How are electrons placed in the atom?

•Introduction

• Everybody knows that there are electrons orbiting atoms, but how are they arranged in the atom? 

• There are actually a set of rules which govern the placement of electrons. 

• The rules are called AUFBAU Principle, Pauli Exclusion Principle and Hund's Rule. 

• However, these rules only apply to the ground state of an atom or ion.

•AUFBAU Principle

• The lowest energy levels are always filled first

• Conversely, electrons are lost from the highest energy levels first 

• The diagram shows the order in which we should fill up the subshells.

http://www.100ciaquimica.net/images/temas/tema3/ima/aufbau3g.gif

•Diagram showing different energy levels of various atoms

Scale/Quantum Number

Arrangement of electrons Orbital Elements

http://www.welsch.com/gallery/vektor/Energieniveauschema_634.jpg

Subshell

•Pauli Exclusion Principle

• If there are 2 electrons occupying the same orbital, the 2 electrons must be of opposite spins so as to stay at ground state

http://santasusana.org/pakelly/G%20Chemistry/Pauli.gif

•Hund's Rule

• If multiple orbitals of the same energy are available, Hund's rule states that unoccupied orbitals will be filled before occupied orbitals are filled up by electrons having different spins.

http://img.sparknotes.com/figures/5/5578bdf1aec90e46e14325a580fdbf6a/fig1_4.gif

Element: Carbon

•Anomalous Electron Configurations

Certain atoms do not follow the orbital-filling rules:  • Chromium's electron configuration is [Ar]3d54s1 instead of the

expected [Ar]3d44s2  if we are to follow the normal rules• Copper's electron configuration is [Ar]3d104s1 instead the

expected [Ar]3d94s2   • There are others in the heavier d-block metals and the f-

block metals. Molybdenum and Rhodium are some examples.

 This occurs when there are enough electrons to produce a half-filled set of degenerate orbitals (as in the case of chromium) or to completely fill a d or f subshell (like copper).

•ANALOGY

The seat a Singaporean would take in a bus.

• Orbital: A set of two seats beside one another.• AUFBAU principle: One would try to be as near the front as

possible since it requires lesser energy.• Pauli exclusion principle: No 2 people are exactly the same.• Hund's Rule: People would rather not to sit next to each

other.• Subshell: A set of orbitals beyond which the force of

attraction towards the front is lower than the force of repulsion from other people, when it is not completely full.