Chemical Behavior – Chemistry

00:00 So the next question becomes why would this individual atoms interact? And I bet you can see already. We've talked about the plus and minus ions. Perhaps positive and negative are attracted to each other. That's one of the types of bonds we'll see. But another type of bond involves purely the electrons. And the behavior of the electrons on the atom. So it all has to do with electron arrangement around the nucleus of the atom. We have energy shells which we describe as somewhat of a railroad track around the center of the atom. Although that's not really the way it works. And electrons can occupy in biology, 1 of 3 electron shells. Each of these electron shells also has subshells or orbitals. The s and p-orbitals, you may have heard of before.

00:56 We're going to look at those in a little bit more detail. And there can only be in each of those orbitals or subshells, there can only be a maximum of 2 electrons. So lets take a closer look at what we mean by energy levels or energy shells. Here you can see that in chemistry there are many energy shells or energy levels. In biology luckily we're only concerned with the first 3.

01:25 The first energy shell has only 1 orbital. It's called the s-orbital. And the s-orbital has a spherical shape. Only contains 2 electrons. And we denote it with a spherical shape because the electrons can essentially be anywhere in the cloud around that nucleus. Now when we move out to the next shell we see that there are p-orbitals. When we move out to the second and third energy levels we have to consider the octet rule. The octet rule is just like it sounds. We have to have 8 electrons in the second and third shells. So, 2 in the first shell, 8 in the second and third.

02:11 We also have the s-orbital in the second shell. But in addition to that we have 3p-orbitals.

02:21 Each of those can be filled with 2 electrons a piece. So with 2 in the s and then 2 in each of 3p-orbitals, we could have a total of 8 electrons in the second and third shell. Now thank goodness in biology we only have to go to the third shell. Otherwise things could become much more complicated.

02:44 But in general we just need to consider the first shell that contains 2 electrons in 1s-orbital.

02:52 The second and third shells contain 1s-orbital and 3p-orbitals. Each of which can only contain 2 electrons which bring us back to the octet rule. So electrons can move between the energy levels.

03:08 And this is precisely what brings us to why atoms are attracted to each other and how they might want to steal electrons or share electrons. In order to fill the octet rule, we might have to gain or lose electrons. But even before gaining or losing electrons, we have our different energy levels as you can see depicted here in our figure. And electrons can actually be pulled out away from their original shell towards a higher level shell. And it takes the energy input to put in that.

03:43 So you think about it as an excited state. When kids get excited about going to a dance, they jump around and there is lots of energy around, right. So in order to have electrons move up an energy level they need to have some energy put in. The thought of going to the dance perhaps could be this energy. So electrons can move up their energy level and then once we release the electron from that energy level back to ground state there is an energy release.

04:13 That energy can then be utilized to fuel biological processes. So again we can put energy in, say from the sunlight to excite electrons and add energy to a molecule. And we can also release energy by moving electrons closer to the nucleus. And utilize that energy for cellular processes.

04:36 as happens in cellular respiration which we will explore later on in this course.

04:43 So electrons can become so excited that they actually leave their original atom to join another atom or one atom could have such a draw for an electron that it steals it from the other.

04:58 So in this case we have A and B. When A loses an electron and gives it to B, then B becomes more negative. Because its becomes more negative we call it reduced. So the process of gaining an electron is called reduction. So, gaining electron is reduction then the other molecule is going to lose an electron. So in this case, A is losing an electron and thus it becomes more positive. When an atom becomes more positive, we call it oxidised. So oxidation is the process of losing an electron. Reduction is the case of gaining an electron. And these two reactions can be paired to be called redox reactions. Generally when one thing loses an electron, another thing gains an electron. So oxidation and reduction or redox reactions are something that we're going to run in to frequently throughout this course.