HYDRIDES: PROPERTIES, TYPES, NOMENCLATURE AND EXAMPLES - CHEMISTRY - 2025

A hydride is hydrogen in its anionic form (H ^-) or the compounds that are formed from the combination of a chemical element (metallic or non-metallic) with the hydrogen anion. Of the known chemical elements, hydrogen is the one with the simplest structure, because when it is in the atomic state it has a proton in its nucleus and an electron.

Despite this, hydrogen is only found in its atomic form under fairly high temperature conditions. Another way to recognize hydrides is when one or more central hydrogen atoms in a molecule are observed to have nucleophilic behavior, as a reducing agent or even as a base.

Aluminum lithiumhydride

Thus, hydrogen has the ability to combine with most of the elements of the periodic table to form different substances.

How are hydrides formed?

Hydrides are formed when hydrogen in its molecular form associates with another element - whether of metallic or non-metallic origin - directly by dissociating the molecule to form a new compound.

In this way, hydrogen forms bonds of the covalent or ionic type, depending on the type of element with which it is combined. In the case of associating with transition metals, interstitial hydrides are formed with physical and chemical properties that can vary greatly from one metal to another.

The existence of free-form hydride anions is limited to the application of extreme conditions that do not occur easily, so in some molecules the octet rule is not fulfilled.

It is possible that other rules related to the distribution of electrons are not given either, having to apply expressions of multi-center bonds to explain the formation of these compounds.

Physical and chemical properties of hydrides

In terms of physical and chemical properties, it can be said that the characteristics of each hydride depend on the type of bond that is carried out.

For example, when the hydride anion is associated with an electrophilic center (generally it is an unsaturated carbon atom), the compound formed behaves as a reducing agent, which is widely used in chemical synthesis.

Instead, when combined with elements such as alkali metals, these molecules react with weak acid (Bronsted acid) and behave like strong bases, releasing hydrogen gas. These hydrides are very useful in organic syntheses.

It is then observed that the nature of hydrides is very varied, being able to form discrete molecules, ionic-type solids, polymers and many other substances.

For this reason they can be used as desiccants, solvents, catalysts or intermediates in catalytic reactions. They also have multiple uses in laboratories or industries with various purposes.

Metal hydrides

There are two types of hydrides: metallic and non-metallic.

Metal hydrides are those binary substances that are formed by the combination of a metallic element with hydrogen, generally an electropositive one such as alkaline or alkaline earth, although interstitial hydrides are also included.

This is the only type of reaction in which hydrogen (whose oxidation number is normally +1) has an extra electron at its outermost level; that is, its valence number is transformed to -1, although the nature of the bonds in these hydrides has not been completely defined due to the discrepancy of those who study the subject.

Metal hydrides possess some properties of metals, such as their hardness, conductivity, and brightness; But unlike metals, hydrides have a certain fragility and their stoichiometry does not always comply with the weight laws of chemistry.

Non-metallic hydrides

This type of hydrides arises from the covalent association between a non-metallic element and hydrogen, so that the non-metallic element is always at its lowest oxidation number to generate a single hydride with each one.

It is also found that these types of compounds are mostly found in gaseous form under standard ambient conditions (25 ° C and 1 atm). For this reason, many non-metallic hydrides have low boiling points, due to van der Waals forces, which are considered weak.

Some hydrides in this class are discrete molecules, others belong to the group of polymers or oligomers, and even hydrogen that has undergone a chemisorption process on a surface can be included in this list.

Nomenclature, how are they named?

To write the formula for metal hydrides, start by writing the metal (the symbol for the metal element) followed by hydrogen (MH, where M is the metal).

To name them, it begins with the word hydride followed by the name of the metal ("M hydride"), thus LiH is read "lithium hydride", CaH ₂ is read "calcium hydride" and so on.

In the case of non-metallic hydrides it is written in the opposite way than in metallic ones; that is, it begins by writing the hydrogen (its symbol) succeeded by the nonmetal (HX, where X is the nonmetal).

To name them, we begin with the name of the non-metallic element and add the suffix “uro”, ending with the words “hydrogen” (“X-hydrogen uro”), thus HBr is read “hydrogen bromide”, H ₂ S is read "hydrogen sulfide" and so on.

Examples

There are many examples of metallic and non-metallic hydrides with different characteristics. Here are a few:

Metal hydrides

- LiH (lithium hydride).

- NaH (sodium hydride).

- KH (potassium hydride).

- CsH (cesium hydride).

- RbH (rubidium hydride).

- BeH ₂ (beryllium hydride).

- MgH ₂ (magnesium hydride).

- CaH ₂ (calcium hydride).

- SrH ₂ (strontium hydride).

- BaH ₂ (barium hydride).

- AlH3 (aluminum hydride).

- SrH2 (strontium hydride).

- MgH2 (magnesium hydride).

- CaH2 (calcium hydride).

Non-metallic hydrides

- HBr (hydrogen bromide).

- HF (hydrogen fluoride).

- HI (hydrogen iodide).

- HCl (hydrogen chloride).

- H ₂ S (hydrogen sulfide).

- H ₂ Te (hydrogen telluride).

- H ₂ Se (hydrogen selenide).

References

1. Wikipedia. (2017). Wikipedia. Recovered from en.wikipedia.org
2. Chang, R. (2007). Chemistry. (9th ed). McGraw-Hill.
3. Babakidis, G. (2013). Metal Hydrides. Recovered from books.google.co.ve
4. Hampton, MD, Schur, DV, Zaginaichenko, SY (2002). Hydrogen Materials Science and Chemistry of Metal Hydrides. Recovered from books.google.co.ve

Sharma, RK (2007). Chemistry of Hidrydes and Carbides. Recovered from books.google.co.ve