Shapes of molecules. How to predict the shapes of molecules Shapes can be predicted from the number of electron pairs around the central atom of a molecule.

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Presentation transcript:

Shapes of molecules

How to predict the shapes of molecules Shapes can be predicted from the number of electron pairs around the central atom of a molecule. Electrons are negative and repel each other. They will arrange themselves so that they are as far away as possible

Two bonding pairs XOOX Linear structure Bond angle – 180 o

Eg; Beryllium chloride. XOOX Be = 1s2 2s2 Cl = 1s2 2s2 2p6 3s2 3p5 BeCl Beryllium forms an electron deficient compound, making two covalent bonds with chlorine

Three bonding pairs XO Bond Angle 120 o Shape; Trigonal planar

Eg; Boron Triflouride XO Boron; 1s2 2s2 2p1 Flourine; 1s2 2s2 2p5 B F FF NB Electron deficient

Four bonding pairs XO Bond Angle o Shape; Tetrahedron (“four faces”).

Drawing 3D structures. Use a simple line to represent bonds in the plane of the paper. Use a wedge, pointing towards the central atom to represent a bond “coming out” of the paper. Use a wedge of stacked lines to represent a bond “going into” the plane of the paper.

Eg For a tetrahedron

Eg; Methane XO Carbon 1s2 2s2 2p2. Hydrogen 1s1 H H H H C

Six bonding pairs XO Bond angle 90 o Shape Octahedron ( eight faces)

Eg; SF 6 XO Sulphur 1s2 2s2 2p6 3s2 3p4 Flourine 1s2 2s2 2p5 S F F F F F F

Three bonding pairs and one lone pair. XX Lone pair XO Lone pairs repel more than a bonding pair as the –ve charge is more dense. Bond angle 107 o Shape; pyramid

Eg; Ammonia XX Lone pair XO Nitrogen 1s2 2s2 2p3 Hydrogen 1s1 N H H H

Two bonding pairs and two lone pairs XX Lone pairs XO Angle o Structure Bent linear or V shape. Two lone pairs cause even more repulsion, lowering the bond angles.

Eg; Water XX Lone pairs XO Oxygen 1s2 2s2 2p4 Hydrogen 1s1