ELECTRON CONFIGURATION

By Hilary Scurlock

Edited by Mrs. Rosenfield

Electron Configurations• Describe the location of electrons• Orbital = region of space (same as electron

cloud)• Principle quantum # (n) = energy level

– n = 1, 2, 3, 4, 5 …with increasing distance from the nucleus

• Energy Sublevels = s, p, d, f (see p. 133-134)– s sphere 1 orbital max 2 electrons– p dumbbell 3 (x, y, z) 6– d clover leaf 5 10– f 7 14

• Max 2 electrons in each orbital

Atomic Orbitals

http://winter.group.shef.ac.uk/orbitron/AOs/1s/index.html

This site shows 3-D versions of the atomic orbitals – I just thought you would be interested

Some important info…• There are 4 quantum numbers that are

associated with electron configurations:– Principal quantum number (n) = energy level

– Angular momentum quantum number (l ) = sublevel, which is the type of orbitals (s, p, d, or f)

– Azmuthal quantum number (m l ) = number of orbitals related to the sublevel

– Electron spin quantum number (ms) = tells if electrons are spinning clockwise or counter clockwise

• We can use the four quantum numbers to label each unique electron in any orbital in any atom, thus giving us electron configuration.

• The quantum numbers are just like your address!

• No two electrons will have the same four quantum numbers

For exampleFor example

the electron configuration for Hydrogen is:the electron configuration for Hydrogen is:

1s1s11

Principal

quantum # (n)Angular momentum

quantum # (l)

# of electrons in orbital or subshell

NOTICE:

For electron configurations ml and ms are not used

Orbital Diagrams

• These show us the number of orbitals and the spin of the electrons in those orbitals.– Here ml and ms are used

H

1s1

This box represents the orbital number

(ml)

It can also be given as lines

or circlesThe arrow

denotes one of the two possible

spinning motions (ms)

Knowing this, now we can draw out a configuration and an orbital notation

• Helium– 2 electrons

• Beryllium– 4 electrons

• Lithium– 3 electrons

• Helium 1s2

___

1s2

• Beryllium 1s22s2

___ ___

1s2 2s2

• Lithium 1s22s1

___ ___

1s2 2s1

An Electron configuration is how the electrons are distributed among the various atomic orbitals.

Some Rules to Configure By:

If you want to get these electron If you want to get these electron configurations/orbital diagrams right there configurations/orbital diagrams right there

are some rules that we must follow.are some rules that we must follow.

PAULI EXCLUSION PRINCIPLE• This principle states that two electrons in

the same orbital must have opposite spins.

• An example of the Pauli exclusion principle:

• He (1s2): ____ ____

1s2 1s2

HUND’S RULE• Electrons entering a subshell containing more

than one orbital (Ex: p, d or f - orbitals) will be the most stable if the electrons are arranged by themselves in separate orbitals alone before being paired up.

• An example of Hund’s rule:• Nitrogen (1s2 2s2 2p3): ___ ___ ___ ___ ___

1s2 2s2 2p3

• What would fluorine look like?

AUFBAU PRINCIPLE

• Electrons start at the lowest energy orbitals first and then continue to fill orbitals of increasing energy.

• How do we know the order????

• The electrons follow a general pattern when filling energy levels and orbitals and guess what object we can thank in helping us figure this out????

• THE PERIODIC TABLE

Those wacky d orbitals!• When dealing with transition metals, which

have d orbitals, always fill the s-orbital in the next energy level before filling the d-orbital.

• For example, the configurations of elements such as Sc are different than we might expect, because, in the cases of this element, the 4s orbital is filled before the 3d orbital.

• 1s2 2s2 2p6 3s2 3p6 4s2 3d1

Another way to do it:

• If the periodic table is too confusing for you, or you just like to memorize things, try:

• Energy level diagram - basically a picture of the aufbau principle– It shows you the order in which the orbitals are filled.

Order of filling orbitals (p.135)

Review of Writing Electron Configurations

Ask yourself these questions every time you have to write an electron configuration or orbital notation:

1. Where is the element on the periodic table?

2. What is the atomic number?

3. How many electrons?

4. What is the row number?

5. How many energy levels?

6. What subshell(s) does the element have?

7. What is the electron configuration?

ONLY CONTINUE IF YOU HAVE TO WRITE ORBITAL NOTATIONS:

8. How many orbitals in each subshell?

9. What is the orbital notation?

Some Examples:

• Give the electron configuration for the following:He BeO NaAr Te

• Give the orbital diagram for the following:Li C

V K

Quick Quiz!

• What is wrong with this configuration?

Al: 1s22s22p43s23p3

A SHORTCUT

• You do not have to write out all the info

Noble gas core

• Li = [He] 2s1

• P = [Ne] 3s23p3

• TRYOxygen

Zinc

Cesium

Exceptions to the Rules• Just as with any good rule there are

exceptions!!!• An atom with almost half or completely -

filled d and f orbitals are more stable when you can fill them from an s-orbital

• Ex: Chromium = [Ar]4s13d5 OR

Copper = [Ar]4s13d10

• You try: – Molybdenum = Mo– Gold = Au– Europium = Eu

Valence Electrons

• These electrons determine the chemical properties of an element

• They are generally the electrons in the atom’s highest energy levels.– Determine the number of valence electrons using electron

configurations:– S [Ne] 3s2 3p4 = 6 valence e-

– Cs [Xe] 6s1 = 1 valence e-

– Fe [Ar] 4s2 3d6 = 2 valence e-

– Br [Ar] 4s2 3d10 4p5 = 7 valence e-

• On the periodic table elements in the same column tend to have the same number of valence electrons.

Electron-dot Structures

• Visual way to show valence electrons of an element for bonding.

• Has element’s symbol in the middle and is surrounded by dots representing the atom’s valence electrons.– Dots are placed one at a time on the four

sides of the symbol and then paired up until they are all used.

Examples of Electron-dot Structures

S

Cs

Fe

Br

See pg 140 – 141 for more examples