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Instantaneous Dipole­ Induced Dipole Forces By Sam Jangala, David Sundaram & Elisha Lai


Introduction  Can be known as:  London Dispersion Forces [LDF]  Named after the German­American physicist Fritz London

 Dispersion Forces  Part of van der Waal’s forces  Weakest type of attractive force  Found between atoms/molecules  The unequal distribution of e­s around a molecule/atom at any  given instant induces a temporary dipole in the molecule


Analogy: Effect of a Magnet on a  Pin

δ-

The pin loses its attraction to the magnet as the magnet moves away from the pin.

A constantly moving magnet (magnetic dipole) comes close to a pin.

The pin is then attracted to the magnet.

The pin becomes temporarily magnetized (induced dipole).

δ+


Strength of Forces

Increases down a group

Increases across a period


Strength of Forces δ-[1 e-] δ+[0e-]

Methane [CH4]

Silane [SiH4]

Germane [GeH4]

•C-6 protons,6 electrons

•Si-14 protons,14 electrons

•Ge-32 protons,32 electrons

Increases with the number of protons and electrons because more chance of dipoles forming. Therefore, the boiling points of these compounds also increase.


Strength of Forces  Continuously varies within a molecule


Daily Life Application #1: Non­Stick  Pans  Polytetrafluoroethane [PTFE]  Commonly known as  Teflon

 LDF between oil/grease & 

PTFE are much weaker than  those present in the oil/grease  itself

 Therefore, oil/grease doesn’t  stick to the PTFE pan


Daily Life Application #2: Graphite Lead in  Pencil  Sheets of carbon atoms are  held together by LDF

 LDF easily overcome  Therefore, the sheets slide  over each other when you  write


Instantaneous dipole-Induced Dipole Forces