If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:38

Video transcript

hello I'm Hank I assume that you are here because you are interested in biology if you are that makes sense because like any good 50-cent song biology is just about sex and not dying and everyone watching this should be interested in sex and not dying being that you are I assume a human being I'm going to be teaching this biology course a little differently than most courses you've ever experienced for example I'm not going to spend the first class talking about how I'm going to teach the class I'm just going to start teaching the class starting right after this next cut first I just wanted to say if I'm going too fast for you the great thing about me being a video and not a person is that you can always go back and listen to what I've said again I promise I will not mind you are encouraged to do this often a great professor of mine once told me that in order to understand any topic you only really need to understand a bit of the level of complexity just below that topic the level of complexity just below biology is chemistry or if you're a biochemist you would probably argue that it's biochemistry so we need to know a little bit more about chemistry and that is where we're going to start I'm a collection of organic compounds called Hank green an organic compound is more or less any chemical that contains carbon and carbon is awesome why lots of reasons I'm gonna give you three first carbon is small it doesn't have that many protons and neutrons almost always twelve rarely it has some extra neutrons making it c-13 or C 14 because of that carbon does not take up a lot of space and can form itself into elegant shapes it can form rings it can form double or even triple bonds it can form spirals and sheets and all kinds of really awesome things that bigger molecules would never manage to do basically carbon is like an Olympic gymnast it can only do the remarkable and beautiful things it can do because it's petite second carbon is kind it's not like other elements that desperately want to gain or lose or share electrons to get the exact number they want no carbon knows what it's like to be lonely so it's not all I could live without your electrons needy like chlorine or sodium is this is why chlorine tears apart your insides if you breathe it in gaseous form and why is sodium metal if ingested will explode carbon now it wants more electrons but it's not going to kill for them it's easy to work with it makes and breaks bonds like a 13-year old mall rat but it doesn't ever really hold a grudge third carbon loves to bond because it needs four extra electrons so it will bond with whoever happens to be nearby and usually it will bond with two or three or four of them at the same time carbon commanda lots of different elements hydrogen oxygen phosphorus and nitrogen and other atoms of carbon it can do this an infinite configurations allowing it to be the core element of the complicated structures that make living things like ourselves because carbon is small kind and loves to bond life is pretty much built around it carbon is the foundation of biology so fundamental that scientists have a hard time even conceiving of life that is not carbon-based silicon which is analogous to carbon in many ways is often cited as a potential element for alien life to be based on but it's bulkier so it doesn't form the same elegant shapes as carbon it's also not found in any gases meaning that life would have to be formed by eating solid silicon whereas life here on earth is only possible because carbon is constantly floating around err in the form of carbon dioxide carbon on its own is an atom with six protons six electrons and six neutrons atoms have electron shells and they need or want to have these shells filled in order to be happy fulfilled atoms the first electron shell called the S orbital needs two electrons to be full then there's the second S orbital which also needs to carbon has this filled as well then we have the first p orbital which needs six to be full carbon only has two left over so it wants four more carbon forms a lot of bonds that we call covalent these are bonds where the atoms actually share electrons so the simplest carbon compound ever methane is carbon sharing four electrons with four hydrogen atoms hydrogen only has one electron so it wants its first S orbital full carbon shares its four electrons with those four hydrogen's and those four hydrogen's each share one electron with carbon so everybody's happy this can all be represented with what we call Lewis dot structures Gilbert Lewis also the guy behind Lewis acids and bases was nominated for the Nobel Prize 35 times and won none this is more nominations than anyone else in history and roughly the same number of wins as everyone else lewis disliked this a great deal he may have been the most influential chemist of his time he coined the term photon he revolutionized how we think about acids and bases he produced the first molecule of heavy water and he was the first person to conceptualize the covalent bond that we're talking about right now but he was extremely difficult to work with he was forced to resign from many important posts and was also passed up for the Manhattan Project so while all of his colleagues worked to save his country Lewis wrote a horrible novel Lewis died alone in his laboratory while working on cyanide compounds after having had lunch with a younger more charismatic colleague who had won the Nobel Prize and worked on the Manhattan Project many suspect that he killed himself with the cyanide compounds that he was working on but the medical examiner said heart attack without really looking into it I told you all that because well the little Lewis structure that I'm about to show you was created by a deeply troubled genius it's not some abstract scientific thing that has always existed someone somewhere thought it up and it was such a marvelously useful tool that we've been using it ever since biology most compounds can be shown in lewis structure form and one of the rules of thumb when making these diagrams is that some elements tend to react with each other in such a way that each atom ends up with eight electrons in its outermost shell that's called the octet rule because these atoms want to complete their octet of electrons to be happy and satisfied oxygen has six electrons in its outer shell and needs to which is why we get h2o it can also bond with carbon which needs four so two double bonds to two different oxygen atoms you end up with co2 that pesky global warming gas and also the stuff that plants and thus all life are made of hydrogen has five electrons in its outer shell here's how we count them there are four placeholders each one's two atoms and like people getting on a bus they prefer start out not sitting next to each other I'm not kidding about this they really don't double up until they have to so we get it out 1 2 3 4 5 so for maximum happiness nitrogen bonds with 3 hydrogen's forming ammonia or with two hydrogen's sticking off another group of atoms which we call an amino group and if that amino group is bonded to a carbon that is bonded to a carboxylic acid group you have an amino acid sometimes electrons are shared equally within a covalent bond like with o2 that's called a nonpolar covalent bond but often one of the participants is more greedy in water for example the oxygen molecule sucks the electrons in and they spend more time around the oxygen than around the hydrogen's this creates a slight positive charge around the hydrogen's and a slight negative charge around the oxygen when something has a charge we say that it's polar it has a positive and negative pole so this is a polar covalent bond ionic bonds occur when instead of sharing electrons atoms just donate or accept an electron from another atom completely and then live happily as a charged atom or ion atoms would in general prefer to be neutral but compared with having the full electron shells it's not that big of a deal most common ionic compound in our daily lives that would be Google table salt NaCl sodium chloride but don't be fooled by its deliciousness sodium chloride as I previously mentioned is made of two very nasty elements chlorine is a halogen or an element that only needs one proton to fill its octet while sodium is an alkali metal an element that only has one electron in its octet so they will happily tear apart any chemical compound they come in contact with searching to satisfy the octet rule but no matter outcome could occur than sodium meeting chlorine they immediately transfer electrons so sodium doesn't have it's extra and the chlorine fills its octet they become na+ and cl- and are so charged that they stick together and that stickiness is what we call an ionic bond these chemical changes are a big deal remember sodium and chlorine just went from being deadly to being delicious they're also hydrogen bonds which aren't really bonds so much so you remember water I hope you didn't forget about water water is important since water is stuck together with a polar covalent bond the hydrogen bit of it is a little bit positively charged and the oxygen is a little negatively charged so when water molecules move around they actually stick together a little bit hydrogen side to oxygen side this kind of bonding happens in all sorts of molecules particularly in proteins and plays an extremely important role and how proteins fold up to do their jobs it's important to note here bonds even when they're written with dashes or solid lines or no lines at all are not the same strength sometimes ionic bonds are stronger than covalent bonds though that's the exception rather than the rule and covalent bond strength varies hugely in the way that those bonds get made and broken is intensely important to how life and our lives operate making and breaking bonds is the key to life itself it's also like if you were to swallow some soda you metal the key to death keep all of this in mind as you move forward in biology even the hottest person you have ever met is just a bunch of chemicals rambling around in a bag of water and that among many other things is what we're going to talk about next time
Biology is brought to you with support from the Amgen Foundation