Modern atomic theory uses probability to describe the motion of the electron. In 1926, Erwin Schrödinger (pictured in glasses) treated the electron as a wave and developed a model of the atom which combines quantum theory and wave theory. The Schrödinger model tries to describe the regions in space where electrons are most likely to be found. Heisenberg's Uncertainity Principle
says that it is impossible to determine both the exact location of the electron and its velocity at the same time. The more certain we are of one, the less certain we can be of the other. Therefore, quantum mechanics uses computers to determine thousands of points where the electron is located.
These points form a three dimensional electron cloud where the probability of finding the electron is high. So, the fixed orbits of the Bohr Theory were replaced with a cloud of electrons around the nucleus. The modern orbital is the region of space where the probability of finding an electron is high.
The size (diameter) of the orbital depends on the energy level. The higher the E level, the larger the cloud. The shape of the cloud depends upon the motion of the electron within the E level. A single electron cloud is called an orbital. Similar orbitals of the same size (diameter) make up energy sublevels. The circular orbits of the Bohr Theory have been replaced with spherical and other shaped electron clouds.
There are 4 types of orbitals.
Each orbital can hold a maximum of 2 electrons.
s |
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p |
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d |
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f |
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Suppose you had a single hydrogen atom and at a particular time you plotted the position of the electron, in the 1s orbital. Then, later you did it again and found the electron to be in a new position. You don't know the path it took to get there, but you keep plotting the position of this electron at different points in time. Eventually, you would build up a 3D map of sorts that shows where the electron is likely to be found. The picture to the left shows just such a map.
The orbital here is a 2s orbital. This is similar to a 1s orbital except that the region where there is the greatest chance of finding the electron is further from the nucleus, on the second energy level.
A p orbital looks like a dumbbell or two balloons stuck together at the ends. Because there are 3 p orbitals, each of the p orbitals is found to lie along an x, y, or z axis.
Here is another representation of the orbital shapes. Remember that these orbitals are not like rigid containers. They are merely a visual representation of the areas of space where the probability of finding an electron is high.
Watch this video from Khan Academy called An Introduction to Orbitals. You only need to view up to 8 minutes and 43 seconds. The remainder of this video will be covered later in the content. Remember to add to your notes on the content as you watch! And, since these topics are very abstract, you may need to read and watch multiple times before things start to make sense.
Orbital Diagrams
We draw orbital diagrams to show the distribution of electrons in a sublevel. Boxes or circles are used to represent the individual orbital. (The shape means nothing!) Arrows are used to represent electrons in the orbital. The first electron in an orbital is represented by a 
and the second by a 
.
Electrons fill orbitals that have the same energy unpaired, until there is no other option but to pair up. Take the 2p subshell, for example. There are 3 orbitals in the 2p subshell. Each can hold 2 electrons, for a total of 6 electrons. Electrons go into each of those orbitals one at a time. When 3 electrons have each occupied an orbital, the 4th electron then pairs up with one of the electrons.
Electron 4 must pair up, but it has an opposite spin.
Electron 5 and 6 would also pair up, each with an opposite spin.
Since electrons all have a negative charge, the question you should be asking is, "What keeps them from flying apart?" Each electron spins on its axis. One spins clockwise and the other spins counterclockwise. When charged particles spin, they act like tiny magnets. Since the two electrons spin in opposite directions, one acts like the north pole of a magnet and the other acts like the south pole. This makes the electrons attract enough to overcome their natural repulsion of one another, but not attract enough to collide.
Hund's Rule
The way we just filled the orbitals within each sublevel is known as Hund's rule. It states that each orbital within a sublevel is half filled before any is completely filled. You can imagine Hund as a card dealer. When you play a game of cards, you must first deal each player one card before you deal each player a second card. Similarly, you must place one electron in each orbital of the same energy level, before you place the second one.
Hopefully you are asking yourself, why do we need to draw orbital diagrams? What information do they tell us?
They are an excellent visual representation that shows the number of paired and unpaired electrons.
Recall that electrons spin and create a magnetic field. When electrons are paired within an orbital, the spins and therefore the magnetic fields cancel each other out. Atoms with unpaired electrons are found to behave like tiny magnets that can be attracted to an external magnetic field. This weak attraction to a magnet is called paramagnetism. Substances in which all the electrons are paired are not attracted to a magnet and are said to be diamagnetic.
