Valence shell electron pair repulsion (VSEPR) theory
Valence Shell Electron Pair Repulsion (VSEPR) theory is a fundamental concept in chemistry used to predict the shapes of covalent molecules based on the repulsion between electron pairs surrounding a central atom. According to VSEPR theory, the arrangement of valence electrons—both bonded and lone pairs—determines the molecular geometry by causing the electron pairs to position themselves as far apart as possible. This theory, introduced by chemists Ronald Gillespie and Ronald Nyholm in 1957, categorizes molecular shapes such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, each corresponding to specific numbers of electron pairs.
The molecular geometry is critical since it influences the compound's chemical properties, including its reactivity and physical characteristics. For instance, bond angles vary based on the number of bonded and lone pairs, affecting how the molecule interacts with others. Understanding VSEPR theory is essential for anyone studying molecular compounds, as it provides a framework for visualizing and predicting the three-dimensional arrangements of atoms within a molecule. This theory is widely applicable in various scientific fields, including chemistry, biology, and materials science, offering insights into the behavior of substances at the molecular level.
Valence shell electron pair repulsion (VSEPR) theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the shape of covalent molecules. Covalent molecules are atoms that are connected by covalent bonds. Molecular geometry refers to the shape of molecules, which is determined by the number and position of valence electrons. Because these electrons repel each other, they try to move as far apart as possible. This causes them to form specific shapes. VSEPR (pronounced VES-PER) is a method to determine their structure. VSEPR was first proposed in 1957 by Ronald Gillespie and Ronald Nyholm, both chemists, in their paper "Inorganic Stereochemistry."
Elements & Atoms
An element is a primary component of matter. It contains only one type of atom and cannot be broken down into a simpler substance. As of 2014, scientists have discovered 118 elements, which are listed in the periodic table.
An atom is the smallest unit of an element. Each atom has a specific number of protons in its nucleus, or center. An element's atomic number is the number of protons within the nucleus of its atoms. For example, oxygen's atomic number is eight, which means its atoms have eight protons.
An atom has the same number of protons and electrons, but its electrons orbit, or spin around, the atom's nucleus much the way planets orbit the sun. Electrons orbit in orbitals, which are commonly called shells. The first shell, the inner shell, can hold two electrons. The next shell can hold eight electrons. The first eighteen elements in the periodic table can hold eight electrons at most in the third, or outer, shell. Valence electrons are the electrons in this shell.
Chemical Compounds
When two or more elements bind, they form a chemical compound. Elements bond in two main ways: ionic bonding and covalent bonding. Withionic bonding, one atom donates valence electrons to the other. When atoms bond with one another, they try to become stable. To do this, some atoms try to acquire the maximum number of electrons in their outer shell. Other atoms try to empty their outer shell so that it does not contain any electrons. This also makes the atom stable because this empty shell is no longer considered its outer shell. For example, lithium fluoride is a chemical compound with an ionic bond. An atom of lithium has one electron in its outer shell and an atom of fluorine has seven. When they bond, a lithium atom gives the electron in its outer shell to a fluorine atom. This fills the outer shell, which makes the fluorine atom stable. It also makes the lithium atom stable; because it does not have electrons in its outer shell, which is its third shell, its second shell becomes its outer shell. This shell has the maximum number of electrons, so it is also full. This is an example of ionic bonding.
Covalent bondingworks in a similar way except the atoms share electrons when they bond. The two atoms from different elements move close together, and an electron from each atom feels an attraction, or pull, from a proton in the other atom's nucleus. This attraction pulls the atoms together, and they share electrons in an area near the atoms' nuclei. Covalent compounds are known as covalent molecules or molecular compounds.
Three-Dimensional Representations
Molecules have different properties. For example, in general, molecular compounds have low melting and boiling points and are flexible and flammable. An individual compound's properties depend on the atoms it contains and the way in which the atoms are arranged. A molecular compound's three-dimensional arrangement is referred to as its molecular geometry.
A molecular compound's geometry is determined by the number of valance electrons and the location of these electrons within the compound. According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, pairs of electrons in a molecular compound (both those that are in covalent bonds and those that are not) repel each other and therefore try to move as far apart as possible. This causes the molecule to take on a specific shape. These are the main postulates of the VSEPR theory:
Molecules with three or more atoms have a central atom. The other atoms are linked to the central atom.
A molecule's geometry is determined by the number of valence-shell electron pairs. These pairs move around the central atom. They may be bonded or not bonded.
All electrons are negatively charged and, therefore, repel each other.
Number of Valence-Shell Electron Pairs: Shape
2: Linear
3: Trigonal planar
4: Tetrahedral
5: Trigonal bypyramidal
6: Octahedral
Determining the number of bonded and lone pairs of electrons helps scientists determine the bond angle formed between the pairs and the central atom. For example, if a molecule has two valence-shell electron pairs and both are bonded, it has a linear shape with a 180-degree bond angle. A molecule with five valence-shell electron pairs, two of which are bonded and two of which are lone, also has a linear shape with a 180-degree bond angle. On the other hand, a molecule with three electron pairs, two of which are bonded and one of which is lone, has a bond angle of 120 degrees.
Bibliography
Boschmann, Erwin. "Molecules." Chemistry Explained: Foundations and Applications. Chemistry Encyclopedia. Web. 23 Dec. 2014. http://www.chemistryexplained.com/Ma-Na/Molecules.html
"Chemical Compound. "Science Daily. ScienceDaily. LLC. Web. 23 Dec. 2014. http://www.sciencedaily.com/articles/c/chemical‗compound.htm
"VSEPR and Molecular Geometry." WzyAnt Resources. WyzAnt, Inc. Wev. 23 Dec. 2014. http://www.wyzant.com/resources/lessons/science/chemistry/lewis‗structures‗vsepr
Wyman, Elizabeth (Nikki). "VSEPR Theory & Molecular Shape." Chemistry 101: General Chemistry Education. Education Portal. Web. 23. Dec. 2014. http://education-portal.com/academy/lesson/vsepr-theory-and-dipole-moments.html