Electronic structure of hydrogen Electronic structure of hydrogen

The hydrogen atom consists of a nucleus containing one proton The hydrogen atom consists of a nucleus containing one proton
and one electron occupying a region outside the nucleus and one electron occupying a region outside the nucleus
• in its ground state, the electron occupies a 1s orbital • in its ground state, the electron occupies a 1s orbital
We can represent this in one of two ways: We can represent this in one of two ways:

Electron configuration
Orbital diagram
For each orbital containing electrons in the ground state:
1. Orbitals are represented by boxes
1. List the principal energy level
2. Electrons contained in the orbitals are represented by up and
2. List each type of orbital and show the number of electrons down arrows (corresponding to positive and negative spin)
contained in the orbitals as exponents
Example: Hydrogen
Example: Hydrogen
Number of electrons in
sublevel’s orbitals
Principal 1s1 1s 2s 2p 3s 3p
energy level Sublevel (type of orbital)

Rules for determining electronic structure

for atoms with more than one electron Atomic structures
1. No more than 2 electrons can occupy an orbital
(Pauli exclusion principle) Element: He
Number of electrons: 2
2. For an atom in its ground state, electrons occupy the lowest
energy orbitals available (Aufbau principle)
-- all of the subshells in a principal energy level must be
completely filled before electrons enter a higher
principal energy level 1s 2s 2p 3s 3p

-- within a given principal energy level, subshells are filled

in order of increasing energy: s < p < d < f
Electron configuration: 1s2
-- within a given principal energy level, a subshell must be
completely filled before electrons enter a higher subshell

3. Within a given subshell, a single electron will enter each orbital First principal energy level (n = 1) is full
before a second electron enters any one orbital (Hund’s law)
 Atomic structures Atomic structures

Element: Li Element: Be
Number of electrons: 3 Number of electrons: 4

1s 2s 2p 3s 3p 1s 2s 2p 3s 3p

Electron configuration: 1s2 2s1 Electron configuration: 1s2 2s2

Atomic structures Atomic structures

Element: B Element: C
Number of electrons: 5 Number of electrons: 6

1s 2s 2p 3s 3p 1s 2s 2p 3s 3p

Electron configuration: 1s2 2s22p1 Electron configuration: 1s2 2s22p2

 Atomic structures Atomic structures

Element: N Element: O
Number of electrons: 7 Number of electrons: 8

1s 2s 2p 3s 3p 1s 2s 2p 3s 3p

Electron configuration: 1s2 2s22p3 Electron configuration: 1s2 2s22p4

Atomic structures Atomic structures

Element: F Element: Ne
Number of electrons: 9 Number of electrons: 10

1s 2s 2p 3s 3p 1s 2s 2p 3s 3p

Electron configuration: 1s2 2s22p5 Electron configuration: 1s2 2s22p6

Second principal energy level (n = 2) is full

 Atomic structures Abbreviated electron configurations
1s 2s 2p 3s 3p
1. Start with the symbol for the previous noble gas in square
Na 1s2 2s2 2p6 3s1 brackets
2. List the valence electrons
Mg 1s2 2s2 2p6 3s2

Al 1s2 2s2 2p6 3s2 3p1 1s 2s 2p 3s 3p

He 1s2
Si 1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p4
O
[He] 2s2 2p4
P 1s2 2s2 2p6 3s2 3p3

S 1s2 2s2 2p6 3s2 3p4

Ne 1s2 2s2 2p6
Cl 1s2 2s2 2p6 3s2 3p5
Mg 1s2 2s2 2p6 3s2
Ar 1s2 2s2 2p6 3s2 3p6 [Ne] 3s2

The spacing between successive principal

Orbital filling sequence energy levels decreases as n increases
n=4
• overall, the n = 4 level is higher in energy
The sequence for filling orbitals is exactly as you would expect up than the n = 3 level, but they are close 4f

through the 3p sublevel: together and there is some overlap 4d

n=3 4p
1s 2s 2p 3s 3p
3d 4s

Ar 1s2 2s2 2p6 3s2 3p6 3p

1s 2s 2p 3s 3p 3s
Energy

n=2
You might expect that the 3d sublevel would be filled next, but this
2p
is not the case The 3d and 4s orbitals are very close in
-- the 4s orbital is filled before the 3d orbitals 2s energy, but the 4s orbitals are actually
slightly lower in energy than the 3d orbitals
K [Ar] 4s1 • electrons will enter the 4s orbitals before
they enter the 3d orbitals
n=1
Ca [Ar] 4s2
1s 2s 2p 3s 3p 4s 1s
 7s 7p 6d 5f
Memory aid for orbital filling sequence

6s 6p 5d 4f

5s 5p 4d 1s 2s 3s 4s 5s 6s 7s
Energy

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

1s

The blocks of elements in the periodic table correspond to filling the

different types of sublevels (i.e., orbitals) Orbital filling sequence
s block (Groups 1A-2A) The sequence for filling orbitals is exactly as you would expect up
p block through the 3p sublevel
1s (Groups 3A-8A) 1s
1s 2s 2p 3s 3p
2s 2p
d block
(Transition Elements)
When the element at the end of Period 3 (argon) is reached, the
3s 3p
3p sublevel has been filled
4s 3d 4p
1s 2s 2p 3s 3p
5s 4d 5p
Ar 1s2 2s2 2p6 3s2 3p6
6s 5d 6p

7s 6d
As Period 4 begins, the 4s orbitals are filled before the 3d orbitals
1s 2s 2p 3s 3p 4s
4f
K [Ar] 4s1
5f

f block (Inner Transition Elements)

Ca [Ar] 4s2
 Period 4: Filling of 3d orbitals
3d 4p
Period 4 (cont’d): Filling of 4p orbitals
4s
Sc [Ar] 4s2 3d1
4s 3d 4p
Ti [Ar] 4s2 3d2 Ga [Ar] 4s2 3d10 4p1

V [Ar] 4s2 3d3 Ge [Ar] 4s2 3d10 4p2

Cr [Ar] 4s1 3d5

As [Ar] 4s2 3d10 4p3
Mn [Ar] 4s2 3d5
Se [Ar] 4s2 3d10 4p4
Fe [Ar] 4s2 3d6

Co [Ar] 4s2 3d7 Br [Ar] 4s2 3d10 4p5

Ni [Ar] 4s2 3d8 Kr [Ar] 4s2 3d10 4p6

Cu [Ar] 4s1 3d10

Zn [Ar] 4s2 3d10

Valence electrons Valence electrons

valence electrons -- the electrons in the outermost valence electrons -- the electrons in the outermost
(i.e., highest energy) principal energy level of an atom (i.e., highest energy) principal energy level of an atom

Only electrons in s and p orbitals are counted as valence electrons

H 1s1
Electrons in d orbitals are not counted as valence electrons
1s 2s 2p 3s 3p

Example: selenium (Se) 1s2 2s2 2p6 3s2 3p64s2 3d10 4p4
N 1s2 2s2 2p3

1s 2s 2p 3s 3p Total electrons: 34 Valence electrons: 6

Ar 1s2 2s2 2p6 3s2 3p6

1s 2s 3s 4s
1s 2s 2p 3s 3p 2p 3p 3d 4p

Valence electrons are involved in the formation of bonds Valence electrons are involved in the formation of bonds
between atoms to form compounds between atoms to form compounds
 Valence electron configuration determines the Noble gas configuration
characteristics of elements in a group
The noble gases (last column in the periodic table) are characterized by
completely filled s and p orbitals
• this is a very stable valence electron configuration
• noble gasses exist as single atoms and do not form compounds with
other elements

1s 2s 2p 3s 3p

He 1s2

Ne 1s2 2s2 2p6

Ar 1s2 2s2 2p6 3s2 3p6

The chemical behavior and properties of elements in a group are
associated with the valence electron configuration of its elements
Etc.

For most elements, attaining a noble gas configuration means having 8

Atoms of other elements seek to attain a valence electrons (two s electrons and six p electrons)
noble gas electron configuration -- this is called a full valence shell (also referred to as an octet )

2
The rest of the elements in the periodic table tend to form 1s2
For the elements in Period 1 (hydrogen and
compounds in combination with other elements He helium), a full valence shell consists of
2 valence electrons (two s electrons)
10
1s2 2s2 2p6 • this is because the first principal energy
When forming compounds, the atoms of these elements lose, gain, Ne level (n = 1) does not have a p sublevel
or share electrons to attain a stable valence electron configuration (i.e., no p orbitals)
18
(i.e., identical to noble gases) 1s2 2s2 2p6 3s2 3p6
Ar
• Atoms that attain a noble gas configuration by losing or gaining 36
electrons (i.e., forming ions) form ionic bonds 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6
Kr
• Atoms that attain a noble gas configuration by sharing electrons 54
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6
form covalent bonds Xe
86
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 5s2 4d 10 5p6 6s2 4f 14 5d 10 6p6
Rn
 Alkali metals are highly reactive because they can obtain Today, there are 115 elements included in the periodic table
a stable noble gas configuration by losing one electron
1 18
Group number
e– 1A 8A
2 13 14 15 16 17
3 2 1 2A 3A 4A 5A 6A 7A
1s2 2s1 1s2
Li He 2
e–
3 4 5 6 7 8 9 10 11 12
11 10
1s2 2s2 2p6 3 3B 4B 5B 6B 7B 8B 1B 2B
1s2 2s2 2p6 3s1
Na Ne Period
e– number 4
19 18
1s2 2s2 2p6 3s2 3p6 5
K 1s2 2s2 2p6 3s2 3p6 4s1
Ar 6
e–
37 36
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 7
Rb 5s1 Kr
e–
55 54
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 Period 4
Cs 5s2 4d 10 5p6 6s1 Xe 5s2 4d 10 5p6
e–
87 86 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6
1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p6 Each horizontal row in the table is called a period
Fr 5s2 4d 10 5p6 6s2 4f 14 5d 10 6p6 7s1 Rn 5s2 4d 10 5p6 6s2 4f 14 5d 10 6p6

Today, there are 115 elements included in the periodic table Today, there are 115 elements included in the periodic table

1 18 1 18
1A
Group number 8A 1A
Group number 8A
2 13 14 15 16 17 2 13 14 15 16 17
1 2A 3A 4A 5A 6A 7A 1 2A 3A 4A 5A 6A 7A

2 2
3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12
3 3B 4B 5B 6B 7B 8B 1B 1s
2B2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 4p 5 3 3B 4B 5B 6B 7B 8B 1B 2B
Period Period
number 4 number 4

1s 2 52s 2 2p 6 3s 2 3p 6 4s 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 5
6 6

7 7

Group 6A
(or Group 16)

The period number corresponds to the Each vertical column in the table is called a group
highest occupied principal energy level -- the elements in a group have similar properties
 Some of the terms in the text are different
than the terms used in the lectures

shell = principal energy level

subshell = sublevel

The elements of a group have the same outermost electron outermost shell = highest occupied
configuration (i.e., valence electron configuration) (valence shell) principal energy level
-- except the electrons are in different principal energy levels

Homework problems

Chapter 3 Problems:

On Pg. 67 of text:
Problems 3.20, 3.21
(write both electron configurations and orbital diagrams)

On Pg. 71 of text: Problem 3.24

On Pg. 72 of text: Problem 3.29

On Pages 76-77 of text:

3.79, 3.84, 3.88, 3.101