If you’ve ever had to answer a question about polarity in a chemistry class, then you’re probably aware of the fact that a molecule can be polar or nonpolar. The difference is based on whether or not the molecule has an occupied space in its bond lines, which is known as the lone pair. When you’re trying to predict whether or not a molecule is polar, you have to know the lone pair repulsion forces. This is important, because the repulsion can affect the properties of the molecule, such as its strength and the ability to form bonds.
lone pair-bond pair repulsion
There are two types of electron pairs in a molecule, bonding and lone. The valence shell electron pair repulsion theory states that bonding and lone pairs are attracted and repelled. These repulsions have a direct effect on the geometry of the molecule.
The shape of the molecule is governed by the arrangement of the electron pairs around the central atom. Lone pairs are positioned closer to the atomic nucleus than the bonding pairs. They also have a stronger repulsion than the bonding pairs.
When the molecule is arranged with a lone pair in the central atom, the lone pair-bond pair repulsion increases and the bond angle decreases. Hence, the shape of the molecule is distorted.
If the lone pair-bond pair consists of two lone pairs, the repulsion is even greater. The bond angle is reduced by about 2.5 degrees. This is in contrast to the repulsion between bonding and lone pairs, which is about a quarter of the repulsion of the lone pairs.
For example, the shape of the NH3 molecule is tetrahedral. In addition, it has three bonds between N and H. It interacts with singly charged cations such as Na+, Li+, and K+.
Unlike NH3, the shape of the BH3 molecule is not tetrahedral. However, it has an attractive repulsive interaction with BeH2 and a less attractive proton attraction with PH3.
The chemistry of the fluorine atom is very similar to the NH3 molecule. However, the fluorine atom is much smaller and has higher electronegativity. As a result, the F — F bond is weaker than expected.
It is important to keep in mind that if there are not empty orbitals, the lone pairs are not involved in bond formation. Instead, they are used to squeeze bonding pairs closer together.
London dispersion forces
The London dispersion force is a temporary dipole that is created by the movement of electrons in a molecule. It’s a subset of the Van der Waals force.
The strength of the London dispersion force depends on the atomic weight of the molecule. Generally, larger atoms have more polarizable electrons and thus, the force is stronger. On the other hand, smaller atoms have less polarizable electrons, and therefore, the force is weaker.
London dispersion forces are present in all types of molecules. However, they are most noticeable in nonpolar molecules. For example, petroleum jelly is composed of alkanes of different lengths. These materials are liquid at room temperature. They also condense to a solid at low temperatures.
Unlike covalent bonds, which are permanent, intermolecular forces are temporary. This is because they take place over short distances. Moreover, they are weaker than covalent bonds.
London dispersion forces are not very strong, and they are very limited. But they are still present between individual atoms. Consequently, they are known as the weakest of all intermolecular forces.
When two molecules are polar, they attract each other through the London dispersion force. However, when a neutral molecule is placed near a molecule with a dipole, the neutral molecule’s electrons shift. In addition, the induced dipole can influence the molecule next to it.
Although the London dispersion force is very weak, it is important to understand it. Besides affecting the viscosity of a molecule, it also determines the boiling point and melting point of a material. Moreover, it is necessary for gases to have this force.
The London dispersion force is often the cause for the condensation of gases from the gas phase to the liquid phase. It is also responsible for the liquification of oxygen, nitrogen and argon gases.
Predicting a molecule’s polarity
The polarity of a molecule is determined by a dipole moment. Dipole moments are a result of the asymmetrical distribution of electrons. Its magnitude and direction is a product of the charge on each bonded atom and the distance between them.
When there is a difference in the electronegativity of an atom, the bond between the atoms is polar. Therefore, it is necessary to know the polarity of an atom to be able to predict the polarity of a molecule.
Polar molecules, also known as dipoles, have partial positive charges on one end and a partial negative charge on the other. Nonpolar molecules have zero dipole moment.
NH3, a tetrahedral molecule with three hydrogen atoms, is a polar molecule. However, its polarity is affected by the presence of nitrogen and the asymmetrical structural geometry.
The valence shell electron pair repulsion (VSEPR) theory is an effective method to determine the polarity of a molecule. It is based on the assumption that electron pairs will repel each other. Consequently, a dipole moment will be predicted.
NH3 is a polar molecule because it has a polar N-H bond. Besides, its trigonal pyramidal structure makes its atoms form an asymmetrical structure. As a result, the atoms have a net dipole moment.
CH2Cl2, a polar molecule with two hydrogen atoms, has a low negative charge on the hydrogen atom region. However, its symmetry cancels out the dipole moments. Similarly, the nonpolar molecule Nz has no dipole moments between nitrogen atoms.
Polar molecules have a permanent electron density distribution. They have lone pairs of electrons at their central atom. This distribution of electrons affects the behavior and reactivity of the molecule.
CCl4 is a symmetrical tetrahedral molecule
Carbon Tetrachloride (CCl4) is a chemical compound that is found in the environment. Its molecule is symmetrical and has four C-Cl bonds. This bond is nonpolar and it can only dissolve in nonpolar solvents.
CCl4 molecule has a negative charge because of its four chlorine atoms. These atoms have a higher electronegativity than carbon atoms. They pull a positive charge in the direction of their position.
The polarity of a molecule is determined by the angle that the bonds have. This angle is called the mutual bond angle. CCl4 has a mutual bond angle of 109.5 degrees.
CCl4 has a Lewis structure with a tetrahedral shape. This is the only way that it is able to have a zero dipole moment. However, there are some lone pairs that are on the central O atom and this contributes to the polarity of the molecule.
A lone pair on the nitrogen atom has a positive charge while one on the hydrogen atom has a negative charge. Unlike covalent bonds, this lone pair takes up more space in the molecule.
Besides asymmetrical bonds, the C-Cl bonds in the molecule have a specific dipole moment value. For example, the CH3Cl bond has a dipole moment of 1.87 D.
When the C-Cl bonds are arranged in a symmetrical tetrahedral structure, the dipole moments in both directions cancel out. This is the reason that the CCl4 molecule has a zero dipole moment.
The lone pairs on the outer atoms are omitted for clarity. However, they are the determining factor in whether a molecule is polar or nonpolar.
When a molecule has a positive dipole moment, it is polar. On the other hand, when a molecule has a negative dipole moment, it is nonpolar.
CS2 is a nonpolar molecule
Carbon disulfide (CS2) is a nonpolar molecule that is a corrosive agent. It is used in various solvent extraction processes. The molecule has two sulfur atoms on either side of the carbon atom. These atoms form a covalent bond with the carbon atom.
This molecule is linear in shape. It is formed by two double bonds that share four electrons. The bond angle is 180 degrees. Because of this, the dipole moment of the molecule is equal and opposite.
The Lewis structure of the CS2 molecule is known as electron dot structure. It is based on the octet rule, which states that a molecule should have eight electrons in the outermost valence shell.
The octet rule is also responsible for the CS2 molecule’s symmetry. Specifically, it means that the central atom of the molecule should be in the center. If the atom is not, the Lewis structure would become assymetric.
To calculate the CS2 Lewis structure, the carbon atom should be in the center. Alternatively, the sulfur atom can be the center. However, the electronegativity difference between the carbon and the sulfur atoms causes the C-S bonds to be polar. Therefore, it is not possible for the sulfur atom to be in the center.
In order to calculate the Lewis structure, you must know the electronegativity difference between the carbon atom and the sulfur atom. By taking the value of the valence electrons of the molecule, you can easily find the difference in electronegativity. When the difference is less than 0.5, the Lewis structure becomes assymetric.
You can also use the octet rule to calculate the electronegativity of the molecule. According to this method, you can see that the CS2 molecule has an electronegativity of 2.55.
To calculate the CS2 Lewis structure, you can follow the VSEPR theory. This theory predicts the trigonal bipyramidal molecule.
