Writing Electron Configurations

Ok...let’s simplify this.

• Every atom has a nucleus.• In that nucleus we have protons (positive

charge) and neutrons (no charge)• Surrounding the nucleus we have electrons.• Electrons have a negative charge, however

they do not “fall” into the nucleus (despite being attracted to the positive proton), why?

Ok...let’s simplify this

• Because electrons exist in “orbitals”• An orbital is the region of space in which you

will find an electron. (For our purposes)• This is where it gets confusing...• There are different levels of orbitals.• We represent these different levels of orbitals

with the letter “n”.

• So if an electron is in the 1st orbital surrounding the nucleus it is n=1.

• If an electron is in the 3rd orbital surrounding the nucleus it is n=3.

• Easy enough, right?

There’s more.

• We also have the letter “l” this represents what type of orbital the electron is in.

• The different types of orbitals are as follows...• S, p, d, f, and g• These letters refer to the shape of the orbital.

• So now, we have two pieces of information to help us determine where an electron is.

• We know the number of orbital it is in and we know the shape of the orbital.

Review: Quantum Numbers

• Principle Quantum Number (n) represents the energy orbital number (how far the orbital is from the nucleus).

• Orbital-Shape Quantum Number Orbital (l) represents the shape of the orbital sublevel (s is round, p is like 2 balloons, 2 d orbitals put together)

• Magnetic Quantum Number represents the direction of the orbital sublevel (l)

• n can be any number from 1 on.

• l can be any number from 0 to (n-1)

• ml is –l to +l

Ok...back to new stuff

• The Fourth Quantum Number (ms) is used to represent the direction of the spin on the electron.

• There are only two possible values for this number, + ½ or – ½ .

• Electrons spin like a top.

Electron Spin

Pauli Exclusion Principle

• We have a new principle to learn!• The Pauli Exclusion Principle states that no

two electrons in any atom will have the same 4 quantum numbers.

• For example you would not find two electrons with n=4, l=3, ml=-3 and ms= - ½

• You could find n=4, l=3, ml=-3, ms= - ½ and n=4, l=3, ml= -3, ms= + ½

Example

Electron Configurations

• An electron configuration can be written for each atom.

• That is, we can write out where we can find each electron in an atom.

• And remember, no two electrons will have the same set of 4 quantum numbers in the same atom.

• Generally when we write electron configurations we write the configuration of the atom in it’s ground state.

• Ground state means that the electron is neutral and is not an isotope.

Example

• Hydrogen, in it’s ground state, has one electron (atomic number 1, mass of 1).

• The electron configuration for Hydrogen is 1s1.• Remember, n=1, l=0 and the small, raised 1

symbolizes that there is one electron in the s sublevel.

• Helium is He 1s2 symbolizing there are 2 electrons in the s sublevel.

• Lithium is 1s22s1 symbolizing that there are 2 electrons in the 1 s sublevel and 1 electron in the 2 s level.

• Fluorine is written as 1s22s22p5

• Why do you think that there are only 2 electrons in the s sublevel, but there can be 5 electrons in the p sublevel?

Remember the Pauli Exclusion Principle?

• What did the Pauli Exclusion Principle tell us?• That no two electrons can have the same 4

quantum numbers in an atom.• Let’s write out the possible values of the 4

quantum numbers in the s sublevel.• How many electrons do you think is possible

in the p sublevel?• How about the d sublevel?

• There is a maximum of 2 electrons in the s sublevel.

• There is a maximum of 6 electrons in the d sublevel.

• There is a maximum of 9 electrons in the p sublevel.

Aufbau Principle

• Each electron occupies the lowest energy orbital available

– For example, all of the “2” orbits must be filled before electrons can go into the 3s orbital

• Atom’s are “built-up”

• Do you think that the direction of the arrow symbolizes anything?

• Why do you think that in the Carbon electron configuration the 2 p level has two electrons in the same direction?

• We can tell the electron configuration of an element just by looking at the periodic table!

• The “long form periodic table” shows us which sublevel contains the atom’s valence electrons.

A helpful guide to what goes where

Let’s try it.

• How would we write the configuration for:– Hydrogen– Helium– Boron– Carbon– Nitrogen

Noble Gas Notation

• The last column of the periodic table is called the Noble Gases.

• The Noble Gases have full valence electron shells.

• So we when we out electron configurations for our elements we can shorten it like this...

Shorten Electron Configuration Notation

• Ne– 1s2 2s2 2p6

• Na– 1s 2 2s2 2p6 3s1

– [Ne]3s1

Try it out!

• In your notebook write out the electron configuration for the first 12 elements.

• We have already done some!

Homework

• Read pages 146-148 • Start working on your assignment.