The types of hydrides

Hydride Ion

Basics of hydrides

Hydrogen is the lightest element of the periodic table. It is found at high temperatures only. In the fundamental form, it is available with two atoms of it, it is called diatomic molecule represented as H2, therefore, also known as dihydrogen. This dihydrogen connects with various elements to make binary compounds, called hydrides. It is usually represented as MHx where M is the element and x are the number of hydrogen atoms. For example- NH3. Hydrogen makes hydrides with many elements except noble gases and also Indium and Thallium. If we talk about the elements of the Lanthanide and Actinide series, they also form hydrides. Transition metals also form the hydrides.

Types of Hydrides

We can divide the hydrides on the basis of their chemical and physical properties. There are 3 different types of hydrides- 

• Ionic or Salt-like saline hydrides.

• Metallic or interstitial hydrides. 

• Molecular or covalent hydrides. 

Ionic hydrides- Ionic hydrides are made by the metals whose measure of electronegativity are significantly lesser than that of hydrogen I.e. (< 2.2). Here We see that all elements present in the group 1 of the periodic table are called the Alkali metals and the elements of group 2 which are called Alkaline earth metals. When we heat any element of group 1 or group 2 at high temperature (595- 973 K) make hydrides. But Berum and Magnesium make the covalent structures of polymers.

In appearance, these hydrides are crystalline solids and contain white colour. When these hydrides are formed, they attain higher density than their parent metals. These also possess high melting and boiling points which helps them in conduction of electricity when they are at the fused state.

Examples are – Ca++H-2, calcium hydride, CsH, NaH etc.

They react with water, e.g. NaH + H2O → NaOH + H2

Metallic Hydrides- In the periodic table, there is a d-block in its central part. In this block, group 3,4,5,10,11,12 and also the elements of block F when heated with the diatomic hydrogen under significant pressure forges hydrides. In the elements of group 6, only Chromium makes hydrides. Such hydrides exhibit properties of their parent metals, that’s why they are called the metallic hydrides. The metals who are the members of group 7,8 and 9 don’t participate in the formation of hydrides, these 3 groups are called the hydride gap. In earlier hypothesis, hydrogen covers small space in the lattice and contorts it but doesn’t alter its type, that’s why it was titled as ‘interstitial hydrides’, but now new outcome of research says, if we exclude the hydrides of Ac, Pd, Ce and Ni, other hydrides of this group have different lattices than their parent metals.

These are hard in appearance and also possess a metallic lustre. These hydrides also show magnetic properties at times. These are good conductors of electricity.

e.g. LiAlH4 (Lithium Aluminium Hydride), LiBH4 ( Lithium Borohydride)

Covalent Hydrides- Covalent hydrides mainly consist of p-block and some elements of s-block also namely Beryllium and Magnesium. It is because of less difference in electronegativity of hydrogen and these elements. It is usually represented as XHn (for elements of s-block) or XH8-n (for elements of p-block), here n is the number of electrons present in the valence shell). These hydrides do not make very strong bonds. They just make weak covalent bonds which are connected by the van der Waals force of attraction, that’s why we call them covalent hydrides also. These hydrides generally add an ‘ane’ after their names, such as azane or oxidane etc. 

Unlike the previous two hydrides, covalent hydrides have a low melting point and low boiling point. Due to this reason, they also are bad conductors of electricity. Hydrides of group 13, are known for their electron deficiency. Hydrides of group 14 consist of the exact number of electrons needed for the formation of the covalent bond. If we talk about the group 15, 16 and 17, these groups possess more electrons than required for the formation of hydrides, so we put them in the category of electron-rich hydrides.

e.g. AlH3 (Aluminium Hydride), NH3 , H2O etc.

GeCl4 + LiAlH4 → GeH4 + LiCl + AlCl3 forms Germane ( GeH4)

Conclusion

Hydrides are basically the compounds made in the presence of hydrogen. As per their bonding with hydrogen we can put them in different categories of ionic, covalent and metallic hydrides. Due to the different number of electrons in different elements of the periodic table, they exhibit different kinds of bond formation which is used in different scenarios as per requirement. Some are good conductors of electricity whereas some are bad at it.