London Forces
Definition and meaning of London Forces in chemistry.
London forces are weak, temporary electrical attractions that happen between all atoms and molecules. They occur because electrons are constantly moving around the center of an atom. Even though they are weak, these forces help hold many everyday materials together.
In more detail
Electrons form a cloudy shell around the positive nucleus of an atom. Most of the time, this electron cloud is perfectly balanced and perfectly even. But electrons move incredibly fast and can sometimes pile up on one side.
This sudden pileup creates a temporary negative charge on that side of the atom. The opposite side of the atom is left with a temporary positive charge. Chemists refer to this sudden, uneven electrical state as an instantaneous dipole.
When this uneven atom nears a normal neighbor, it pushes the neighbor's electrons away. This pushing creates a matching uneven charge in the neighboring atom. The positive side of one atom now attracts the negative side of the other.
This temporary electrical pull is known as a London dispersion force. These weak forces are the only thing that holds completely nonpolar molecules together. The total strength of the pull depends heavily on the size of the molecule.
Larger atoms have bigger, looser electron clouds that can shift their shape very easily. Because their clouds shift easily, large molecules create much stronger London forces than small molecules.
Key facts
| Field | General Chemistry |
|---|---|
| Also called | London dispersion forces |
| Cause | Sudden shifts in the electron cloud |
| Present in | All atoms and molecules |
| Strength factor | Larger electron clouds make stronger forces |
We can see the power of London forces by looking at the halogen elements. Fluorine and chlorine are both small atoms with very tight electron clouds. Their London forces are very weak, making them both gases at room temperature. Bromine is a moderately larger atom with a much looser electron cloud. Its stronger London forces pull the molecules together to form a visible liquid. Iodine (I2) is an even larger molecule with a massive, floppy electron cloud. Its electrons shift easily to create very strong and frequent temporary charges. These strong London forces completely lock the heavy iodine molecules together. Because of this locking effect, iodine exists as a solid crystal at room temperature.
Frequently asked questions
Do London forces happen in water?
Yes, London forces happen in every single molecule, including water. However, water also has much stronger pulls called hydrogen bonds that hide the weak London forces.
Why are London forces temporary?
The electrons in an atom never stop moving. The uneven pileup that causes the force only lasts for a tiny fraction of a second before the electrons move again.
How does molecule shape affect London forces?
Long, flat molecules can lay right next to each other and touch in many places. This large surface area creates stronger London forces than round, bumpy molecules.