Electron configuration

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Atomic Structure and
Electron Configuration

HS Ch i t R id Learning Series
Chemistry Rapid L
i
S i
Wayne Huang, PhD
Kelly Deters, PhD
Russell Dahl, PhD
Elizabeth James, PhD

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© Rapid Learning Inc. All rights reserved.

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1


Learning Objectives
By studying this tutorial you will learn…
Basic structure of atoms.
How to determine the
number of electrons.
How to place electrons in
energy levels, subshells
and orbitals.
How to show electron
configurations using three
methods.
How to write and
understand Quantum
Numbers.
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Concept Map
Previous content
Chemistry

New content
Studies

Quantum Numbers
Matter
Location described by
Made of

Electrons

Chemical properties
determined by

Atoms

3 ways to show configurations

Boxes and Arrows

Spectroscopic
Notation

Noble Gas
Notation

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2


Atomic Structure

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Definition: Atom

Atom - Smallest piece
p
(basic unit) of matter that
has the chemical
properties of the element.
Often called the
“Building Block of Matter”.

Graphical Rendering of an Atom
p
g
Protons
Neutrons
Electrons

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3


What’s in an Atom?
An atom is made of three sub-atomic particles.
Particle

Location

Mass

Charge

Proton

Nucleus

1 amu =
1.67×10-27 kg

+1

Neutron

Nucleus

1 amu =
1.67×10-27 kg

0

Electron

Outside the
nucleus

0.00055 amu
9.10×10-31 kg

-1

1 amu (“atomic mass unit”) = 1.66 × 10-27 kg

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The Atom
Electron
Cloud

Nucleus

Mass =
M
# of protons
+ # of neutrons

Charge =
# of protons

Charge =
Ch
- (# of
electrons)

Very small
relative mass

Overall Charge =
# of protons
# of electrons
f l t

Overall Mass =
# of protons
+
# of neutrons
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4


Protons Versus Electrons
Protons

Electrons

+ Charge

- Charge

Contributes to mass of
atom.

Not contribute significantly
to mass of atom.

Found in nucleus.

Found outside nucleus.

# determines the “identity”
of the atom (atomic
number).

# and configuration
determine how the atom will
react.

Cannot be lost or gained
without changing which
element it is (nuclear
reaction).

Can be lost or gained—
results in an atom with a
charge (ion).

The ratio of protons to electrons determines the charge on
the atom (since neutrons are “neutral”).
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Electron
Locations

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5


Definition: Electron Cloud
Electron cloud – It is
the area outside of the
nucleus where the
electrons reside (i.e.
the probability of
finding electrons).

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Electron Clouds
Electron
cloud

Principal
energy levels

The electron cloud is made
of energy levels (n).

Subshells

Energy levels are
composed of subshells (l).

Subshells have orbitals (ml).

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6


Definition: Subshell and Orbital
Subshell – A set of orbitals with equal
energy.
gy
Orbital – Area of probability of an electron
being located.

Each orbital can hold 2 electrons
(spin up and down).

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Types of Subshells
There are 4 types of subshells that electrons reside in
under ordinary circumstances.

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Begins in
energy level

Number of
equal energy
orbitals

Total number
of electrons
possible

s

1

1

2

p

2

3

6

d

3

5

10

f

Energ Increases
gy

Subshell

4

7

14

Subshell Mnemonic: spdf = Smart People Don’t Fail.


7


Pictures of Orbitals

1 s orbital
3 p orbitals

5 d orbitals
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Electron
Configuration

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8


Definition: Electron Configurations
Electron Configurations –
Shows the grouping and
g
p g
position of electrons in an
atom.
Since the number of electrons and their
configuration determines the chemical properties of
the atom, it is important to understand them.

Box (and Arrow) Notation: Electron
configurations use boxes for orbitals and
arrows for electrons.
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Aufbau Principle
The first of 3 rules that govern electron configurations:

1

Aufbau (building-up) Principle: Electrons must fill
(
)
gy
subshells (and orbitals) so that the total energy of
atom is at a minimum.

What does this mean?
Electrons must fill the lowest
available subshells and orbitals
before moving on to the next
higher energy subshell/orbital.

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9


Energy and Subshells
The energy diagram below shows the relative energy
levels.
6p
6s

5d

5p

4f

4d

5s
4p
3d

4s
3p
3s
2p

Energy

2s

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Subshells are filled from the lowest
energy level (1s) to increasing energy
levels (follow the arrows).
Not that this does not always go in
numerical order.

1s

Hund’s Rule
The second of 3 rules that govern electron configurations.

2

Hund s
Hund’s Rule: Place electrons in unoccupied
orbitals of the same energy level (spin up)
before doubling up.

How does this work?
If you need to add 3 electrons to a p subshell
subshell,
add 1 to each (in parallel spins) before
beginning to double up.

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10


Pauli Exclusion Principle
The last of 3 rules that govern electron configurations.

3

Pauli Exclusion Principle: Two electrons that
occupy th same orbital must have different spins.
the
bit l
th
diff
t i
“Spin” describes the angular
momentum of the electron.
Spin
Up

“Spin” is designated with an up
or down arrow.
Spin
Down

How does this work?
If you need to add 4 electrons to a p
subshell, you’ll need to double up. When
you double up, make them opposite spins.

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Determining the Number of Electrons
In order to properly construct an electron configuration,
you must be able to determine how many electrons to
use.
Charge = # of protons – # of electrons
Atomic number = # of protons

Example:
Br1-

How many electrons does the following have?
Charge = -1
Atomic number for Br = 35 = # of protons
-1 = 35 - Electrons
Electrons = 36

35Br

1-

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11


Another Example
In order to properly construct an electron configuration,
you must be able to determine how many electrons to
use.
Charge = # of protons – # of electrons
Atomic number = # of protons

Example:
Cl

How many electrons does the following have?
No charge written

Charge is 0

Atomic number for Cl = 17 = # of protons
0 = 17 - Electrons

17Cl

Electrons = 17
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Applying the Rules
Use the 3 rules of electron configurations.

2

Aufbau Principle: Electrons must fill subshells (and orbitals)
so that the total energy of atom is at a minimum.
Hund s
Hund’s Rule: Place electrons in unoccupied orbitals of the
same energy level before doubling up.

3

Pauli Exclusion Principle: Two electrons that occupy the same
orbital must have different spins.

1

Example:

17Cl

Give the electron configuration for a Cl atom.
No charge written

0 = 17 - Electrons
Place 17 electrons 4 0
9
8
17
16
15
14
13
12
11
7
6
5
1
3
2
1s

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Charge is 0

Atomic number for Cl = 17 = # of protons

2s

2p

Electrons = 17

3s

3p

Electron Configuration Rules Mnemonic: Aufbau (stays low); Hund (does not
double up); Pauli (spins up and down) = “Alligator stays low; Hippo does not pair
up and Penguin jumps up and down.”


12


Spectroscopic
Notation

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Definition: Spectroscopic Notation
Spectroscopic Notation – Shorthand way of
showing electron configurations.
g
g
The number of electrons in a subshell are shown
as a superscript after the subshell designation.

1s

2s

2p

3s

3p

Box
Notation

Number of Electrons

1S2
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1s2 2s2 2p6 3s2 3p5

Spectroscopic
Notation

Subshell (l)
Principal Energy Level (n)


13


Writing Spectroscopic Notation
1

Determine the number of electrons to place.

2

Follow Aufbau’s Principle for filling order.

3

Fill in subshells until they reach their max (s = 2, p = 6, d = 10,
f = 14).

4

The total of all the superscripts is equal to the number of
electrons.

Example: Give the spectroscopic notation for S.

16S

No charge written
Charge is 0
Atomic number for S = 16 = # of protons
Electrons = 16
0 = 16 - Electrons

Place 16 electrons

2 + 2 + 6 + 2 + 4 = 16
1s 2 2s 2 2p6 3s 2 3p4

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Electron Configurations
and the Periodic Table

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14


Configurations Within a Group
Look at the electron configurations for the Halogens
(Group 7).
F

1s 2s 2p
1 22 22 5

Cl

1s2 2s2 2p6 3s2 3p5

Br

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5

I

All of the elements in Group 7 end with 5 electrons in
a p subshell.
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Configurations and Periodic Table
In fact, every Group ends with the same number of
electrons in the highest energy subshell.
Each area of the periodic table is referred to by the
highest
hi h t energy subshell that contains electrons.
b h ll th t
t i
l t
p-block
s-block
IA

Group

s1 s2

IIIB IVB

VIIIA

d-block

IIA

VB

VIB

VIIB VIIIB VIIIB VIIIB

IIIA

IB

IIB

IVA

VA

VIA

VIIA

p1 p2 p3 p4 p5 p6

d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

f-block

f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14

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15


Periodic Table as a Road-Map - 1
Wondering how to remember the order of filling of
the subshells?
Just use the periodic table as a mnemonic device.

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In order to do this, the “f” block needs to be placed in atomic
order.
(It’s usually written below to fit it on the paper).

Periodic Table as a Road-Map - 2
To see the filling order of subshells, read from left to right,
top to bottom!
This tool shows that the 3d energy level is filled after the 4s energy level!
1s

1s

2s

2p

3s

3p

4s

3d

4p

5s

4d

5p
6p

6s

4f

5d

7s

5f

6d

s subshells begin in level 1, so begin the s-block with “1s”.
p subshells begin in level 2, so begin the p-block with “2p”.
d subshells begin in level 3, so begin the d-block with “3d”.
f subshells begin in level 4, so begin the f-block with “4f”.
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16


Another Tool for Filling Order
There is another tool (mnemonic device) commonly
used to remember orbital filling order.
1s
2s

2p

3s

3p

3d

4s

4p

4d

4f

5s

5p

5d

5f

6s

6p

6d

7s

Building-Up Principle:
To read the chart, start
with 1s and follow the
arrows. Move down one
diagonal as far as
possible, then jump to
the top of the next
diagonal and keep
di
l dk
going.

7p

8s
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Electron
Configurations
of Ions

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17


Definition: Ion
Ion – an atom (or group of
atoms) that has gained or
)
g
lost electrons resulting in
a net charge.
Atoms gain and lose electrons to be in a more stable state.
Usually, the “more stable state” is a full valence shell.
Outermost shell of electrons

35/56

Full Valence Shell Ions
Look at the electron configurations for the following
(#p = # of protons and #e = # of electrons):
Br-

#p = 35

-1 = 35 - #e

#e = 36

1s 2 2s 2 2p6 3s 2 3p 6 4s 2 3d 10 4p 6
O2-

#p = 8

-2 = 8 - #e

#e = 10

+1 = 11 - #e

#e = 10

+2 = 20 - e

Charge
= p-e

#e = 18

1s 2 2s 2 2p6
Na+

#p = 11
1s 2 2s 2 2p6

Ca2+

#p = 20

1s 2 2s 2 2p6 3s 2 3p 6
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18


Full Valence Shell Ions
What do you notice about each of these
configurations?
They all end with full p subshells.
Br -

1s 2 2s 2 2p6 3s 2 3p 6 4s 2 3d 10 4p 6

O2-

1s 2 2s 2 2p6

Na+

1s 2 2s 2 2p6

Ca2+

1s 2 2s 2 2p6 3s 2 3p 6

Notice that O2- and Na+ have the
same number and configuration of
electrons.
This makes them isoelectric.

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Noble Gas
Configuration

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19


Definition: Noble Gas Notation
Noble Gas – Group 8 of the Periodic
Table. They contain full valence shells.
Noble Gas Notation – Noble gas is used
to represent the core (inner) electrons
and only the valence shell is shown.
35Br
Spectroscopic Notation: 1s 2 2s 2 2p6 3s 2 3p 6 4s 2 3d 10 4p 5
Noble Gas Notation:

[Ar] 4s 2 3d 10 4p 5

The “[Ar]” represents the core electrons and only the valence electrons are shown.
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Which Noble Gas Do You Choose?
How do you know which noble gas to use to
symbolize the core electrons?
Think: Price is Right.
How d you win on the Price is Right?
H
do
i
th P i i Ri ht?
By getting as close as possible without going over.
Choose the noble gas that’s closest without going over!

Noble Gas
He

2

Ne

10

Ar

18

Kr
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# of electrons

36

Xe

54


20


Where Does the Noble Gas Leave Off?
How do you know where to start off after using a
noble gas?
Use the periodic table!
He

1s
2p

2s

Ne

3p

Ar

4s

3d

4p

Kr

5s

4d

5p

Xe

6p

Rn

3s

6s

4f

5d

7s

5f

6d

The noble gas fills the subshell that it’s at the end of.
Begin filling with the “s” subshell in the next row to show
valence electrons.
41/56

Noble Gas Notation Example
1

Determine the number of electrons to place.

2

Determine which noble gas to use.

3

Start where the noble gas left off and write
spectroscopic notation for the valence electrons.

Example:

As
33A

Give the noble gas notation for As.

No charge written
Charge = 0
Atomic number for As = 33 = # of protons.
0 = 33 - electrons
Electrons = 33
Place 33 electrons.
Closest noble gas: Ar (18)
Ar (1s22s22p63s23p6) is full up through 3p.
18 + 2 + 10 + 3 = 33
[Ar] 4s 2 3d10 4p 3

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21


Comparing the
Different Notations

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Pros and Cons of Each Notation
Each notation has it’s advantages and disadvantages.
Pro

Con

“Boxes and
arrows”

Shows if electrons
are paired or
unpaired.

Longest method.

Spectroscopic
Notation

Quicker than “Boxes
and arrows”.

Does not show
pairing of electrons.
Does not show core
electrons.

Noble Gas
Notation

Allows focus on the
valence electrons
(that control
bonding).
Quickest method.

Does not show
pairing of electrons.

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22


Exceptions to the
Aufbau Rule

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Stability of d Subshells with 5 or 10
d subshells have 5 orbitals…
They can hold 10 electrons.
According to the Aufbau principle, Cr should have the
following valence electron configuration:
4s2 3d4

But a half-full or completely full d subshell is more stable
than the above configuration, so it is:
4s1 3d5

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23


Elements with Exceptions
The following elements are excepts to the Aufbau
Principle:
Element

Should be

Actually is

Cr

4s2 3d4

4s1 3d5

Mo

5s2 4d4

5s1 4d5

W

6s2 5d4

6s1 5d5

Cu

4s2 3d9

4s1 3d10

Ag
g

5s2 4d9

5s1 4d10

Au

6s2 5d9

6s1 5d10

They are the two groups on the periodic table that begin with Cr and Cu.

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Quantum
Numbers

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24


Definition: Quantum Numbers

Quantum Numbers – A set of 4
numbers (n, l, ml, & ms) that
describes the electron’s
placement in the atom.

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4 Quantum Numbers
ml

n

n=2
ms = ½(up)

2, 1, -1, +½

or

2p
-1
1

l
Quantum
Number

Symbol

Principal

n

Azimuthal
A i th l
(Angular)

l

Magnetic
Spin

ms

0 +1

l=1

ml = -1

Describes

Possible
Numbers
Whole # ≥ 1

ml

Shell Number
(Size)
Subshell
S b h ll
Type (Shape)
Orbital
(Orientation)

ms

Spin (up or
down spin)

Whole # < n
(0

-l

n-1)

+l

+½ or –½

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25


Determining Quantum Numbers
4p 3

n: principal energy level
Give the number of the shell.

l: subshell

s=0
p=1
d=2
f =3

Coding system: 0,1… n-1.

ml: orbital

s

p

0

-1

0

Number-line system of identifying orbitals.
0 is always in the middle.
Number line from –l to + l
l
l.

ms: spin

d

1

f

-3

-2

-2

-1

0

1

2

-1

0

1

2

3

↑ = + ½ (spin up)
↓ = - ½ (spin down)

Coding system

51/56

Quantum Number Examples
Example: Give the quantum numbers for the red arrow.
1s

2s

2p

3s

3p
0

It s
It’s in level “3”.
3

It’s in subshell “s” - the “code” for “s” is “0”.
It’s in orbital “0”.
___, ___, ___, ___
3
0
0 -½

It’s a down arrow.

Example: Give the quantum numbers for the red arrow.
1s

2s

It’s in level “2”.

2p

3s
-1

3p

0 +1

It’s in subshell “p”—the “code” for “p” is “1”.
It’s in orbital “-1”.
It’s an up arrow.

___, ___, ___, ___
2
1 -1 +½

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26


Identifying Incorrect Quantum Numbers
Example: What’s wrong with the following sets of quantum numbers?
1, 1, 0, +½

2, 1, -2, -½

n = 2…OK as n can be any whole # >0
l = 1…subshell is “p”.
OK as level 2 has “p”, i.e. “2p”.
ml = -2…on the “-2” orbital
“p” subshell has 3 orbitals: ___ ___ ___
-1 0 +1
No “-2” orbital in a “p” subshell.
ml must be between –l and l (i.e. -1, 0, +1), not -2.
l
1,
1),
2.

1, 0, 0, -1

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n = 1…OK as n (energy level) can be any whole # > 0
l = 1…subshell is “p”, but if n = 1 so l must be 0 (i.e. s subshell).
There is no p subshell in energy level 1.
1

n = 1…OK as n can be any whole # >0
l = 0…subshell is “s”.
OK as level 1 has an “s”.
ml = 0…on the “0” orbital
OK as “s” has 1 orbital and it’s “0”.
ms = -1
ms must be either +½ or -½, not -1.

Learning Summary
Electron
configurations can
be shown with
boxes and arrows,
in spectroscopic
notation, or noble
gas notation.

Atoms are made of
protons, neutrons
and electrons. The
configuration of the
g
electrons
determines the
chemical properties
of the atom.

Electrons are organized in
levels, subshells and
orbitals.

Quantum numbers
describe the
ocat o o a
location of an
electron in an atom
and are a series of
4 numbers.

Electron configurations
f
are written following the
Aufbau principle, Hund’s
Rule and the Pauli
Exclusion Principle.

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27


Congratulations
You have successfully completed
the core tutorial

Atomic Structure and
Electron Configuration

Chemistry :: Biology :: Physics :: Math

What’s N t
Wh t’ Next …

Step 1: Concepts – Core Tutorial (Just Completed)
Step 2: Practice – Interactive Problem Drill
Step 3: Recap – Super Review Cheat Sheet

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