Quantum Theory is a theory describing the behavior and interactions of elementary particles or energy states based on the assumptions that energy is subdivided into discrete amounts and that matter possesses wave properties. Energy of electrons is quantized (they can only have certain amounts of energy). Electrons exhibit wavelike behavior. It is impossible to know the exact position as well as momentum of an electron at any given instant. The quantum-mechanical model of an atom includes all of these ideas. Let's begin, shall we?
The electron is a particle that must be in an orbital in the atom. It is a cloud of negative charge or wave function. The cloud is most dense where the probability of finding the electron is highest. Electron density is the density of an electron cloud. An atomic orbital is a region around the nucleus of an atom where an electron with a given energy is likely to be found. In other words, orbitals are in 3D space where the electrons most probably are. The energy of an electron is in its vibrational modes - like notes on a guitar string.
Photons are produced when high energy modes change to lower energy modes.
Like the Bohr model, the energy of the electron increases as n (principal energy levels) increases from 1 to 2 to 3, and so forth. Unlike the model, however, each principal energy level is divided into one or more sublevels, and each sublevel consists of one or more orbitals. There are several different kinds of orbitals. Each orbital has a different fundamental shape. They are designated by the letters s (spherical), p (dumbbell shaped), d, and f. Each orbital holds a maximum of 2 electrons.
S ORBITALS
The s orbital is a spherically-shaped region describing where an electron can be found, within a certain degree of probability.There is one sub-orbital. Each orbital holds a maximum of 2 electrons. Thus, there are 2 electrons in total.
P ORBITALS
The p orbital is a dumbbell-shaped region describing where an electron can be found, within a certain degree of probability. There are 3 sub-orbitals. Each orbital holds a maximum of 2 electrons. Thus, there are 6 electrons in total.
The p orbital is a dumbbell-shaped region describing where an electron can be found, within a certain degree of probability. There are 3 sub-orbitals. Each orbital holds a maximum of 2 electrons. Thus, there are 6 electrons in total.
D ORBITALS
The d orbital has 5 sub-orbitals. Each orbital holds a maximum of 2 electrons. Thus, there are 10 electrons in total.
The d orbital has 5 sub-orbitals. Each orbital holds a maximum of 2 electrons. Thus, there are 10 electrons in total.
F ORBITALS
The f orbital has 7 sub-orbitals. Each orbital holds a maximum of 2 electrons. Thus, there are 14 electrons in total.
ELECTRON SPIN
Electrons behave as though they were spinning on their own axis. The Pauli exclusion principle states that each orbital in an atom can hold at most 2 electrons and that these electrons must have opposite spins.
The most stable, lowest energy state of the atom is called the ground state. This is the way atoms normally exist.
AUFBAU PRINCIPLE
"Aufbau" is a German word meaning "building up." This principle states that electrons are added one at a time to the lowest energy orbitals available until all the electrons of the atom have been accounted for.
PAULI EXCLUSION PRINCIPLE
This principle states that an orbital can hold a maximum of 2 electrons. The 2 electrons must spin in opposite directions. The electrons are said to be paired when they have opposite spins, and occupy an orbital. An unpaired electron is a single electron present in an orbital.
HUND'S RULE
This rule states that electrons occupy equal-energy orbitals so that a maximum number of unpaired electrons results.
EXAMPLE:
*The atomic number of carbon is 6. The first 2 electrons go into the 1s orbital with their spins paired. The next 2 go into the 2s orbital with their spins paired. The fifth and sixth electrons occupy two separate 2p orbitals.
ELECTRON CONFIGURATION
The above is an orbital diagram, which shows how electrons populate orbitals. An up-arrow symbolizes an electron spinning in a counterclockwise direction. A down-arrow symbolizes an electron spinning in a clockwise direction. The direction of the arrows is based on the magnetic properties of the spinning electron, and the first electron entering an orbital is usually represented by an up-arrow.
There is an even more compact form
of showing the electron configuration.
The pattern is: Poe
P is the principle quantum number. o is the orbital type (s, p, d, or f). e, the superscript, is the number of electrons in the orbital. The sum of the superscripts equals the number of electrons in the atom. The diagram below can help to write this notation. Start from the top and go across, working your way down. For the most part, the principle quantum number is the period number.
Notice that in the above periodic table, the d-orbitals (yellow) are labelled 3 in the fourth row. This is because the 4s sublevel is actually lower in energy than the 3d. So, the d-orbitals are an exception.
EXAMPLES:
*How many and what type of electrons are in cesium?
Starting from the top, moving across, and working your way down from the periodic table, you should arrive at:
1s22s22p63s23p64s23d104p65s24d105p66s1
But this is rather long, so there is a shortcut method. Write the chemical symbol of the preceding noble gas. Then, add in the electrons that are remaining.You should arrive at:
[Xe]6s1
*Using the shortcut method, write the configuration for Barium.
[Xe]6s2
*What about an ion, like Ba2+?
*What about an ion, like Ba2+?
[Xe]
So, we are able to conclude that the chemical species Ba2+, Xe, and I- are isoelectronic, meaning , having the same electron configuration.
Next time we will discuss isotopes and mass spectrometers!
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