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Melting and Boiling Point Periodic Trends

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by

Jennie Yang

on 12 December 2014

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Transcript of Melting and Boiling Point Periodic Trends

The melting point is the amount of energy required to break a bond to change the solid phase of a substance to a liquid.
Alkali and Alkali Earth Metals
Nonmetals
Melting and boiling points generally increase as it goes down the group.
Metalloids
Melting and boiling points generally decrease as it goes down diagonally along the staircase.
Trends Across the Period
for Melting Point
Excluding metalloids, melting point generally increases until group 6 and then decreases as you continue across the period.
Transition Metals
Melting and boiling points generally increase as it goes down the group.
Melting
and
Boiling
Point Periodic Trends

Melting Point
Boiling Point
The boiling point is the amount of energy required to break a bond to change the liquid phase of a substance to a gas.
General Trends Down the Groups
Melting and boiling points generally decrease as it goes down the group.
Trends Across the Period
for Boiling Point
Excluding metalloids, the boiling point generally increases until group 5 and then decreases as you continue across the period.
Group 11 and 12
Exceptions
The elements in groups 11 and 12 form intermolecular bonds in the way that alkali and alkaline earth metals form intermolecular bonds. Their melting and boiling points generally decrease as you go down the group.
Things that Affect Melting and Boiling Points
Metals: Metallic Bonding
Nonmetal & Metalloid Solids: Covalent Bonding
Nonmetal Gases: Van der Waals Forces
BONDS
Valence electrons become delocalized (don't stay with their nuclei), forming a "sea of electrons." The metal is held together by the attraction between the positive atomic cores and negative free electrons.
When a metal has more valence electrons, the metallic bonds are stronger, because the "sea" is more dense with electrons and the nuclei/core electrons left behind have a higher charge.
Metals: Melting v. Boiling Point
When metals melt, their metallic bonds are loosened, but not actually broken.
When metals boil, their metallic bonds are finally broken. It takes a lot of energy for this to happen.
Valence electrons are shared between adjacent atoms. Some elements, like carbon, form lattices with themselves as it bonds.
In general, covalent bonds weaken as you move down a group, because the electrons have to hold together larger atoms; the bonding region becomes smaller compared to the atoms' sizes.
Nonmetal/Metalloid Solids: Melting v. Boiling Point
Such nonmetal/metalloid solids often don't melt or at least have very high melting points because their crystal structure is so tight.
For example, carbon does not exist as a liquid unless it is under very, very high pressure. It instead goes directly from solid to gas (sublimates) when heated to high temperatures.
Gas molecules experience Van Der Waals intermolecular forces,
Dipole-Dipole
London Dispersion
Carbon has an extremely high melting point
Fluorine is a gas at room temperature because the attractions are not strong enough to make fluorine solidify

Iodine is a solid because there is not enough kinetic energy to escape its attractive forces, so the attractions cause the iodine will solidify.
Helium has extremely low melting and boiling points
Gaseous Elements: Melting v. Boiling Point
Gases only have relatively weak Van der Waals forces betwen their molecules, not actual bonds, so they have very low melting/boiling points.
Some gases, like helium, cannot even exist as a solid unless subjected to extremely high pressures.
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