London Dispersion Force: Definition

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• The London dispersion force weakly attracts all atomsAtoms

  All atoms of any element that have electrons will attract to any other atoms via the London dispersion force.

  Example:

  Even hydrogen with only one electron attracts to other atoms by the London dispersion force. The same force causes even noble gas atoms to attract together

  to each other
• The attraction is caused by the electrostatic force, so the London dispersion force is not actually a fundamental force

  There are four fundamental forces which are thought to cause all other forces. These are gravity, the electrostatic force, the weak force and the strong force.

  Example:

  The London dispersion force is not listed becasue it is caused by the electrostatic force

  !
• As electrons move around the nucleus

  We do not actually know the path that electrons travel as the move around the nucleus. But we do know that they move to different positions near the nucleus. Thus, they will be found at different places at different times

  Example:

  We know that the electrons of a helium atom are found somewhere within a small sphere (orbital) around the nucleus 90% of the time. But we do not know the direction and speed of the electrons as they travel

  , there are times

  The electrons are constantly moving, so at any given moment, you might find them collected on one side of the nucleus, on the other side or scattered evenly. Any arrangement is very temporary as the electrons soon move to a new position

  Example:

  When both of the electrons of a helium atom are breifly on one side of the nucleus, that side gains a negative charge, while the other side has a postive charge

  when more of them are on one side than the other. This causes a temporary dipole

  A dipole is anything with two opposite charges separated by a distance. In this case, an atom with net positive charge on one side and negative charge on the other. The dipole is temporary becasue the electrons will soon move to a new position which may no longer form a dipole

  Example:

  A helium atom with both electrons on one side forms a dipole, but if the electrons are close to evenly spaced around the nucleus, the dipole is minimal

  where the side of the atom with more electrons has a partial negative electric charge

  The electrical charge of an atom defines how strongly oppositely-charged particles attract to one another via the electromagnetic force. Protons have a +1 charge and electrons have a -1 charge. The partial overlap of charges in atoms often casues areas of partial (non-integer) charges

  Example:

  Atoms like sodium and chlorine have +1 and -1 electric charges and attract one another strongly. Two argon atoms attract very weakly becasue they only have partial charges caused by electron dispersion

  and the side with fewer electrons has a partial positive charge
• The negative charge of the electron-dense side of the temporary dipole repels electrons in nearby atoms, causing them to temporarily form dipoles as well
• This is true of both free atoms and atoms incorporated in molecules

  Although atoms that are part of molecules share electrons through bonds, these electrons are still capable of moving to create partial charges and attractions via the London dispersion force

  Example:

  Even H₂ gas (which has no electrons which are not involved in bonds) is capable of weakly attracting other molecules of H₂. The weak attraction causes H₂ molecules to attract one another to form solid hydrogen at very low temperatures

  , so entire molecules also attract via the London dispersion force