Main Difference – Metal Halide vs High Pressure Sodium
Metal halides are compounds composed of a metal ion chemically bonded to a halide. The chemical bond between a metal and a halogen can be either a covalent bond or ionic bond. Ionic compounds such as sodium chloride, covalent compounds such as palladium chloride and some coordination compounds are considered as metal halides. Metal halides are used in metal halide lamps. High pressure sodium is a term used to describe a type of lamps that are used to produce light used in lightening streets, industrial lightening and as a plant growth stimulator. The main difference between metal halide and high pressure sodium is that metal halide lamps have a high efficiency and color rendition of light whereas high pressure sodium lamps have a poor color rendering.
Key Areas Covered
1. What is a Metal Halide
– Definition, Different Types, Metal Halide Lamps
2. What is High Pressure Sodium
– Definition, High Pressure Sodium Lamp
3. What is the difference Between Metal Halide and High Pressure Sodium
– Comparison of Key Differences
Key Terms: Covalent Compound, High Pressure Sodium Lamp, Ionic Compound, Low Pressure Sodium Lamp, Metal Halide, Metal Halide Lamp, Palladium Chloride, Sodium, Sodium Chloride
What is a Metal Halide
Metal halides are compounds between metals and halogens. Metals are the elements in the s block and d block. Halogens are the elements in the group 7. Metals form cations by removing one or more electrons from their outermost electron shell. Halides are anions of halogens. Halogens can accept electrons to their outermost electron shell. Therefore, these two elements can bind via an ionic bond. An ionic bond is a type of chemical bond which has an electrostatic attraction between a cation and an anion. Here, a metal halide has electrostatic attractions between metal cations and halide anions. For example, sodium chloride is an ionic metal halide compound. It exists in a crystal structure as shown in the below diagram.
Figure 1: Sodium Chloride Crystal Structure
Some metal halides have covalently bonded metals and halogens. These metal halides are often polymeric structures. For example, palladium chloride is a metal halide which exists as a polymeric structure. This structure has infinite chains of palladium chloride units.
Figure 2: Palladium Chloride Polymeric Chain
Ionic metal halides are often formed from a combination of halide ions with metal ions of group 1 and group 2 (alkali and alkaline earth) elements. These compounds have high melting and boiling points. These metal halides are highly soluble in water forming the aqueous metal ion and the halide ion. Some transition metal halides also dissolve in water.
Some coordination complexes are also considered as metal halides since there are metal ions bonded to halide ions via coordinate covalent bonds. Here, the halide ion is called a ligand. For example, Titanium tetrachloride [TiCl4], chromium(III) chloride [CrCl6]3−, etc.
Metal Halide Lamps
A metal halide lamp is an electrical lamp. It produces light by electrical breakdown of a gaseous mixture of vaporized mercury and metal halides. The metal halides used for this purpose are metal bonded with bromine or iodine. These metal halides improve the efficiency and color rendition of the light.
What is High Pressure Sodium
The term high pressure sodium describes a form of light that is used mainly in industrial lightening and is a type of growth light commonly used for indoor gardening. This light can stimulate the growth of plants. This is by diffusing electromagnetic radiation required for photosynthesis.
The low pressure sodium lamp was the first sodium lamp to be developed. Later, the high pressure sodium lamp was developed due to the disadvantages of the low pressure sodium lamp. High pressure sodium lamps contain more components such as mercury which is absent in low pressure sodium lamps. Unlike low pressure sodium lamp, the high pressure sodium lamp has an acceptable color.
Figure 3: High Pressure Sodium Lamps can be used to Lighten Streets
Advantages and Disadvantages of High Pressure Sodium Lamps
There are several advantages and disadvantages of high pressure sodium lamps. Good efficiency, smaller size than low pressure sodium lamp, better bulb life are some of the advantages. The major disadvantage is that it still has a bad color when compared to a metal halide lamp.
Difference Between Metal Halide and High Pressure Sodium
Definition
Metal Halide: Metal halides are compounds between metals and halogens.
High Pressure Sodium: The term high pressure sodium describes a form of light that is used mainly in industrial lightening, and is a type of growth light commonly used for indoor gardening.
Applications
Metal Halide: Metal halides are used in metal halide lamps along with mercury.
High Pressure Sodium: High pressure sodium lamps are used for lightening purposes and in plant growth stimulation.
Advantages
Metal Halide: Metal halide lamps have a high efficiency and color rendition of light.
High Pressure Sodium: High pressure sodium lamps have a poor color rendering.
Components
Metal Halide: Metal halide lamps are composed of mercury along with some metal halides including metal bromides and metal iodides.
High Pressure Sodium: High pressure sodium lamps have sodium, mercury, xenon, etc. inside a tube made of aluminum oxide.
Conclusion
Metal halides are inorganic compounds composed of a metal ion and a halide ion. These compounds are used in metal halide lamps. High pressure sodium lamps are an improved version of low pressure sodium lamps. The main difference between metal halide and high pressure sodium is that metal halide lamps have a high efficiency and color rendition of light whereas high pressure sodium lamps have a poor color rendering.
References:
1. “Halides | Metal Halides | Uses of Halides | Examples | Chemistry | Byju’s.” Chemistry, Byjus Classes, 18 Sept. 2017, Available here.
2. “Metal-Halide lamp.” Wikipedia, Wikimedia Foundation, 21 Nov. 2017, Available here.
Image Courtesy:
1. “Sodium chloride crystal” By Walkerma – Own work by the original uploader (Public Domain) via Commons Wikimedia
2. “Alpha-palladium(II)-chloride-xtal-3D-balls” By CCoil (talk) – Own work (CC BY 3.0) via Commons Wikimedia
3. “High Pressure Sodium Lamps” By Swisstack – Own work (Public Domain) via Commons Wikimedia
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