Electron Configuration

Objectives Describe the relationship between orbitals and energy levels for the electrons of an atom Describe how to write the electron configuration for an atom Explain quantum numbers and how they affect electron configuration of atoms Explain why the actual electron configurations for some elements differ from those predicted by the Aufbau principle

Important Vocabulary Atomic orbital Quantum numbers Principal quantum number angular momentum quantum number Magnetic quantum number Electron configuration Pauli exclusion principle Hund’s rule Aufbau principle

Atomic Orbitals An atomic orbital is best though of as a region of space in which there is a high probability of finding an electron Electrons are found in orbitals within energy levels Within each level, electrons occupy orbitals that have the lowest energy 4 different kinds of orbitals s, p, d and f

s Orbitals Are the simplest Are spherical in shape Have the lowest energy Hold only 2 electrons

p Orbitals Are dumbbell shaped They can be oriented three different ways in space Has more energy than a s orbital Each p orbital can hold 2 electrons for a total of 6 electrons

d & f Orbitals Are much more complex There are 5 possible d orbitals which are clover leaf shapes There are 7 possible f orbitals f orbitals have the greatest energy Each orbital holds a maximum of 2 electrons

Orbitals

Relationship between Levels & Sublevels 1st energy level has 1 sublevel = s So it contains 1 orbital and holds 2 electrons 2nd energy level has 1 s orbital and 3 p orbitals So it can hold 8 electrons 3rd energy level has s, p, & d orbitals So it can hold 18 electrons 4th energy level has s, p, d, & f orbitals So it can hold 32 electrons

Periodic Table Orbitals

Quantum Numbers Scientists have defined the region in which electrons can be found by using 4 quantum numbers A quantum number is a number that specifies the properties of electrons The principal quantum number, n, indicates the main energy level occupied by an electron Values of n are positive integers As n increases, the electron’s distance from the nucleus and its energy increases

orbital

Quantum Numbers Continued Main energy levels can be subdivided The sublevels are represented by the angular momentum quantum number, l This number indicates the shape or type of orbital of a particular sublevel Chemists use a letter code for this quantum number l = 0 = an s orbital l = 1 = an p orbital l = 2 = an d orbital l = 3 = an f orbital

Magnetic Quantum Numbers Symbolized by m Is a subset of the angular quantum number It also indicates the numbers and orientations of orbitals around the nucleus The value of m is in whole-number values but depends on the value of l 1 s orbital, 3 p orbitals, 5 d orbitals and 7 f orbitals

Spin Quantum Number Symbolized by + ½ and – ½ or ↑ and ↓ It indicates the orientation of an electron’s magnetic field relative to an outside magnetic field A single orbital can hold a maximum of 2 electrons, which must have opposite spins

Principal Quantum Number Angular Momentum Quantum Number Magnetic Quantum Numbers Spin Quantum Numbers

Electron Configurations Electrons in atoms tend to assume arrangements with the lowest possible energies An electron configuration is the written arrangement of electrons in an atom It shows the lowest-energy arrangement of the electrons for an element It is a shorthand notation 3 rules determine it

Pauli Exclusion Principle Was established by the German chemist Wolfgang Pauli in 1925 It states that each orbital can hold a maximum of 2 electrons In other words, no two electrons in the same atom can have exactly the same four quantum numbers In addition, if one electron has a spin quantum number of + ½ than the other must be – ½

The Aufbau Principle Also helps to write the electron configuration for an atom It states that electrons fill orbitals that have the lowest energy first Order for filling orbitals is as follows: 1s ‹ 2s ‹ 2p ‹ 3s ‹ 3p ‹ 4s ‹ 3d

Hund’s Rule States that orbitals of the same n and l quantum numbers are each occupied by 1 electron before any pairing occurs

Electron Configuration Example Let’s do magnesium (Mg) It has 12 electrons

Electron Configuration 1s2 2s2 2p6 3s2 or [Ne]3s2

Electron Configuration Practice Let’s try a few together Na Ar Ca

Electron Configuration

Electron Configuration Practice Now you try! F S

Exceptional Electron Configurations The elements after Vanadium (atomic #23) do not follow the Aufbau principle Filled energy sublevels are more stable than partially filled sublevels. Exceptions to the aufbau principle are due to subtle electron-electron interactions in orbitals with very similar energies. For Example: Cu and Cr

Exceptions to Aufbau’ Principle INCORRECT CORRECT