6.3 |
Aufbau Principle |
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The aufbau principle deals with the filling up of electrons in energy levels. The relative order of various levels has to be known to begin with. We fill the orbitals (with two electrons of opposite spins in each orbital) from the lowest to the highest so that the atoms have maximum stability. The electronic configuration of atoms can be constructed by using these two principles, together with Hund's rule. Related to the spin multiplicity or degeneracy of orbitals. |
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In a nitrogen atom, there are seven electrons. The first two can be placed in the 1s orbital, the second two in 2s and the next three are placed in 2px, 2py and 2pz orbitals. They are not placed such that two are in 2px and one in 2py because the pairing in 2px (which takes up some energy) will reduce the spin multiplicity. By avoiding pairing, atoms attain higher spin multiplicity and a lower total energy. |
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Spin pairing is |
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The two electrons will have opposite spins. |
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It is the pairing in the same space that cause, repulsion and hence increase in energy. |
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