Electronic Structure of the Atom

electron configuration is the arrangement of electrons of an atom, a molecule


Shells
    -The electron shells are labelled K, L, M, N, O, P, and Q; going from innermost shell   outwards. 
    -Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells.

Subshells
    -Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. 

    - Example: The first (K) shell has one subshell, called "1s"; the second (L) shell has two subshells, called "2s" and "2p"; the third shell has "3s", "3p", and "3d"; and so on.

• Each s subshell holds at most 2 electrons
• Each p subshell holds at most 6 electrons
• Each d subshell holds at most 10 electrons
• Each f subshell holds at most 14 electrons
• Each g subshell holds at most 18 electrons

Notation
   -Notation consists of a sequence of atomic orbital labels (e.g. for phosphorus the sequence 1s, 2s, 2p, 3s, 3p) with the number of electrons assigned to each orbital (or set of orbitals sharing the same label) placed as a superscript.

   -Examples:

       Hydrogen :  1s¹       (has one electron in the 1s-subshell)                                                  

       Lithium    : 1s² 2s¹   (has two electrons) 

       Phosphorus: 1s² 2s² 2p⁶ 3s² 3p³   (has fifteen electrons)    

                                            

Valence Electron
    - the electrons in the last shell or energy level of an atom
    - the electrons of an atom that can participate in the formation of chemical bonds with other atoms
    -The valence electrons increase in number as you go across a period
    

Periodic table group	Valence electrons
Group 1 (I)	1
Group 2 (II)	2
Groups 3-12	See note *
Group 13 (III)	3
Group 14 (IV)	4
Group 15 (V)	5
Group 16 (VI)	6
Group 17 (VII)	7
Group 18	8**

* The general method for counting valence electrons is generally not useful for transition metals. Instead the modified d electron count method is used.

** Except for helium, which has only two valence electrons.

    -Example: Valence electron of Phosphorus(1s² 2s² 2p⁶ 3s² 3p³)   : 2+3 = 5                                        

    
     
Core Notation
    - Instead of writing out all of the electrons in the configuration, you can write out just the ones since the last noble gas
    -Example: electron configuration of Magnesium is 1s 2 2s 2 2p 6 3s 2
                                               ---->［Ne］3s 2


Ground State
    -The condition of an atom, ion, or molecule, when all of its electrons are in their lowest possible energy levels, not excited. 
    -When an atom is in its ground state, its electrons fill the lowest energy orbitals completely before they begin to occupy higher energy orbitals, and they fill subshells in accordance with Hund's rule 


Excited State
  -An excited state is any state with energy greater than the ground state

  -Excited states tend to have short lifetimes; they lose energy either through collisions or by emitting photons to "relax" back down to their ground states. 

Hund's Rule
    -Developed by the German scientist,
    -A rule of thumb stating that subshells fill so that the number of unpaired spins is maximized. Also known as therule of maximum multiplicity.
    - Electrons are added to the lowest available energylevel (shell) of an atom.


Pauli Exclusion Principle
    -it was formulated by the Austrian-born physicist Wolfgang Pauli in 1925. 
    -The Pauli exclusion principle states that each electron must have its own unique set of quantum numbers that specify its energy.
    -no two electrons can occupy the same quantum state at the same place and time