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Oct 31, 2018

Electronic Configuration of Elements

Orbital Energy Ordering in Multi-electron Atoms:

The orbitals of a multi-electron atom are not likely to be quite the same as the hydrogen atom orbitals. For practical purpose, however, the number of orbitals and there shapes in multi-electron cases may be taken to be the same as for the hydrogen orbitals. In multi-electron atoms experimental studies of spectra shows that orbitals with the same vale of n but different l values have different energies.
Orbital Energy Ordering in Multi-electron Atoms
Orbital Energy Diagram For Multi-Electron Atom
The 3S orbital is lower energy then 3P orbitals which again are of lower energy then 3d orbitals. However orbitals belonging to a particular type ( P or d or f ) will be of equal energy (degenerate) in an atom or an ion. For example the three P orbitals or the five d orbitals originating from same n will be degenerate. The separation of orbitals of a major energy level into sub-levels is primarily due to the interaction among the many electrons.
This interaction leads to the following relative order of the energies of each type of orbitals: 

  • 1S ㄑ2S ㄑ2Pㄑ 3S ㄑ3P4S3d4P5S4d5P6Sㄑ4fㄑ5d6P ㄑ7S ㄑ5f ....
Admittedly it is often difficult for the readers to remember the orbital energy diagram . A trivial but distinctly more convenient way is to make give as,

trivial orbital energy diagram
Orbital Occupancy Order
The different orbitals originating from the same principle quantum number n are written in the horizontal lines. Now inclined parallel lines are drown through the orbital according to the above picture. Filling up the different orbitals by electrons will follow these lines.

A element which contain 36 electrons the electronic configuration of this element according to the above diagram is,

  • Multi-electron Atoms : the Aufbau or Building Up Principle:
The question that arise now how many electrons can be accommodated per orbital. The answer to this follows from Pauli's Exclusion Principle.

  • Pauli's Exclusion Principle:
No two electrons in an atom can have the same four quantum numbers. 
This principle tells us that in each orbital maximum of two electrons can be allowed. The two electron have the same three quantum numbers namely the same n, same l and the same mⳑ. Any conflict with Pauli Principle can now be avoided if one of the electrons has the spin quantum numbers is (+1/2) and (-1/2).
An alternative statement of the Pauli's Exclusion Principle is ,
No more than two electrons can be placed in one and the same orbital.
when two electrons with opposite spins exists in an orbital, the electrons are said to be paired. These two electrons per orbital given the maximum accommodation of electron in an atom.

  • Capacities Of Electronic Levels:
The total number of electrons for a particular n is given by 2n².

Capacities Of Electronic Levels
Capacities Of Electronic Levels

To determine the electronic configuration of elements the procedure is to feed electrons in different orbitals obeying certain rules.
The Aufbau or Building up Principle is bases on the following Rules:
  • Electrons are fed into orbitals in order of increasing energy (increasing n ) until all the electrons have been accommodated.
  • Electrons will tend to maintain maximum spin. So long orbitals of similar energy are available for occupation electrons will prefer to remain unpaired. in other words electrons tend to avoid the same orbital, that is, hate to share space. This rule is Known as Hund's Rule of maximum spin Multiplicity.
  • Spin pairing can occur only when vacant orbitals of similar energy are not available for occupation, and when the next available vacant orbital is of higher energy.  

  • Electronic Configuration of Elements:

Electronic Configuration of Elements H to Ne:

Hydrogen (Atomic Number 1) has its only one electron in the 1s orbital and this electronic configuration represented below. the bar on the pictorial representation indicates the orbital and the arrow ↑ a single spinning electron in the orbital. In helium (Atomic Number 2) the second electron occupy the 1S orbital since the next 2S orbital is much higher energy. Obeying Pauli principle the configuration 1S² represented below.

  • From the above rule we can represented the electronic configuration from H to Ne.
  • Hydrogen (H ) 1S¹
  • Helium (He) 1S²
  • Lithium(Li)  1S²2S¹
  • Beryllium (Be)  1S²2S²
  • Boron (B)  1S²2S²2P¹
  • Carbon (C)  1S²2S²2P²
  • Nitrogen (N)  1S²2S²2P³
  • Oxygen (O)  1S²2S²2P⁴
  • Fluorine (F) → 1S²2S²2P⁵
  • Neon (Ne)  1S²2S²2P⁶
Pictorial Representation of H, He, Li, Be, B
Pictorial Representation of H, He, Li, Be, B

  • Electronic Configuration of Elements Na to Ar:
After neon the next available orbital is 3S being followed by 3P. The orbitals are then progressively filled by electrons. Thus the electronic configuration of Na to Ar given below.

  • Element  Symbol  Atomic Number  Electronic Configuration
  • Sodium  Na → 11  1S²2S²2P⁶3S¹
  • Magnesium  Mg → 12  1S²2S²2P⁶3S²
  • Aluminium  Al → 13  1S²2S²2P⁶3S²3P¹
  • Silicon  Si → 14  1S²2S²2P⁶3S²3P²
  • Phosphorus  P 15  1S²2S²2P⁶3S²3P³
  • Sulphur  S → 16  1S²2S²2P⁶3S²3P⁴
  • Chlorine  Cl → 17  1S²2S²2P⁶3S²3P⁵
  • Argon  Ar → 18  1S²2S²2P⁶3S²3P⁶
Pictorial Representation of sodium to argon
Pictorial Representation of Na to Ar

  • Electronic Configuration of Elements K and Ca:
Then 4S orbital, being of lower energy then the 3d, is filled. the elements involves potassium and calcium are represented as,
  • Element  Symbol  Atomic Number  Electronic Configuration
  • Potassium  K → 18  1S²2S²2P⁶3S²3P⁶4S¹
  • Calcium  Ca  18  1S²2S²2P⁶3S²3P⁶4S²

pictorial representation of potassium and Calcium
Pictorial Representation Of K and Ca
  • Electronic Configuration of the First Series of Transition Elements:
In scandium the twenty first electron goes to 3d orbital, the next available higher energy orbital. There are five 3d orbitals with the capacity of ten electrons. From Scandium to Zinc These d orbitals are filled up. Presence of partly filled d orbitals generates very special properties.. Elements with partly filled d or f orbitals in elementary state or in ionic state are called transition elements. The electronic configuration of the elements from Scandium To Zinc are,
  • Element  Symbol  Atomic Number  Electronic Configuration
  • Scandium  Sc → 21  1S²2S²2P⁶3S²3P⁶4S²3d¹
  • Titanium  Ti  22  1S²2S²2P⁶3S²3P⁶4S²3d²
  • Vanadium  V  23  1S²2S²2P⁶3S²3P⁶4S²3d³
  • Chromium  Cr  24  1S²2S²2P⁶3S²3P⁶4S²3d⁴

Pictorial Representation of Sc To Cr
Pictorial Representation of Sc To Cr
In reality experimental studies on chromium reveals thus the electron configaration of Chromium represented as, 
Chromium  Cr  24  1S²2S²2P⁶3S²3P⁶4S¹3d
This reordering of electrons is due to an extra stability associated with a half filled or a filled sub shell. For Chromium Half filled Sub shell associated this extra stability. 
  • Manganese  Mn  25  1S²2S²2P⁶3S²3P⁶4S²3d⁵
  • Iron  Fe  26  1S²2S²2P⁶3S²3P⁶4S²3d⁶
  • Cobalt  Co  27  1S²2S²2P⁶3S²3P⁶4S²3d⁷
  • Nickel  Ni  28  1S²2S²2P⁶3S²3P⁶4S²3d⁸
  • Copper  Cu  29  1S²2S²2P⁶3S²3P⁶4S²3d⁹
For Copper Filled subshell associated this extra stability and the electronic conflagration is,
Copper  Cu  29  1S²2S²2P⁶3S²3P⁶4S¹3d¹

  • Zinc  Zn  30  1S²2S²2P⁶3S²3P⁶4S²3d¹                                                                                                                  

Pictorial Representation of Sc To Cr
Pictorial Representation of Mn To Zn
  • Electronic Configuration of Elements Ga to Kr:
The next available higher energy orbital is 4p and it contain six electrons thus the electronic Configuration from Ga to Kr are,
  • Gallium  Ga  31  1S²2S²2P⁶3S²3P⁶4S²3d¹⁰4P¹
  • Germanium  Ge  32  1S²2S²2P⁶3S²3P⁶4S²3d¹4P²
  • Arsenic  As  33  1S²2S²2P⁶3S²3P⁶4S²3d¹4P³
  • Selenium  Se  34  1S²2S²2P⁶3S²3P⁶4S²3d¹4P⁴
  • Bromine  Br  35  1S²2S²2P⁶3S²3P⁶4S²3d¹4P⁵
  • Krypyon  Kr  36  1S²2S²2P⁶3S²3P⁶4S²3d¹4P⁶