Electron configurations
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ELECTRON CONFIGURATIONS. Electron Configuration:. A method for communicating the location and number of electrons in the electron energy levels. 3p 5 Principal Quantum Number = 3 Orbital = p Number of electron(s) in the orbital(s) = 5.

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Electron configuration
Electron Configuration:

A method for communicating the location and number of electrons in the electron energy levels.


Principal Quantum Number = 3

Orbital = p

Number of electron(s) in the orbital(s) = 5

Electron configurations

  • The arrangement of electrons follows the Aufbau Principle.

    (means building-up in German) in the ground state, the electrons will fill the atomic orbital of lowest energy.

  • An electron configuration is a listing of the number and kinds of electrons in order of increasing energy, written in a single line.

  • Example: Li 1s2 2s1

  • The order, from left to right, is the order of increasing energy of the orbitals

Aufbau diagram
Aufbau Diagram:

Procedure for writing and electron configuration
Procedure for Writing and Electron Configuration:

  • Step 1: Determine the position of the element in the periodic table and the total number of electrons in the atom or simple ion.

  • Step 2: Start assigning electrons in increasing order of main energy levels and sublevels.

  • Step 3: Continue assigning electrons by filling each sublevel before going on to the next sublevel, until all the electrons are assigned.

Electron configurations

ElementElectron configuration

1H 1s1

2He 1s2

3Li 1s22s1

5B 1s22s22p1

7N 1s22s22p3

13Al 1s22s22p63s23p1

17Cl 1s22s22p63s23p5

21Sc 1s22s22p63s23p64s23d1

23V 1s22s22p63s23p64s23d3

Electron configurations


Chromium (24 electrons)

EC = 1s2 2s2 2p6 3s23p64s23d4 is INCORRECT

EC = 1s2 2s2 2p6 3s23p64s13d5 is CORRECT!

The d-orbital becomes half-filled so, it changes to 4s13d5 instead.

The next element, Manganese will have an EC of:

1s2 2s2 2p6 3s23p64s23d5

This exception also appears near the end of the d orbital filling
This exception also appears near the end of the d-orbital filling:


EC = 1s2 2s2 2p6 3s23p64s23d9 is INCORRECT

EC = 1s2 2s2 2p6 3s23p64s13d10is CORRECT

Additional exceptions are Mo 5s14d5; Ag 5s14d10; Au 6s15d10 That is reasonable considering their position on the periodic chart.

Expanded and condensed electron configurations
Expanded and Condensed Electron filling: Configurations

  • What you had just learned is the expanded notation of writing EC.


    Na: 1s2 2s2 2p6 3s1Expanded form

    Na: [Ne] 3s1Condensed form

    The “ [Ne] ” is called a kernel = [closest noble gas]


Electron configurations


Similar to EC, except, you are now expected to draw out the orbitals and place the electrons to show proper spinning.

Rules for drawing orbital-Box diagrams

  • The Pauli Exclusion Principle-

    each orbital can hold 2e-, where those 2e- are always in oppositespin.

    2) Hund's Rule –

    Electrons occupy all the orbitals of a given sublevel singly before pairing begins. Spins of electrons in different incomplete orbitals are parallel in the ground state.The most stable arrangement of electrons in the subshells is the one with the greatest number of parallel spins.

Electron configurations

Suppose we want to draw the orbital-box diagram of filling:carbon:

Step 1: Draw the orbitals.

Step 2: Fill the electrons in the 1st two s-orbitals, showing opposite spin. These electrons must fill the lower energy orbitals first before advancing to the next higher energy level (AUFBAU’s PRINCIPLE).

Step 3: Fill the 2p sublevels one at a time (HUND’s RULE).

Orbital diagrams examples
Orbital Diagrams filling:Examples

Electron configurations

1s filling:



Electron configurations
IONS filling:


The electrons in the outermost shell of the Bohr diagram.

Lose electrons = become positively charged (Cation)

Gain electrons = becomes negatively charged (Anion)

Electron configurations


Electrons do notcome out the same way as we put them in according to the Aufbau Principle.

Electrons leave the outer most shell first.

Let's look at V vs V2+

23V 1s22s22p63s23p64s23d3

23V2+ 1s22s22p63s23p63d3