What is molecular geometry?

The three-dimensional shape of a molecule is an important factor in the chemical and physical properties of the molecule. This includes its reactivity, color, biological activity, and state of matter (in a solid, liquid, or gas). The geometry also affects how the atoms are connected together through chemical bonds. The atomic position of an atom in space can be specified by bond lengths, bond angles, and torsional angles. 

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Molecular Geometry and Electron Pair Geometry

A molecule’s molecular geometry is the three-dimensional arrangement of the atoms in a molecule. This can be determined by using a theory of how electrons arrange around a central atom. This is called Valence Shell Electron Pair Repulsion, or VSEPR, and is the main method used to predict the shape of a simple molecular compound. 

VSEPR is used to calculate the geometrical properties of many compounds that have a central atom, like oxygen and carbon dioxide. This theory proposes that the repulsion between the electron pairs will cause them to arrange in a way that maximizes the distance between them. This results in the molecules having a different geometric shape from what they would if there were no bonded or lone pairs of electrons. 

Depending on whether there are lone pairs or bonded electrons, the molecules can be linear, trigonal planar or angular. For example, carbon dioxide has a linear shape because the bonded electrons are arranged in a straight line. Water has a trigonal planar shape because the bonded electrons are positioned in a flat plane. 

Electron Pair Geometry and Molecular Geometry

In a molecule without lone pairs, electrons will be arranged around the central atom in a way that minimizes their repulsion and maximizes the distance between them. This will make the bond angles in the molecule slightly smaller than they would be with bonded electrons. 

However, when there are lone pairs of electrons in the molecule, they will repel each other stronger than bonded ones. This will change the angle of the molecule, and so the atoms will be placed in a different shape than they would be without lone pairs. 

There are four basic types of geometries: linear, trigonal planar, angular and octahedral. The bond angles in these geometries can be different as well, making the molecules able to exist in a variety of states (or conformations). 

Linear: In a linear model, the atoms are arranged in a straight line with the bond angles set at 180deg. This is the geometry of most common molecules. Examples include carbon dioxide and nitric oxide. 

Trigonal Planar: A trigonal planar molecular geometry has a flat structure with the bond angles set at 120deg. This is the geometry of most boron molecules, such as boron trifluoride. 

Angular: In an angular molecular geometry, the atoms are not arranged in a straight line but are arranged in a non-linear manner, such as water’s. These molecules are sometimes known as bent molecules, but the shape is actually non-linear.