An electron in an atom has four quantum numbers-n,l,ml,ms - associated with it as discussed in one of our tutorial.
An electron configuration of an atom is a perticular distribution of electrons amoung available subshells.
The notation of the configuration lists the subshell symbols,one after the other,with a superscript giving the number of electrons in that subshell.
Pauli exclusion Principle : Not all of the concievable arrangments of electrons amoung the orbitals of an atom are physically possible.Pauli exclusion priciple,which summarizes experimental observation,states that no two electrons in an atom has the quantum numbers n=1,l=0,ml=0,ms=+1/2, no other electron can have same quantum number.
Building up principle or Afbau principle :A scheme used to reproduce the electron configurations of the ground states of atoms by successively filling subshells with electrons in a specific order.Following this principle,you obtain the electron configuration of an atom by successively filling subshells in the following order:1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f. You need to memorize this order
The number of electrons in a neutral atom equal to the atomic number, you can obtain ground state configuration by placing the single electron into 1s orbital 1s1 , Similarly for He 1s2.For lithium first two electrons give configuration similar to He but the third goes into next higher orbital in the building up order because 1s is full the next electron should go to next higher orbital, this results in 1s2 2s1 configuration of Li .
Exceptions to the building-Up or Aufbau Principle:
There are some exceptions like Chromium , as per Aufbau rule its electronic configuration should be [Ar]3d94s2 , although the correct one is found exprimentally to be [Ar] 3d5 4s1 . These two configurations are actually very close in total energy because of the closeness in energies of the 3d and 4s orbitals.For that reason,small effect can influence which of the wto configurations is actually lower in energy.
Copper (Z=29) is another exception to the builiding-up principle,which predicts the configuration [Ar] 3d9 4s2 , although experiment shows the ground state configuration to be [Ar] 3d10 4s1.
Find the electron onfiguration of sulfur and iron atoms.
Step 1: Remember the sulfur atom has atomic number 16,so there are 16e- .
Step 2: Remember the afbau principle and start the fill the electrons into sublevels: Two electrons go into 1s sublevel,two into 2s sublevel,and six into 2p sublevel.Two more go into the 3s sublevel,and the remaining four go into the 3p sublevel.The electron configuration of the s atom is
1 s22s2 2p63s23p4
similarly, Iron atom has 26 electrons>proceeding as above,the electronic configuration must be,
The fill 4s sublevel before 3d sublevel