Electron Configuration

Revised by Ferguson

Fall 2014

How do the chemical properties arise from the structure of atoms?

pages144 ->

• Our first understanding of the arrangement of electrons around the nucleus began in 1920, by Neils Bohr, and is called the Quantum Theory.

• The quantum theory provides insight into why some atoms tend to “gain” electrons and other atoms “donate” electrons to form ions in chemical bonds.

What is a “quantum” state?• a specific combination of values of

variables such as energy and position that is allowed by quantum theory.

• Electrons are grouped into different “quantum states” as a way to describe the electron arrangement around a nucleus of an atom.

• Quantum-Mechanical Model of the Atom–Describes the probability that the electron

will be in a certain region of space at a given instant.

–Orbitals are regions of different energies where the electrons can be found.

• The quantum-mechanical model uses 3 quantum numbers to describe an orbital: –The principal quantum number (n)

•Can be any positive integer

(n=1, n=2, n=3, etc.)•Describes the energy level

– The angular momentum quantum number (l)Lowercase cursive

L• Can have values from 0 to (n–1)• Defines the shape of the orbital

– The magnetic quantum number (ml)

• Can have integer values between – l and + l • Describes the orientation of the orbital in space

Value of l 0 1 2 3

Orbital shape s p d f

Value of n

Possible values of l

Subshell possible values of ml

# of orbitals in subshell

1 0 1s 0 1

2 0 2s 0 1

1 2p -1, 0, 1 3

3 0 3s 0 1

1 3p -1, 0, 1 3

2 3d -2, -1, 0, 1, 2 5

4 0 4s 0 1

1 4p -1, 0, 1 3

2 4d -2, -1, 0, 1, 2 5

3 4f -3,-2, -1, 0, 1, 2,3 7

Orbitals• Orbitals hold electrons, and show the

area in which the electrons can be found.

• “s” orbitals can hold 2 electrons• “p” orbitals can hold 6 electrons• “d” orbitals can hold 10 electrons• “f” orbitals can hold 14 electrons

Quantum States and Orbitals• There are 4 shapes associated with

quantum states: s, p, d, and f.

s orbitals

• Have a spherical shape.

• There is one orbital in each s subshell.

p orbitals• Dumbbell shaped orbitals• Each p subshell contains 3 orbitals.• Each of the 3 orbitals is oriented along a different

axis (x, y, or z)

d orbitals• Each d sublevel contains 5 orbitals.• 4 of the 5 orbitals have a four-leaf clover

shape. The dz2 orbital has lobes on the z

axis and a “doughnut” shape in the x-y plane.

f orbitals• An f sublevel will contain 7 orbitals

Electron Configurations• Electron configuration = a

description of which orbitals contain electrons for a particular atom.

• All orbitals in the same subshell are said to be degenerate, meaning that they have the same energy. (i.e. the 3 orbitals in the 2p sublevel each have the same energy).

• A maximum of 2 electrons can be located in a given orbital.

Subshell # of orbital Max # of Electrons

s 1 2

p 3 6

d 5 10

f 7 14

Finding Electron Configuration:

• The complete list of orbitals and the order in which they fill for all of the currently discovered elements is:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

Aufbau Principle: orbitals are filled in order of increasing energy (lowest energy orbitals filled first).

• The fourth quantum number– The magnetic spin quantum number (ms)

– Can have a value of either + ½ or – ½ – Represents the electron’s spin about its axis, which

can generate a magnetic field in two possible directions.

• The Pauli Exclusion Principle– No two electrons in an atom can have the same

quantum numbers. – Therefore two electrons found in the same orbital will

have opposite spins (+ ½ and – ½)

How to find electron configuration:

Hund's rule of maximum multiplicity

• When more than one orbital of equal energies are available then the electrons will first occupy these orbitals separately with parallel spins.

• The pairing of electrons will start only after all the orbitals of a given sub level are singly occupied.

Arrow Notation• We have learned the long method of

writing out electron configuration.• We can also write electron config.

using the orbital diagram or arrow method.

For example, the three electrons that are filled into the three 'p'

orbitals can be represented in two different ways:

• Valence Electrons – outer-shell electrons – Include s and p electrons in the highest energy level– Electrons that are involved in chemical bonding

• Core Electrons = inner-shell electrons• For example, a phosphorus atom has 15 total electrons

– Phosphorus has 5 valence electrons (the electrons in the 3s and 3 p orbitals)

– Phosphorus has 10 core electrons (found in the first and second energy levels)

• The octet rule atoms tend to form chemical bonds to reach a full outer-shell of 8 valence electrons.

• For example, the phosphorus atom on the previous slide would gain three electrons to reach a full outer shell. This P3- ion would have the same electron configuration as the element argon.

• When an atom and an ion have the same electron configuration, they are said to be isoelectronic.

• Transition metal ions– d and f electrons are not considered valence

electrons.– Outer shell s and p electrons will be lost first.

• For example:

Fe

Fe2+

Fe3+

Nobel Gas Notation (Short-Hand)