Back To CourseCLEP Biology: Study Guide & Test Prep
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We learned already that electrons are negatively charged subatomic particles. We also talked about how the electrons are found in clouds around the nucleus. You've probably seen a lot of symbols for atoms, probably something like this where the electrons are represented as having specific orbits. What actually is the case is that the electron cloud is a prediction of the area in which the electrons can be found.
So, if we have our nucleus in the middle and we think about this circle as being a 3-dimensional sphere around the nucleus, we can say that any given time, if I was to take a snapshot of this atom, that the electron would be found somewhere in this sphere or this cloud around the nucleus. But I couldn't be sure exactly where it is. If I were to take different photos at different times, then the electron would be in an unpredictable, nonrandom place somewhere in that cloud.
The electron cloud is just giving us a prediction of the area in which the electrons can be found. There's also energy associated with the electron clouds. The electron clouds are also known as electron shells, and they're also known as energy levels.
Let's just make a representation in 2-D of an atom. If we have our nucleus here, I can draw one energy level like this, and I can put an electron here. Now, think about subatomic particles in terms of their charge. The electron has a negative charge, and the protons in the nucleus have a positive charge. Negative and positive things attract each other.
If I had two powerful magnets here, and one end has a positive charge, and one end has a negative charge, they're going to want to move together. Now, the fact that the electron is orbiting out here outside of the nucleus, that it's found out here away from the positive charge, means that it's moved away, farther away than that charge would probably prefer to be.
If I was to take this negative magnet and move it across the table and if these are powerful magnets, then I'm going to have to exert some force to keep this negative magnet away from the positive one. Otherwise, it's going to move rapidly toward that positive magnet. What I've created here is something called potential energy. By moving this magnet away, I've created this scenario where this magnet wants to move; it wants to release this potential energy as kinetic energy, or it wants to move towards the positive magnet.
Now, when I have this nucleus think about it in terms of energy levels, we think about it in terms of energy levels because it took energy to have this electron out here at this orbital. So, there's some sort of energy associated with this energy level, and if I have multiple energy levels, I'm going to have even more energy associated with this second energy level. As I move out with each energy level away from the nucleus, the electrons that occupy these energy levels have more and more energy because they've been pulled farther and farther away from this nucleus.
If I was to talk about energy levels in terms of the first energy level or the second energy level or the third energy level, there's more energy associated with energy levels as they increase in number. So, energy is increasing as we increase the number of the energy levels.
We can also talk about energy levels in terms of the number of electrons that can occupy those levels. I'll draw my first energy level, and it's referred to as 1s, so one for the energy level, and the letter that is associated with the number is basically defining the shape of the electron cloud. It's not going to be important for our purposes to actually know what all the shapes are, but just be aware that this is what that is describing.
So, each orbital has a different kind of shape. An s orbital has a specific kind of shape, and in a moment we'll talk about a p orbital, which has yet another kind of shape. Each orbital surrounding the nucleus can be occupied by two electrons. Let's draw our nucleus again, and I'll have orbital one. I can have an electron here, and I can have an electron here. So, this would be a full orbital. If I then go out to the second energy level, the second energy level is defined by 2s and then three kinds of 2p's for a total of eight electrons.
Just remember that this is a representation of an atom, that this is a way for us to count electrons when we want to talk about atoms and how they interact with other atoms.
It's important to be able to define the electrons because the electrons are going to be able to define the reactivity of atoms with other atoms. We talk about the electrons in the outermost shell. In this case, let's say I have one electron shell and have one electron in this orbital. This electron level is called the valence electron level, and the electrons in this orbital are the ones that are going to interact with other atoms.
Now, atoms want to have full electron shells. We've talked about 1s as being able to hold two electrons, so this atom with only one electron in this valence level is unstable, and it wants to have a full valence level. Take another example - if there are two electron levels here, and I have one electron out here, this valence level has one electron. If I added another electron, that's not going to help me, because at the second energy level, I'll want to have eight electrons. So, I would've had to add seven other electrons to fill this valence level. So, I could also just remove this electron and end up with two electrons in the 1s level - that then would be more stable, because this second electron level has been emptied.
If we take a look again at the periodic table, we can see now how the organization of the periodic table is going to help us predict the reactivity of various kinds of atoms. Generally speaking, in biology, we're going to talk about the first, second and 13th-18th columns. What you can see here is that these first, second and these other columns, three, four, five, six, seven, and eight, are sometimes referred to as groups 1-8, signifying atoms that have 1-8 valence electrons.
All of the atoms are aspiring to be like atoms in the eighth column, because all of these atoms have eight electrons and are most stable. These atoms are referred to as Noble gases.
We've learned a lot about electrons in this lesson. We've learned that the electron shell defines the space around the nucleus where the electrons are most likely to be found. We've also learned that this can be referred to as an energy level, and we've also learned that the electrons in the outermost energy levels are referred to as valence electrons. Finally, we've learned a little bit about the organization of the atoms in the elements in the periodic table, and how the atoms without a complete valence shell aspire to be more like the Noble gases, which have a full valence level.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 224 lessons