Study Guides
Big Picture
When Dmitri Mendeleev was organizing the elements into the periodic table, he placed elements with similar physical
and chemical properties in the same group (vertical column). Since electrons play a key role in determining the
properties of elements, the periodic table actually reflects the repeating pattern in the electron configurations of atoms.
An element’s location on the periodic table will indicate the principal and azimuthal quantum numbers for that element.
Key Terms
Chemistry
Periodic Table: Quantum Numbers
Periodic Law: When the elements are arranged in order of increasing atomic number, the properties of elements are
“periodic” - they repeat from left to right across each period.
Group: Vertical column on the periodic table.
Period: Horizontal row on the periodic table.
Noble Gases: Rightmost group of the periodic table.
Valence Electron: Electron in the outermost principal quantum level of an atom.
Core Electron: Electron that is not in the outermost principal quantum level of an atom.
Principal & Azimuthal Quantum Numbers
The periodic law is the result of the the electron configurations. The periodic table can be split up into blocks where
each block tells you which sublevel (the azimuthal quantum number l) the last electron is in.
• The number of groups (columns) in each block is equal to the number of electrons that sublevel can hold.
• The s-block contains 2 groups – s sublevels can have a maximum of 2 electrons
• The p-block contains 6 groups – p sublevels can have a maximum of 6 electrons (2 in each of the three p orbitals)
• The d-block contains 10 groups – d sublevels can have a maximum of 10 electrons (2 in each of the five d orbitals)
• The f-block contains 14 groups – f sublevels can have a maximum of 14 electrons (2 in each of the seven f orbitals)
Helium is part of the s-block even though it is drawn in Group 18 (Group 8A).
The period gives the highest value of the principal quantum number.
• For elements in the main group, the period also indicates the highest occupied energy level.
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Disclaimer: this study guide was not created to replace
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Figure: A periodic table with labeled blocks
Chemistry
Periodic Table: Quantum Numbers
cont .
Noble Gas Configuration
In the electron configurations for the elements in group 18 (group 8A), the s and p sublevels of the highest occupied
energy level are completely filled.
Electron Configuration for Noble Gases
Element
Electron Configuration
Helium (He)
Neon (Ne)
Argon (Ar)
Krypton (Kr)
Xenon (Xe)
Radon (Rn)
Writing out the electron configurations for elements with high atomic numbers can be a tedious and lengthy process.
To simplify the electron configuration for an element:
1.Look for the noble gas that precedes the element.
• On a periodic table, look at the period above the period that the element is and look at the rightmost column.
2.Write the chemical symbol of the noble gas in square brackets [].
• Examples: [He], [Ne], [Ar], [Kr], [Xe], [Rn]
3.Write out the configuration for the electrons not contained in the noble gas configuration.
Example: oxygen (Z = 8)
• Entire electron configuration:
• Noble gas configuration:
• [He] means that the configuration is identical to the electron configuration of helium.
This does not mean that there is a helium atom inside the carbon atom. It just means that the configuration of
the first 2 electrons in carbon is the same as the configuration for helium.
Example: sulfur (Z = 16)
• Entire electron configuration:
• Noble gas configuration:
• Sulfur is in the same group
as oxygen. They have similar configurations for the electrons not contained in the
noble gas configuration – 2 electrons in the s orbitals and 4 electrons in the p orbitals.
Valence and Core Electrons
Valence electrons are electrons that affect the chemical properties of an element. The number of valence electrons
can be seen in the electron configuration and the periodic table. For main group elements:
• The
valence electrons are the ones not present in the previous noble gas. Ignore the electrons in the d and f
sublevels.
• The number of valence electrons equals to the group number when using the American system.
• Helium is the exception: helium has two valence electrons, not eight.
The main groups in the periodic table have the following number of valence electrons:
• Group 1 (Group 1A): 1 valence electron
• Group 2 (Group 2A): 2 valence electrons
• Group 13 (Group 3A): 3 valence electrons
• Group 14 (Group 4A): 4 valence electrons
• Group 15 (Group 5A): 5 valence electrons
• Group 16 (Group 6A): 6 valence electrons
• Group 17 (Group 7A): 7 valence electrons
• Group 18 (Group 8A): 8 valence electrons (except for helium, which has 2)
Core electrons do not affect the chemical properties.
• The electrons in a noble gas configuration are part of the core electrons.
Valence electrons for transition metals do not follow a clear pattern and are not useful for predicting the chemical
properties of transition metals.
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