Wedge and Dash: The Ideal Model for Molecular Geometry

When drawing a three-dimensional structure of molecules on the paper, the wedge and dash model is used to point out the position and dimension of atoms in a molecule. It was proposed by the biologist Thomas Huxley in 1869.

This model describes that when two bonding atoms are positioned on the ends of a linear (or more appropriately, planar) arrangement, one atom (the one attached to the more electronegative atom) will be wedged into the respective angle, while the other atom makes the dotted or dash arrangement (attached to the less electronegative atom) and will extend out at an angle opposite to the side of wedge angle.

For example, when a three-dimensional representation of a molecule is drawn on the paper. These are three types of lines used in the diagram to represent the dimension and angle of atoms in the molecule.

  • Solid lines represent the angle of atoms in the same plane of the paper.
  • The wedge-shaped line that shows the atoms pointed toward the viewers.
  • Dash line that represents the atoms pointed away from the viewers.

wedge and dash model in chemistry

What are wedge and dash diagrams?

Wedge and dash diagrams are used in chemistry to determine molecular geometry. Wedge lines are used to denote the atoms facing viewers while dotted or dashed lines are used to show the atoms pointed away from the viewer. They are often used in conjunction with a Newman projection of a molecule or polyatomic ion.

However, successful wedge-and-dash diagrams have three important components that can make or break their overall appearance.

  1. Arrows should point in one direction only.
  2. All dashed lines should be at least two times longer than any wedge lines.
  3. Every wedge should touch two other wedges (except for terminal ones).

If these rules are followed consistently throughout your diagram you will create a visually appealing representation of molecular geometry.

Inorganic chemistry also uses wedge and dash diagrams but they may look different from those in organic chemistry because some bonds can’t be distinguished between single and double. For example, both NaCl and KCl contain linear ions with equal numbers of shared electrons so it’s impossible to tell which one has more electrons between them without more information about them. This is not true for most organic compounds because they tend to have unique geometries that allow us to distinguish between single/double bonded pairs using wedge-and-dash drawings alone.

Traditional VSEPR Model

Molecules are more complex than a simple cube or sphere. When working out molecular geometry, there are two most common models used to represent three-dimensional shapes.

  • Wedge and dash are commonly referred to as VSEPR (valence shell electron pair repulsion).
  • Lewis dot diagrams.

The wedge and dash model is based on valence shell electron pair repulsion (VSEPR). One of the central tenets of VSEPR theory is that when bonding occurs between atoms with lone pairs of electrons, they will be arranged so that these lone pairs lie along one axis. For example, they will be pushed apart by their mutual electrostatic attraction. This results in a shape with bonds pointing away from that axis.

An atom with one lone pair bonded to another atom that has its own lone pair along that same axis creates an overall triangular shape called wedge bond formation. This is because it results in a triangle pointing away from that central axis. Similarly, if add a third atom with its lone pair along that same axis, then all three pairs of electrons will form bonds

pointing away from that center point and hence creating a tetrahedron shape.

Cases where wedge and dash diagrams are more effective than traditional VSEPR

Wedge and dash model can be used to predict where an atom will be most effective in donating a pair of electrons. For example, Sulfur has five valence electrons. It can form bonds with oxygen or nitrogen atoms. However, it is more electronegative than oxygen or nitrogen. In this case, VSEPR can be used to determine which elements will likely bond to sulfur. It will point at sulfur itself, not necessarily in a location that makes sense chemically.

By using wedge and dash diagrams rather than VSEPR rules, sulfur can be placed next to either oxygen or nitrogen, because they are more electronegative than sulfur. This would make both O-S-O and N-S-N bonds possible. This is a very important property of wedge and dash models, as it allows chemists to have much greater control over how atoms bond with each other.

This also means that wedge and dash models work well when predicting how molecules will react. In general, wedge and dash models predict chemical reactions better than VSEPR because they take into account electron density and electronegativity differences between atoms. Electronegativity is a key factor in determining whether or not elements will bond with each other.

With these examples in mind, it’s clear that wedge and dash diagrams are more useful than traditional VSEPR rules in a variety of situations making them an ideal model for molecular geometry. It’s important to note that wedge and dash diagrams are not completely perfect either, there are many cases where you can’t use them effectively due to overlap issues between multiple bonds or lone pairs of electrons.

Why don’t we always use a ball and stick model instead of a wedge and dash model?

There are some cases where a wedge and dash diagram may be more appropriate than a ball and stick model. It all comes down to simplicity and chemistry. In any given wedge and dash model, each atom is shown as either a circle or a square. These circles or squares represent bonds (so if you see an empty circle or square, you know there is no bond). In addition to showing bonds, circles, and squares can also represent lone pairs of electrons.

However, when representing atoms in real life (like hydrogen) wedges are used instead of circles or squares. That’s because we don’t have many examples of hydrogen bonds (if any), so using wedges will give us less clutter on our diagrams without sacrificing accuracy. The number next to each wedge represents how many lone pairs an atom has.

This is useful because it allows us to quickly see which atoms have more or less electron density than others. In addition, we can use these numbers to determine where atoms are likely to form bonds with other atoms.

Concepts Berg

What Is a wedge and dash projection in chemistry?

Wedge and dash projection is a way to represent the 3d molecules on the paper. There are three types of lines are used in the diagram to show the dimension of atoms in the molecules. These are the lines,

  • Solid lines
  • Dash lines
  • Wedge lines

How do you draw wedge-and-dash structures?

We can draw this kind of structure on paper. Those atoms that are pointed toward or away from the viewers are represented by wedge and dash lines.

What does dash bond mean?

These are lines in the form of stacks on each other. These are very small lines and sometimes they are shown in the form of dots.

What does the dashed line represent?

Dashed lines represent the point out away from the viewers.

What does the wedged line mean?

Wedge lines mean atoms are facing the viewers.

Is there a way to represent double bonds in a dash and wedge diagram?

It can be used to represent the double bonds also. Dashed lines are used in some causes to used to denote a double bond if it is out of the plane.

How do you know when to put dash or wedge when you were given the name of the compound?

When we want to draw molecules in three-dimensional shapes, we use the dash and wedge model to show the orientation of atoms in the molecules.

Reference links

Wedge and dash model (thoughtco.com)

Wedge and dash projection (onlineorganicchemistrytutor.com)

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