Trends in Melting and Boiling Temperatures in
The Periodic Table
Introduction
The melting and boiling points of a substance are determined by its structure and the strength of
intermolecular forces.
Giant structures generally have high melting and boiling points.
Simple molecular structures generally have low melting and boiling points. The trend for
melting point is the same as the trend for boiling point.
Trends Across a Period
When examining trends across a period (from left to right), the type of structure and bonding
changes significantly, leading to distinct trends in melting and boiling points.
Metallic Elements (Groups 1-3)
Structure: Giant metallic structure (regular rows of positive ions surrounded by delocalized
electrons).
Trend: Melting and boiling points generally increase from Group 1 to Group 3 (e.g., Na to Mg
to Al).
Explanation: As you move across these groups, the number of delocalized outer shell
electrons per atom increases (1 in Group 1, 2 in Group 2, 3 in Group 3). This leads to stronger
attractive forces between the positive ions and the delocalized electrons, requiring more
energy to overcome.
Group 4 Elements
Structure: Giant macromolecular structures (e.g., diamond and graphite for carbon, silicon).
These structures involve many strong covalent bonds between atoms in a three-dimensional
network.
Trend: Group 4 elements typically have the highest melting and boiling points in a period.
Explanation: A large amount of energy is required to break the numerous strong covalent
bonds within these giant structures.
Groups 5-8 Elements
Structure: Simple molecular structures. These can be diatomic molecules (e.g., N₂, O₂, F₂),
polyatomic molecules (e.g., P₄, S₈), or monatomic atoms (e.g., Ne, Ar).
Trend: Melting and boiling points are low. There can be a slight increase from Group 5 to
Group 6/7 due to increasing molecular size and therefore stronger intermolecular forces (Van
der Waals forces). Group 8 elements (noble gases) have very low melting and boiling points.
Explanation:
The forces between these simple molecules are weak intermolecular forces (Van der Waals
forces). Only a small amount of energy is needed to overcome these weak forces.
The Periodic Table
Introduction
The melting and boiling points of a substance are determined by its structure and the strength of
intermolecular forces.
Giant structures generally have high melting and boiling points.
Simple molecular structures generally have low melting and boiling points. The trend for
melting point is the same as the trend for boiling point.
Trends Across a Period
When examining trends across a period (from left to right), the type of structure and bonding
changes significantly, leading to distinct trends in melting and boiling points.
Metallic Elements (Groups 1-3)
Structure: Giant metallic structure (regular rows of positive ions surrounded by delocalized
electrons).
Trend: Melting and boiling points generally increase from Group 1 to Group 3 (e.g., Na to Mg
to Al).
Explanation: As you move across these groups, the number of delocalized outer shell
electrons per atom increases (1 in Group 1, 2 in Group 2, 3 in Group 3). This leads to stronger
attractive forces between the positive ions and the delocalized electrons, requiring more
energy to overcome.
Group 4 Elements
Structure: Giant macromolecular structures (e.g., diamond and graphite for carbon, silicon).
These structures involve many strong covalent bonds between atoms in a three-dimensional
network.
Trend: Group 4 elements typically have the highest melting and boiling points in a period.
Explanation: A large amount of energy is required to break the numerous strong covalent
bonds within these giant structures.
Groups 5-8 Elements
Structure: Simple molecular structures. These can be diatomic molecules (e.g., N₂, O₂, F₂),
polyatomic molecules (e.g., P₄, S₈), or monatomic atoms (e.g., Ne, Ar).
Trend: Melting and boiling points are low. There can be a slight increase from Group 5 to
Group 6/7 due to increasing molecular size and therefore stronger intermolecular forces (Van
der Waals forces). Group 8 elements (noble gases) have very low melting and boiling points.
Explanation:
The forces between these simple molecules are weak intermolecular forces (Van der Waals
forces). Only a small amount of energy is needed to overcome these weak forces.
