intermolecular forces

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  • Created by: Leigh. x
  • Created on: 23-10-12 12:27

electronegativity and polarity

Is a measurement of the attraction of a bonded atom for the pair of electrons in a covalent bond

The greater the difference in electronegativity between the bonded atoms, the greater the permanent diople

The greater the electronegative an atom is it will posses the s- charge and the lesser an atom is electron negative it will posses(

Atomic radius decreases > Ionization energy increases > Electronegativity increases

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Intermolecular forces are attractive forces between molecules. These forces are normally weak attractive forces.

1. dipole-dipole forces

2. temporary dipole-induced dipole forces

3. hydrogen bonding forces

all molecules have Van der Waals forces. 

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ionic and covalent are strong

ionic bonds hold ions together in a giant lattice

  • at room temp all ionic compounds solid

covalent molecules are held together by shaired pairs of electrons

  • many are small molecules


intermolecular forces are week

  • act between differnt molecules
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perminant dipole dipole

  • Occur between polar molecules.


  • permanent dipole attracts another permanent dipole in another molecule


  • +ve end of one molecule is near the -ve end of another.


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Van Der Waals or induced dipole dipole

more electrons on one side of the molecule than on the other.

Instantaneous dipole will result and will induce dipoles in the neighbouring molecules by causing electrons in the neighbouring molecule to arrange themselves so that the force is attractive.

small induced dipole attract one another causing weak intermolecular forces 'van der waals'

Force increases with > more electrons- lager induced dipoles >greater attractive force between molecules > boiling point increases- as you go down noble gasses

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Hydrogen Bonding

occurs in F, O and N because they are very electronegative atoms.

Strongest of the intermolecular forces.

High melting pt & boiling pt.

When covalently attached to a very electronegative atom, X hydrogen atom can form a hydrogen bond with another very electronegative atom, Y which has a lone pair of electron.

Hydrogen has no electrons other than its shared pair of electrons in covalent bond because these are being pulled away from it by F, O and N. Now H atom has no inner shell of electrons, the single proton is readily available for any form of dipole-dipole attraction.

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strucutre of ionic compounds

high meltthing and boilng point

solid at room temp

strong forces between ions> large amount of energy needed to break the electrostatic forces ( hold oppositly charged ions together in strong lattice)

electrical conductivity

ions are in a fixed psition so they cant move> doesnt conduct

when melted or disolved in water> solid lattice breaks down> ions free to move> now a conductor


dissolves in polar substances such as water > in water H+ attracts - and O- attracts + ions

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structure of covalent compounds

Simple molecular structure

  • molecules held by weak intermolecular forces (van der waals)> low melthing and boiling point
  • atoms within each molecule are bonded strongly together by covalent bonds
  • no charged particles to move free so> doesnt conduct electricity
  • soluable in non polar solvents> van der waals forces form between them
  • van der waals forces weaken the lattice structure
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structure of covalent compounds

Giant covalent structure

  • high mlting and boiling points> high temps are needed to break the strong covalent bonds in the lattice
  • no free charged particles except in graphite> dont conduct elcricity
  • insoluable in both non and polar solvents > covalent bonds in the lattice too strong to be broken
  • Graphite
  • delocalised electrons between layers> conducts electricity
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properties of water

hydrogen bond in water has 5% of the strength of an O-H covalent bond

less dnse than water

ice is less dense than water> ice has an open lattice with hydrogen bonds holding the water molecules apart 

when ice melts the rigid hydrogen bonds collapse> H2O molecules move closer together

reletivley high melting and boiling point

strong bonds/ H bonds are extra forces> they have to be over come in order to melt or boil

hight surface tension

extra intermolecular bonding

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