What Are Covalent Hydrides Definition Types Bonding and Key Examples
Dihydrogen, which is better known as H2, is the most abundant element in the world and it can combine with almost anything and form carbohydrates, proteins, hydrides, hydrocarbons and many other compounds. Dihydrogen or H2 combines with almost every element on earth, except noble gasses, and forms binary compounds called hydrides under certain conditions. MgH2, B2H6 are two examples of hydrides. These hydrides are broadly classified into three categories:
Ionic or saline or salt like hydrides
Covalent or molecular hydrides
Metallic or non-stoichiometric hydrides
Ionic hydrides
These compounds of H2 are mostly formed with s-block elements which are highly electropositive in nature. Although lighter metal hydrides such as LiH, BeH2 have significant covalent character.
LiH is unreactive at a moderate temperature with O2 or Cl2, so it is used in the synthesis of other useful hydrides.
Covalent hydrides
H2 forms molecular bonds with most of the p-block elements and the most common compounds are H2O, NH3, CH4 and HF.
Covalent hydrides are further divided into three categories according to the relative numbers of electrons and bonds in their Lewis structure:
(i) electron-deficient - all the group 13 elements form electron deficient compounds with hydrogen and act as electron acceptor (Lewis acids), eg, B2H6,
(ii) electron-precise - all group 14 elements form electron precise compounds and see tetrahedral in geometry, eg, CH4,
(iii) electron-rich hydrides - elements from groups 15 through 17 form such bonds that have excess electrons which are present as lone pairs, eg, NH3 where 1 lone electron is present.
Metallic hydrides
Hydrides formed by many elements of d and f-block with H2 are called metallic hydrides. Elements from groups 7,8 and 9 do not form bonds with H2, and only Cr from group 6 forms a bond. Some of the metals can accommodate a large number of hydrogen making them highly potential for hydrogen storage, eg, Pt, Pd. In this type of hydride, hydrogen occupies the metal lattices in the interstitial spaces.
FAQs on Covalent Hydrides in Chemistry Definition Structure and Properties
1. What are covalent hydrides?
Covalent hydrides are compounds of hydrogen with non‑metals in which hydrogen is bonded by covalent bonds. In these hydrides, hydrogen shares electrons with elements such as carbon, nitrogen, oxygen, or halogens. Examples include CH4 (methane), NH3 (ammonia), and H2O (water). They are typically molecular in nature and may exist as gases, liquids, or volatile solids.
2. How are covalent hydrides formed?
Covalent hydrides are formed when hydrogen shares electrons with a non‑metal atom to complete their valence shells. The formation involves:
- Overlap of atomic orbitals between hydrogen and the non‑metal.
- Sharing of one or more pairs of electrons.
- Formation of stable molecules such as 2H2(g) + O2(g) → 2H2O(l).
The type of bonding depends on the electronegativity difference between hydrogen and the other element.
3. What are the types of covalent hydrides?
Covalent hydrides are mainly classified into three types: electron‑precise, electron‑deficient, and electron‑rich hydrides.
- Electron‑precise hydrides: Obey the octet rule (e.g., CH4, NH3).
- Electron‑deficient hydrides: Have incomplete octets (e.g., B2H6).
- Electron‑rich hydrides: Contain lone pairs on the central atom (e.g., H2O, NH3).
This classification helps explain their structure and reactivity.
4. What is the difference between covalent hydrides and ionic hydrides?
The key difference is that covalent hydrides contain shared electron pairs, while ionic hydrides contain hydride ions (H-).
- Covalent hydrides: Formed with non‑metals, molecular, lower melting points (e.g., CH4).
- Ionic hydrides: Formed with highly electropositive metals, contain H-, high melting points (e.g., NaH).
Ionic hydrides react vigorously with water, while covalent hydrides show varied reactivity.
5. What are electron-deficient hydrides with example?
Electron‑deficient hydrides are covalent hydrides in which the central atom has fewer than eight electrons in its valence shell. A common example is B2H6 (diborane).
- Diborane contains three‑center two‑electron (3c–2e) bonds.
- It does not follow the octet rule for boron.
- It is highly reactive and used in organic synthesis.
6. Why is water considered a covalent hydride?
Water is considered a covalent hydride because hydrogen forms covalent bonds with oxygen by sharing electrons. The molecule H2O has:
- Two O–H covalent bonds.
- Two lone pairs on oxygen.
- A bent molecular geometry due to VSEPR theory.
Since oxygen is a non‑metal and bonding involves electron sharing, water is classified as a covalent (molecular) hydride.
7. What are electron-rich hydrides?
Electron‑rich hydrides are covalent hydrides in which the central atom has one or more lone pairs of electrons. Examples include NH3 and H2O.
- They follow the octet rule.
- They contain excess electron density as lone pairs.
- They often act as Lewis bases due to available lone pairs.
8. What are some common examples of covalent hydrides?
Common examples of covalent hydrides include CH4, NH3, H2O, HF, and H2S. These compounds:
- Are formed between hydrogen and non‑metals.
- Exist as discrete molecules.
- Show varying polarity depending on electronegativity differences.
They are widely studied in general and inorganic chemistry.
9. How does electronegativity affect covalent hydrides?
Electronegativity difference determines whether a covalent hydride is polar or nonpolar. If the non‑metal is highly electronegative:
- The bond becomes polar (e.g., H–F in HF).
- Partial charges develop (δ+ on H, δ- on the non‑metal).
- The molecule may exhibit hydrogen bonding.
If the electronegativity difference is small, as in CH4, the hydride is largely nonpolar.
10. What are the general properties of covalent hydrides?
Covalent hydrides generally have low melting and boiling points and exist as discrete molecular compounds. Their main properties include:
- Weak intermolecular forces (except hydrogen bonding cases).
- Poor electrical conductivity.
- Variable polarity depending on structure.
- Often gases or volatile liquids at room temperature.
These properties distinguish them from ionic and metallic hydrides in inorganic chemistry.
