Periodic Table for Biology – Chemistry

00:00 So of course we can't leave the periodic table out of the conversation when we're talking chemistry.

00:07 But thankfully, as biologist we only have to look at the first three periods and these first three periods correspond with three energy levels that we've talked about previously. Lets look at carbon for example. Carbon is atomic number 6 which means by definition it has 6 protons. We also see that it has an atomic mass of 12. This atomic mass tells us that there are also 6 neutrons.

00:41 Each of those has an atomic mass of 1, 6+6 is 12. So now, lets look at how we can tell from looking at any atom in the periodic table what its electron arrangement might be in the energy shells.

01:00 Here we're looking at the second period. Carbon is the second row down in the table.

01:05 It's the second period. This means the carbon has two electron shells. The first shell is full with two electrons. The second shell is going to be full with 8 but it only has 4.

01:22 Hence atomic number 6. Also we can see that carbon is in the fourth column. Again, we only need to look at the blue part. In the second period meaning we have 2 electron shells and 4 spots over meaning that there are 4 electrons in that outermost shell presently. Now how would carbon be most comfortable. The octet rule applies here to say that carbon would probably be most comfortable if it had its outer electron shell full. In which case it has the potential to get 4 more electrons in its outer shell. So in biology, there are only six major elements that we need to know as well as some electrolytes like potassium and calcium and sodium. But the main 6 elements are called the SPONCH elements. For ease of memory, SPONCH makes complete sense. We have sulfur, we have phosphorus, we have oxygen, nitrogen, carbon, the cornerstone of biological molecules and then hydrogen.

02:40 So once again luckily we only have to deal with a certain smaller portion of chemistry in our study of biology. But our understanding of those atoms is pretty important. So in order to consider bonding arrangements, we really are concerned mostly with the outermost shell. We call this the valence shell. The number of electrons in the valence shell is how many valence electrons we have.

03:08 We know that the first shell can carry two electrons. So that's illustrated in period 1.

03:15 Period 2 shows us that the second shell can be filled with 8 electrons and period 3 also can be filled with 8 electrons. In order to fulfill the octet rule, an atom is looking to be happy having 8 electrons in its outermost shell. How many electrons that an atom would like to have is considered to be its valence. So we say that an atoms valence is how many electrons it would need to satisfy the octet rule. So when an atoms valence shell is full, its non-reactive because it doesn't need to find anymore electrons to fill that shell. For example here, helium has its innermost shell its only shell full with two electrons. So lets consider nitrogen. How many valence electrons does it have? We can count the electrons in the outermost shell. We know that it has 5 electrons in its outermost shell. But lets contrast that to the question of what is the valence of nitrogen? The valence again is how many would it like to satisfy the octet rule. In this case, it would like 3 in order to satisfy that octet rule. So it has a valence of 3. It would like to find 3 more electrons.

04:42 What about helium? It has an outer shell, one shell. It's the innermost shell. It's full with 2 electrons.

04:51 So it has no valence. It has no desire to bond or associate with other atoms. From this lecture, you should have gained a great understanding of atomic structure. At this point, you should be able to diagram an atom including the protons, neutrons and electron arrangements.

05:12 In addition, you should have a great understanding of what valence means so that in the next lecture we can move on and explore the chemical foundations of biology a little further.