The word Aufbau is German for building up.
The rules of placing electrons within shells is known as the Aufbau principle . The Aufbau principle states that the electrons added one at a time to the lowest energy orbitals available then proceeding to the one with higher energy. If an atom is ‘excited’, e.g., by being heated, one or more of its electrons may temporarily be transferred to an orbital of higher energy, but it will soon return to its ground state.
Hund's rule refer to a set of rules used to determine which is the term symbol that corresponds to the ground state of a multi-electron atom. They were proposed by Friedrich Hund.
The Hund’s rule states that in filling up a set of degenerate orbitals, the orbitals are occupied by one electron at a time with the electrons having the same spin.
Pauli’s Exclusion Principle
The Pauli’s Exclusion Principle is a quantum mechanical principle formulated by the Austrian physicist Wolfgang Pauli in 1925. It states that no two electrons in the same atom can have the same set of four quantum numbers. The consequence of this principle can be stated simply as: ‘Only two electrons may occupy an orbital, and they must have different spins’.
Quantum Theory / Mechanics
Types of Quantum No.
An orbital can be described by the four quantum numbers n, l, ml and ms.
Principal Quantum Number, n
The principal quantum number, n, was proposed by Niels Bhor. It Describes the electron shell or main energy level of an orbital in which the electron is present.. The value of n ranges from 1 to "n", where "n" is the shell containing the outermost electron of that atom.
By knowing these quantum numbers many things like energy of electron, number of sub shells in an orbit, number of orbital’s in an orbit and number of electrons in a orbit can be calculated
As the value of 'n' increases, the distance from the nucleus as well as the energy of the electrons increases.
This number also represents the radial distance that the region where the electron is most likely to be found extends out from the nucleus. The higher the n, the greater is the radial distance.
- Maximum number of electrons in n is 2 n 2
Azimuthal Quantum Number, l
The azimuthal quantum number, l, is also called angular momentum number, orbital angular momentum quantum number, ORBITAL TYPE quantum number , or subsidiary number. It was introduced by Sommerfeld. It represents energy sublevels in n to which the electron belongs and can have values beginning with zero and increasing until the integer n-1 is reached (0 = s orbital, 1 = p orbital, 2 = d orbital, 3 = f orbital, etc.).
The azimuthal quantum number also defines the shape of the orbital. Orbitals have shapes that are best described as spherical (l = 0), polar (l = 1), or cloverleaf (l = 2). Orbitals even take on more complex shapes as the value of the angular quantum number becomes larger.
Magnetic Quantum Number, ml
The magnetic quantum number, ml,proposed by Lande. It describes the specific orbital within that subshell. and can have an integral value from –l to +l, including 0.
A faster way of determining the number of ml values per sublevel is by using the formula 2l+1.
Spin Quantum Number, ms
describes the spin of the electron within that orbital. Uhlenbeck and Goudsmith are the persons who proposed this quantum numbers.
There are only two possible values for this quantum number, ms: ms = + ½ and ms = - ½ . There two values corresponds to the only two possible orientations for the electron spin. These two orientations are loosely designated as clockwise and counterclockwise spins. These two orientations are also sometimes represented by an arrow pointing up and an arrow pointing down. These are also referred to as 'spin up' and 'spin down'.
When an electron is assigned to an orbital in atom, it may take either of the two possible orientations. If a second electron is assigned to the same orbital, it can only have an orientation opposite to that of the first.