Learning objectives

In this article, the author has explained what are halides, covalent halides, ionic halides, metals halides, and polymeric halides.

What are Halides?

The binary compounds of halogens with other elements of the periodic table are known as halides.

Definition of Halides

For example NaCl, AlCl₃, CCl₄

Classification of halides on the basis of nature of bonding

There are three types of halides in this respect.

• Ionic halides
• Bridge type halides or polymeric halides or metal halides
• Covalent halides

Let us discuss Ionic halides one by one in detail.

Ionic halides

Those halides in which there is ionic bond between the metal and the halogen atom are called ionic halides.

Definition of ionic halides

Explanation of ionic halides

The metals of the group I-A and II-A are sufficient electropositive. These have low ionization energy values and can create ionic bonds with F₂, Cl₂, Br_2, and I2. However, the salts of bromides and iodides are less ionic than these of fluorides and chlorides.

General properties of ionic halides

1. These are crystalline substances like NaCl
2. These have high melting and boiling points
3. These have three-dimensional lattices consisting of discrete ions
4. These have lattice energies, but the lattice energies of the fluorides are maximum. This is due to the very small size of the fluoride ion.
5. These are soluble in water and insoluble in organic solvents
6. These allow the electrical current to pass through them in the solution state and molten state.
7. The order of melting and boiling points is as follows

Metal fluorides > Metal chlorides > metal bromides > metal iodides

Polymeric halides

Definition of polymeric halides

Those halides which have the polymeric structure and have partially ionic bonding along with the layer of chain lattice

Explanation of polymeric halides

The elements like Be, Ga, and Al form polymeric halides. The properties of these halides are intermediate between ionic and covalent halides.

Metal halides

Metals halides in their lower oxidation states are ionic in character, while in their higher oxidation states they give covalent halides.

Examples of metal halides

The nature of the bonding and oxidation state of the central element is closely linked with metal halides.

PbCl₂, is mainly ionic while PbCl₄ is fairly covalent. Because Pb₄⁺ has a greater charge density. Its high polarizing power and the possibility of the covalent character in PbCl₄ becomes greater.

Covalent halides

Those halides in which the bonds are produced due to the sharing of electrons between halogens and other elements are called covalent halides.

Definition of covalent halides

Explanation of covalent halides

The electronegative elements or non-metals of periodic table are mostly responsible for such type of halides. Metalloids also give the covalent halides. For example the elements of the group IV-A, V-A and VI-A give covalent halides.

 General properties of covalent halides

1. These are mostly soluble in water but less soluble than the ionic halides
2. Some of these are gases, some are liquids and the majority are solids
3. Their melting and boiling points are less than those of ionic halides
4. Physical properties of covalent halides are influenced by the size and polarizability of halogen atoms
5. Iodides have the largest sizes and greater polarizabilities. Greater the wander wall’s forces higher the melting and boiling points than other covalent halides

Variation of properties of halides in the periodic table

There is a gradual change in the properties of halides in groups and periods

Variation of halides in the period

When we move from left to right in a period the electronegitivity difference between the halogens and the elements decreases. The bonding electrons becomes localized and percentage of covalent character increases.

There is gradual change in the bond type and melting points of the chlorides. It is clear from the chlorides of third period.

Property	NaCl	MgCl2	AlCl3	SiCl4	PCl3	SCl2
Melting Point oC	808	715	192	-68	-93	-80
Bonding type	ionic	Partially ionic	Partially ionic	polar covalent	polar covalent	polar covalent

Variation of halides in group

Electropositive character increases down the group, so the percentage of ionic character increases down the group. KCl is more ionic than NaCl and CsCl has maximum ionic character.

Sizes of halogens increase down the group from Fluorine to iodine, and then electronegativity decreases. For this reason, AlF3 is a purely ionic compound with a melting point 1290 °C. It is a good calculator for electricity. AlI3 ( aluminum tri-Iodide) is predominantly covalent and has a melting point of 198°C. It is not a conductor of electricity.

Applications of halides

1. These are used in the preparation of paper and phosphoric films such as silver halide
2. These are used in High-intensity metal discharge lamps
3. In greenhouses, these are used as a light provider
4. These are used as solid pastors.