthest from its nucleus. 66 00:02:28,283 --> 00:02:30,166 But back to covalent bonding. 67 00:02:30,166 --> 00:02:31,692 Carbon has four electrons 68 00:02:31,692 --> 00:02:32,797 that are free to bond, 69 00:02:32,797 --> 00:02:34,133 nitrogen has three, 70 00:02:34,133 --> 00:02:35,412 oxygen two. 71 00:02:35,412 --> 00:02:37,274 So, carbon is likely to form four bonds, 72 00:02:37,274 --> 00:02:38,106 nitrogen three, 73 00:02:38,106 --> 00:02:39,603 and oxygen two. 74 00:02:39,603 --> 00:02:41,303 Hydrogen only has one electron, 75 00:02:41,303 --> 00:02:43,410 so it can only form one bond. 76 00:02:43,410 --> 00:02:44,890 In some special cases, 77 00:02:44,890 --> 00:02:46,432 atoms can form more bonds 78 00:02:46,432 --> 00:02:47,522 than you'd expect, 79 00:02:47,522 --> 00:02:49,998 but they better have a really good reason to do so, 80 00:02:49,998 --> 00:02:52,077 or things tend to fly apart. 81 00:02:52,077 --> 00:02:53,099 Groups of atoms 82 00:02:53,099 --> 00:02:55,112 that share electrons covalently with each other 83 00:02:55,112 --> 00:02:56,819 are called molecules. 84 00:02:57,557 --> 00:02:58,578 They can be small. 85 00:02:58,578 --> 00:03:00,612 For example, every molecule of oxygen gas 86 00:03:00,612 --> 00:03:03,087 is made up of just two oxygen atoms 87 00:03:03,087 --> 00:03:04,523 bonded to each other. 88 00:03:04,523 --> 00:03:06,192 Or they could be really, really big. 89 00:03:06,192 --> 00:03:09,379 Human chromosome 13 is just two molecules, 90 00:03:09,379 --> 00:03:13,022 but each one has over 37 billion atoms. 91 00:03:13,022 --> 00:03:14,210 And this neighborhood, 92 00:03:14,210 --> 00:03:15,183 this city of atoms, 93 00:03:15,183 --> 00:03:18,230 is held together by the humble chemical bond.