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HOW DO THE ORBITALS FILL UP WITH ELECTRONS?

An Introduction to Electron Configurations

The Big Questions

• Now that we know how electrons are arranged into atoms, how are they arranged into orbitals and sub-orbitals?

• How can we communicate the arrangement of atoms in orbitals?

• What are valence electrons and why are they more important than other electrons?

• Complete the activity “Welcome to Atomos Apartments!”

• Answer the post-lab questions:

• What did the students represent?

• What were the floors analogous to?

• What were the types of rooms analogous to?

Assigning an Electron’s Address Explore

• Before we begin… • How do you determine the number of electrons in

an atom again? • In a NEUTRAL atom, # of protons from periodic

table = # of electrons

• We show the way electrons are arranged in atoms by writing electron configurations • The electron configuration of an atom is the

complete description of the orbitals occupied by all of its electrons

• There are rules to follow!

Predicting Electron Locations

• Electrons are added one at a time to the lowest energy orbitals, or subshells, available until all the electrons of the atom have been accounted for

• “aufbau” is German for ‘build up or construct’

Rule #1 – Aufbau Principle

1s

3s

2s 2p

3d 3p

4s 4p 4d 4f

5s 5p 5d 5f

Energies of Electron Orbitals

• An orbital can hold only two electrons

• In other words, no two electrons can ever be in the same place at the same time

• The electrons MUST HAVE OPPOSITE SPINS

• Electrons are associated with “spin”, either one way or the other – like a top

• These spins are called “spin up” and “spin down”

• So, maximum number of electrons held in each orbital is as follows: • 2 for s

• 6 for p (2 x 3 p-orbitals)

• 10 for d (2 x 5 d-orbitals)

• 14 for f (2 x 7 f-orbitals)

Rule #2 – Pauli’s Exclusion Principle

• “Electrons must fill a subshell such that each orbital has a spin up electron before they are paired with spin down electrons”

• More energetically favorable

• A bus analogy:

• If you enter a bus and don’t know anyone on it, you will pick a seat that is completely empty rather than one that already has a person in it

• i.e. Electrons are unfriendly!

Rule #3 – Hund’s Rule

• Electrons fill in order from lowest to highest energy • Ground floor first then up

• The Pauli exclusion principle holds • An orbital can hold only two electrons

• Two electrons in the same orbital must have opposite signs (spins)

• You must know how many electrons can be held by each orbital

• Hund’s rule applies • The lowest energy configuration for an atom is the one having the

maximum number of unpaired electrons for a set of orbitals

• By convention, all unpaired electrons are represented as having parallel spins with the spin “up”

In Summary…

Making the Connection…

• Thinking back to the “Welcome to Atomos Apartments!” activity:

• Complete the following table:

Apartment Rules Electron Rules

From the Bottom Up: Rooms must be filled from the ground floor up. Fill the one room on the first floor before starting to put new students on the second floor. At higher floors, the order might change a bit

Singles First: Singles are placed in rooms before pairing them with a roommate.

Opposite Gender Only: No men may room together and no women may room together. Roommates must be opposite genders.

Applying the Electron Rules • Lucky for you, you don’t have to memorize the order in which electrons fill

the orbitals (subshells)

• You can just use the periodic table!

• The PT follows the Aufbau principle – notice (n-1) d-orbitals are filled after ns and before np orbitals AND (n-2) f-orbitals are filled after ns!

• Each element square represents ONE electron in that particular orbital

• Start with H and move through the table in order until the desired element is reached!

Illustrating Electron Configurations • There are several ways to represent

electron configurations: • Full electron configurations

• Condensed (noble gas) electron configurations

• Orbital diagrams

Full Electron Configurations

• In most cases, it is sufficient to write a list of all of the occupied subshells and indicate the number of electrons in each subshell with a superscript.

H 1s1

C 1s2 2s2 2p2

Ar 1s2 2s2 2p6 3s2 3p6

Practice!

• #7 and #10 on page ___________

• Noble gases have full valence shells, so you can condense electron configurations to “eliminate” the electrons already accounted for by the closest noble gas

It cuts down on a lot of writing, and that’s a good thing!

• To use noble gas notation:

• Write the symbol for the preceding noble gas [in brackets] to represent all of the electrons in its electron configuration

• Add the rest of the electrons at the end • Example -the full configuration for As-(Arsenic) is:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3

• Notice, the part in red is the same as Argon’s configuration:

1s2 2s2 2p6 3s2 3p6

• The noble gas configuration will start with the gas in the row before it

[Ar] 4s2 3d10 4p3

There’s a Quicker Way…Introducing Condensed (Noble Gas) Notation

Practice!

• #13 and #17 on page ___________

Orbital Diagrams

• An orbital box diagram goes one step further by also illustrating the spins of the electrons

• This notation uses boxes to represent orbitals

• One arrow (↑) represents 1 e-

• 2 arrows (↑↓) represent 2 e-

• Same rules as full electron configurations apply!

Phosphorus Full Electron Configuration

1s2 2s2 2p2 2p2 2p2 3s2 3p1 3p1 3p1

Phosphorus Orbital Diagram

• The orbital box diagram indicates that the three electrons in the 3p subshell all have parallel (unpaired) spins

1s 2s 2p 3s 3p

Example

Order of Orbital Filling

Electron Configuration for Platinum (Element 78)

1s2 2s2 2p6 3s2 3p6 4s2 4p6 3d10 5s2 4d10 5p6 6s2 4f14 5d8 1s

2s

3s 4s

5s

6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

6d

4f

5f

7p

Examples - Orbital Diagrams

• Hydrogen

• e- configuration:

1s1

1s

• Helium

• e- configuration:

1s2

1s

• Carbon

• e- configuration is:

1s2 2s2 2p2

Examples – Orbital Diagrams

1s 2s 2p

• Oxygen

• e- configuration is:

1s2 2s2 2p4

Examples – Orbital Diagrams

1s 2s 2p

Practice!

• # 1, #10 #18 on page ______________

EXCEPTIONS TO THE FILLING RULES

Exceptions of the Rules

• Remember, (n – 1)d orbitals are generally filled after ns orbitals and before np orbitals due to differences in energies of the electrons

• There are some exceptions to this rule:

• When it is possible to half-fill or fill the (n-1)d shell, the ns subshell can be left half-filled

• This is an example of Hund’s Rule

• The ns and (n-1)d orbitals are very close in energy, so the more parallel spins, the better!

Example

Sc [Ar] 4s23d1

- 4s filled before 3d

Ti [Ar] 4s23d2

V [Ar] 4s23d3

4s 3d

4s 3d

4s 3d

Example

Cr You might expect… [Ar] 4s23d4 • Physical properties indicate that this is not the electron

configuration – It is actually…

Cr [Ar] 4s13d5

• Notice the 3d subshell is half-filled • This configuration maximizes unpaired electrons - Hund’s Rule

4s 3d

4s 3d

Exception to the Rules

• Having a filled subshell is also energetically favorable, so copper has an unexpected configuration…

Cu [Ar]3d104s1

• The energetic stability gained from having either a filled or a half-filled subshell has an effect on the reactivity of different elements

4s 3d

4s

• When considering the principal electron shells (n = 1,2,3,…), there are two types of electrons:

• Core Electrons: electrons in the filled “inner” shell(s) of an atom

• Valence Electrons: electrons in the unfilled “outer” shell of an atom

• All elements in the same group have similar chemical properties because they have the same number of valence electrons in their outer shell!

What About Valence Electrons?

What Orbitals Correspond to Valence Electrons Location? • For elements in the first three periods: • The core electrons are those in the preceding noble gas

configuration

• The additional electrons in the outer shell are the valence electrons

• Example:

Full Configuration - B 1s2 2s2 2p1

Noble Gas Config - B [He] 2s2 2p1

Core: 1s2 Valence: 2s2 2p1

(Shell with n = 1) (Shell with n = 2)

Location of Valence Electrons • For elements in the fourth period and below in groups

3A – 7A, the filled d subshells are also part of the core, even though they are not included in the noble gas configuration

Full Config - Se 1s2 2s2 2p6 3s2 3p6 4s2 3d104p4

Noble Gas Config - Se [Ar] 4s2 3d104p4

Core: Valence:

1s2 2s2 2p6 3s2 3p63d10 4s2 4p4

• ONLY ELECTRONS IN HIGHEST NUMBERED s- AND p- ORBITALS ARE VALENCE ELECTRONS!