Secondary Interactions and Determining Structure and Properties Of Matter - Session 1

Polarization.It is a deformation of a symmetric electron charge cloud of a cation and anion. It is the act of dividing something that contains different opinions into two different opposing groups. An ionic bond is formed by gain or loss of electrons and as a result anions and cations are formed. So the cation distorts the shape of the cloud of the anion. This distortion of an electron cloud of the negatively charged ion by the positively charged ion is called polarization. It refers to the tendency of matter to acquire an electric dipole moment in an electric field in proportion to that applied field.

In an ionic crystal, the positive electric field of a cation pulls the electronic cloud of an anion from an equal distance. Polarizability of cations.Polarizing power depends upon the capability of a cation to distort the electronic cloud of an anion. Cations with smaller sizes and with high positive charges have high polarizing power.Polarizing Power of anions.It is a property to be polarized by cation deformation of electronic cloud. Its polarizability rises with an increment in anionic size and negative charge.By this, the covalent characteristic of the molecule also grows.

It is a measure of the polarity of a molecule. It is produced when atoms unequally share electrons in a molecule. dipole moment is produced in a molecule when charge separation occurs. Charge separation occurs when the electronegativity difference between atoms is high. Let's see this through the hydrogen chloride molecule. Hydrogen has a positive charge and chlorine has a negative charge. They have a high electronegativity difference. Because of this, dipoles are created in it and we can measure its dipole moment. dipole moment is a vector quantity represented by µ.

It is equal to the distance between the charges multiply by the number of charges. Its direction is always from a less electronegative atom to a more electronegative atom. In molecules having a lone pair, the dipole moment direction is towards the lone pair. The dipole moment of a non-polar molecule is zero. The dipole moment is not equal to zero for a polar molecule.Heteroatomic molecules.Carbon dioxide’s dipole moment is zero because the effect of polarity of one side oxygen is canceled out by second on the other side oxygen. Thus it becomes non-polar.

Ammonia.We know Ammonia has one lone pair, and three bond pairs. The polarity of the lone pair is in the direction of the lone pair. Meanwhile, between hydrogen and nitrogen, nitrogen is more electronegative. Therefore, the direction of polarity is toward nitrogen. So ammonia becomes a highly polar molecule. It has partial negative charge on nitrogen and partial positive charges on three hydrogen atoms. Carbon Tetrachloride. Carbon Tetrachloride is a non-polar molecule. This is because the polarity of chlorine-carbon bonds are canceled out by each other.

Beryllium Chloride.Beryllium has an electronegativity of 1.57. Chlorine has a electronegativity of 3.16. Since chlorine is more electronegative, the direction of polarity is towards chlorine. It has polar bonds but the molecule overall is non-polar. The effect of polarity of one side of chlorine is canceled out by the second side of chlorine. So it has zero dipole moment.

Ion-dipole interaction.Ions are atoms or molecules that have a net charge. For example, when chlorine gains an electron, it becomes a chloride ion. It is an ion because it now has a net negative charge. Similarly when sodium loses an electron it gets a positive charge and becomes an ion. What's the difference between that, and a dipole? In dipole, the charge is separated into different ends of the molecules resulting in a partially positive end and a partially negative end. Water is a very polar molecule having oxygen which is quite electronegative and covalently bonded to two hydrogen atoms. So electrons spend more time around oxygen than around hydrogen.

In the interaction of ions and dipoles, the coulomb forces are an important factor. The partial negative part of a dipole would be attracted to a positively charged ion. Similarly, in a chloride anion, the negative ion will be attract to the partially positive end of the molecule. This is one of the reasons that sodium chloride dissolves easily in water. These ions can be separated and then attracted to polar water molecules which have molecular dipoles. These interaction forces are coulomb forces so the strength of the charges matter. A stronger ion-dipole force needs to have stronger charges on the ions.

Dipole-Dipole.A dipole is a molecule having partially positive and negative poles. In HCl, chlorine is more electronegative. So partial dipole direction is towards chlorine due to which electronic cloud tilt towards chlorine atom. As a result, a partial positive charge appears on hydrogen and a partial negative charge on chlorine. Let's consider two HCl molecules next to each other. The negative pole of one HCl will attract the positive pole of the other HCl. This attractive force generated between the two molecules is called dipole-dipole force. We usually represent dipole-dipole force with a dotted line.

In an ICl molecule, the more electronegative chlorine atom has a partial negative charge, and iodine has a partial positive charge. So the partial positive iodine of one ICl molecule is attracted towards the partially negative chlorine of another ICl molecule. The covalent bonds between the HCL are very strong. Chlorine is more electronegative so it has a partial negative charge and hydrogen has a partial positive charge. The Dipole direction is from chlorine towards hydrogen. The positive end of one molecule is attracted towards the negative end of the second HCL and vice versa.

Hydrogen Bonding.An attractive force called a hydrogen bond can exist between some molecules. These bonds are weaker than ionic or covalent bonds because it takes less energy to break these types of bonds. However, a large number of these bonds can exert a strong force. Hydrogen bonds are the result of unequal charge distribution on a molecule. These molecules are said to be polar.

If we look at a water molecule we can see the oxygen atom shares electrons with 2 different hydrogen atoms. They have a total of 8 electrons, 4 shared between the oxygen atom and two hydrogen atoms. There are 4 other non-shared electrons from the oxygen atom. These are in the valence shell, paired in two lone pairs. The two lone pairs repell each other and they also repell bond pairs between hydrogen atoms and oxygen atom. Oxygen being highly electronegative and due to presence of lone pair of electrons have partial negative charge.

Thus, a water molecule is partially positive on the hydrogen side and partially negative on the oxygen side of the molecule where the lone pairs are. The hydrogen atom of one water molecule is attracted to the oxygen atom of a neighboring water molecule. This is how hydrogen bonding in water occurs.

Hydrogen Fluoride.Hydrogen fluoride is made by covalent bonding between hydrogen and fluorine. In these bonds the atoms in which they share electrons. Hydrogen has one electron while fluorine contains seven valence electrons. Thus it requires one electron to become stable. So hydrogen has a partial positive charge while fluorine atom has a partial negative charge due to more electronegativity. In this way when the hydrogen of one molecule gets attracted to the fluorine of a neighboring molecule hydrogen bonding occurs.