Electron Configuration Study Guide
Quantum Numbers
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Quantum numbers are an address system for electrons.
Each electron has four quantum numbers that give it a specific address within the atom.
The quantum numbers are represented by the letters n, l, m and ms.
Principal Quantum Number (n)
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The principal quantum number, designated “n,” indicates the average distance from the
nucleus where an electron resides—or the principle energy level in which it resides.
The set of orbitals within the same n is called an electron shell.
An electron’s “n” is determined by locating the row of the periodic table in which the
element is located.
Example: Potassium is on row 4 of the periodic table, thus having a principal quantum
number (n) of 4.
Azimuthal Quantum Number (l )
 The azimuthal quantum number, l (a lowercase letter L), describes the sublevels of each
principle energy level.
 The sublevels are symmetrical shapes that surround the nucleus.
 Each sublevel has a unique shape represented by the letters s, p, d, and f.
 The sublevels are assigned the following numerical values for l :
s=0
p=1
d=2
f=3
 The set of orbitals that have the same n and l values is called a subshell.
 Each subshell is written with a number (n value) and a letter (l value) of s, p, d, or f.
 Example: the orbitals that have n=4 and l =2 are called “4d orbitals” and are in the 4d
subshell.
You can find an atom’s l by identifying which “block” of the periodic table it is in.
Magnetic Quantum Number (m)
 Describes the spatial orientation of the orbitals within an atom
 For each l , there are 2 l + 1 orbitals
◦ Example:
For l =2, there are 2(2) + 1 = 5 orbitals
 “m” can be anywhere from –l to +l.
◦ Example:
For l =2, m can be: -2, -1, 0, +1, +2
Electron Spin Quantum Number (ms)
 Each orbital holds only two electrons.
 These two electrons have opposite spin.
 Values for the ms are either +1/2 or -1/2
Orbital Types (a.k.a. Energy Sublevels)
 s orbital:
◦ Simplest of all orbitals
◦ Has a spherical shape
◦ Unlike other sublevels, the s sublevel contains only one orbital. It can hold 2
electrons.
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p orbitals:
◦ Contains three dumbbell shaped orbitals arranged around the x, y, and z axes.
◦ Each shell of n=2 or greater has three p orbitals
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d orbitals:
◦ There are five orbitals in the d sublevel.
◦ Any shell of n=3 or greater has five d orbitals
◦ Since each orbital holds two electrons, there can be a maximum of ten electrons
in each d sublevel.
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f sublevel:
◦ There are seven orbitals in the f sublevel.
◦ Since each orbital can hold two electrons, each f sublevel can hold a maximum of
14 electrons.
◦ Only ground state atoms with n=4 or greater have an f sublevel.
◦ Very complicated orbital shape—much more complicated than d orbitals.
Electron Configuration
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The way an atom’s electron’s are distributed among its orbitals is called its “electron
configuration.”
The electrons want to be in the lowest possible energy level, but they can’t all crowd into
the 1s orbital.
The Pauli Exclusion Principle tells us that each orbital can hold, at most, 2 electrons.
Electrons fill orbitals in order of increasing orbital energy.
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Diagonal Rule: helpful hint to remember the order in which orbitals are filled.
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1s
2s
3s
4s
5s
6s
7s
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2p
3p
4p
5p
6p
7p
3d
4d 5f
5d 6f
6d
Practicing electron configuration:
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Lithium:
 Lithium has 3 electrons (element 3 on periodic table).
 The orbital with lowest possible energy, the 1s, is filled first and it holds
2 electrons.
 The remaining electron must fill the next available orbital, the 2s.
Li: __ __ __ __ __ __ __
1s
2s
2p
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Lithium has an electron configuration of 1s22s1
More practice with electron configuration:
Electron configuration of Boron:
Boron has 5 electrons (element 5 on the periodic table).
B: ____
1s
____ ___ ___ ___
2s
2p
Summary of Boron’s electron configuration: 1s22s22p1
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More practice….
◦ Nitrogen has seven electrons (element 7 on periodic table).
N: ____ _____ ___ ___ ___
1s
2s
2p
Summary: 1s22s22p3
**Notice, that the electrons are unpaired in the 2p orbitals. Hund’s Rule says that for like
orbitals, the lowest energy is achieved when the number of electrons with the same spin is
greatest.
In other words, don’t pair electrons until you have to!
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Practice using Hund’s Rule:
◦ Electron configuration for oxygen:
◦ Oxygen has 8 electrons (element 8 on periodic table).
O: ____ ____ ____ ____ ____
1s
2s
2p
Summary: 1s22s22p4
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Silicon (element 14 on periodic table):
Si: ____ ____ ____ ____ ____ ____ ____ ____ ____
1s
2s
3s
2p
3p
Summary form Si: 1s22s22p63s23p2
We can also do Si another way:
Abbreviated Electron Configurations
We can use a short-hand way of showing Si’s electron configuration….
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The 2p subshell is filled at neon; neon has a stable electron configuration, with eight
electrons in its outermost shell forming an “octet”
When 8 electrons fill the outermost shell, the atom is in a very stable configuration.
To use condensed electron configuration, we use the nearest noble-gas element that has a
lower atomic number. (To find our which noble gas to use, look at the last column and
use the noble gas right above the one on the same row as your element)
For example: Lithium would be Li: [He]2s1
Oxygen would be O: [He]2s22p5.
Silicon would be Si: [Ne]3s23p2
Valence Electrons- the electrons that are in the outermost energy of an atom. Determines the elements
characteristics and chemical behavior.
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Not necessarily the last electrons to fil an orbit.
They are the electrons that have the highest principal energy level.
Add all the electrons in the highest principal energy level (n) orbitals
o Example: Argon [Ne} 3s23p6 Valence electrons = 8
Valence electrons will never be in the d or f sublevel
Electron Dot (Lewis) Diagrams
 The octet rule
1. All elements except hydrogen ( hydrogen have a duet of electrons) have octet of electrons
once they from ions and covalent compounds.
 The Lewis dot symbols for atoms and ions shows how many valence electrons they have
 Basic rules drawing Lewis dot symbols
1. Draw the atomic symbol.
2. Treat each side as a box that can hold up to two electrons.
3. Count the electrons in the valence shell.
Start filling box - don’t make pairs unless you need to.
 Elements try to complete their valence shell to achieve noble-gas electron configurations. The stability
of noble/inert gases is due to their filled valence shell. Elements either loose/gain or share electrons for
this purpose.
 A Lewis symbol is a symbol in which the electrons in the valence shell of an atom or simple ion are
represented by dots placed around the letter symbol of the element. Each dot represents one electron.
Ions- Atoms that have either gained or lost electrons- charged atom
Anion- negative ions- gained electrons
Cation- Positive ions- lost electrons
Basic Atomic Structure, Isotopes, and Atomic Mass Atomic Structure
atom: the smallest piece of an element that retains the properties of that element.
nucleus: a dense region in the center of an atom. The nucleus is made of protons and neutrons, and
contains almost all of an atom’s mass.
proton: a subatomic particle found in the nucleus of an atom. It has a charge of +1, and a mass of
1 atomic mass unit (amu).
neutron: a subatomic particle found in the nucleus of an atom. It has no charge (is neutral), and has a
mass of 1 amu.
electron: a subatomic particle found outside the nucleus of an atom. It has charge of −1 and a mass of
0 amu (really about 1/2000 amu). Atoms can gain, lose, or share electrons in chemical reactions.
charge: positive and negative charges cancel each other out, so the charge of an atom is the difference
between the number of positive charges.
Atomic number (Z) Used to identify an element. Equal to the number of protons in the atom.
Mass Number (A) Sum of the protons and neutrons in an atom
Atomic Mass- The weighted average of all the elements isotopes.
Isotope- element with the same number of protons but with a different number of neutrons.
Write the complete symbol for an atom that has 92 protons 146 neutrons:
Write the complete symbol for an atom whose mass number is 21 and which has 11 neutrons:
Isotopes & Atomic Mass
What's the atomic mass of the 12C isotope?
If the 13C isotope is 1.0836129 times the mass of the 12C isotope, what is the atomic mass of the 13C
isotope?
If in every living thing on earth, 98.89% of the carbon is 12C and 1.11% of the carbon is 13C, what is the
average atomic mass of carbon?
What is the significance of average atomic mass?
Example:
Fill in the remaining cells in the following table, and perform calculations after the table:
Isotope % Abundance Atomic mass (amu)
Ne
90.00
19.99212
Ne
0.27
20.99316
Ne
9.73
21.99132
20
21
22
Calculate the atomic mass of neon.
# of protons
# of electrons
# of neutrons