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Molecular geometry - Lewis structures, names, polarity
Molecular geometry - Lewis structures, names, polarity
Shapes and Lewis Structures
Shapes and Lewis Structures
Lewis structures are 2 dimensional visual representation of atoms, and more specifically molecules which show the bonds and valence shell electrons.
Lewis structures are 2 dimensional visual representation of atoms, and more specifically molecules which show the bonds and valence shell electrons.
Here are a few examples of how to draw simple Lewis structures.
Here are a few examples of how to draw simple Lewis structures.
Names and polarity
Names and polarity
To determine whether a molecule is polar or not the bonds as well as the overall shape need to be considered. A molecule can contain polar bonds (discussed above) but can be non-polar depending on the shape. The key is whether dipoles cancel out across the molecule. For example the shape of the molecule can be symmetrical (more later) but if the bonds are different then the dipoles will not cancel out-the molecule will be polar. Looking at the image below: CCl₄ and CH₃Cl illustrate this point. They both have a symmetrical shape (tetrahedral) and contain polar bonds. However CCl₄ contains the same bonds (C-Cl x 4 )which cancel out; whereas CH₃Cl contains different bonds (C-Cl x 1; C-H x 1) which do not cancel out. CCl₄ is non-polar whereas is CH₃Cl polar.
To determine whether a molecule is polar or not the bonds as well as the overall shape need to be considered. A molecule can contain polar bonds (discussed above) but can be non-polar depending on the shape. The key is whether dipoles cancel out across the molecule. For example the shape of the molecule can be symmetrical (more later) but if the bonds are different then the dipoles will not cancel out-the molecule will be polar. Looking at the image below: CCl₄ and CH₃Cl illustrate this point. They both have a symmetrical shape (tetrahedral) and contain polar bonds. However CCl₄ contains the same bonds (C-Cl x 4 )which cancel out; whereas CH₃Cl contains different bonds (C-Cl x 1; C-H x 1) which do not cancel out. CCl₄ is non-polar whereas is CH₃Cl polar.
key points
key points
Polar molecules have permanent dipoles and stronger intermolecular forces
Polar molecules have permanent dipoles and stronger intermolecular forces
This means one end of the molecule has a permanent negative charge and the other end a permanent positive charge
This means one end of the molecule has a permanent negative charge and the other end a permanent positive charge
Asymmetrically shaped molecules will be polar
Asymmetrically shaped molecules will be polar
Symmetrically shaped molecules, such as tetrahedral, will be non polar, unless they contain different bonds
Symmetrically shaped molecules, such as tetrahedral, will be non polar, unless they contain different bonds
molecular shapes
molecular shapes
Shapes of molecules can be determined from the Lewis structure and the VSEPR theory.
Shapes of molecules can be determined from the Lewis structure and the VSEPR theory.
Here is a table showing the molecular shapes - at this level you only need to know: linear, bent, trigonal planar, trigonal pyramidal and tetrahedral. The other shapes, are needed for Level 3 Chemistry.
Here is a table showing the molecular shapes - at this level you only need to know: linear, bent, trigonal planar, trigonal pyramidal and tetrahedral. The other shapes, are needed for Level 3 Chemistry.
Remember: if atoms are in the same group they have the same valency and will have the same shape. For example CO₂ has the same shape as CS₂ as the O and the S are in the same group; likewise H₂O has the same shape as H₂S.
Remember: if atoms are in the same group they have the same valency and will have the same shape. For example CO₂ has the same shape as CS₂ as the O and the S are in the same group; likewise H₂O has the same shape as H₂S.
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