Dhristi JEE 2022-24

What is Variable Valency?

Before discussing what is variable valency, the concept of valency has to be understood. So, what is valency? Elements generally do not exist independently in nature and combine with the other elements. Valency is the combining capacity of one element with another element. Elements combine with one another to attain a stable state. It determines the number of electrons an element can donate or accept to form a stable electronic configuration.

Types of Valency

  1. Valency in ionic compounds

  2. Valency in covalent compounds

  3. Variable valency

Valency in Ionic Compounds or Electrovalent Compounds

Ionic compounds or electrovalent compounds are compounds that form by the combination of metal ions and non-metal ions. For example, sodium chloride (NaCl). In this compound, sodium (Na) is the metal, and chloride (Cl) is the non-metal. It is formed by the transfer of electrons. What is electrovalency? This type of chemical bonding between metal and non-metal ions is called electro valency, and these compounds are called electrovalent compounds. Electro valency can be defined as the number of electrons lost or gained by the atoms in an ionic compound.

For Example: In NaCl

Na → Na⁺ + 1 e⁻

Cl + 1 e- → Cl⁻

Na⁺ + Cl⁻ → NaCl

In the above example of sodium chloride (NaCl), the electro valency of sodium (Na) and chloride (Cl) is 1, as 1 electron is lost by sodium and gained by chloride ion. 

Valency in Covalent Compounds

Covalent compounds are compounds formed by the chemical bonding of non-metals. These compounds are formed by the sharing of electrons between atoms. 

Non-metal + Non-metal → covalent compounds.

The valency of these compounds can be defined as the number of bonds by which the atom is directly attached to the other atom. These compounds exhibit co-valency.

For Example

Methane (CH₄)

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The carbon is attached with the four hydrogens with the help of four single covalent bonds. Therefore, the co-valency of carbon in methane is four. One hydrogen is attached with the one-carbon by the single covalent bond and hence, the co-valency of hydrogen is one. 

What is Variable Valency?

Some elements show more than one type of valency; these types of valency are called variable valency. These types of compounds show valency in one compound and another valency in other compounds. Variable valency is shown by elements like Iron, mercury, and copper. Transition elements show variable valency. For example: in some cases, iron shows a valency of 2 like ferrous sulfate (FeSO₄), and in some, it shows valency of 3 like ferric chloride (FeCl₃). Copper shows two types of valencies 1 and 2. Mercury shows two types of valency 1 and 2.

Why do some Elements show Variable Valency?

Let’s see an example of iron:

Iron (Fe)

The atomic number of iron = 26

Electronic configuration of iron = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

4s² 3d⁶ shown electronic configuration makes iron an unstable molecule. The two electrons will be first removed from the 4s orbital. Then the electronic configuration of iron becomes 4S⁰ 3d⁶. 3d⁶ is not a stable electronic configuration. After losing one electron from the d subshell it will become a half-filled subshell. This 3d⁵ is a stable electronic configuration. Hence the element of iron shows 2 and 3 valencies. These valencies are called iron variable valencies. The elements show variable valency to acquire stability. The half-filled subshell shows more exchange energy which further lowers the energy of the compound. Therefore, the compound becomes more stable.

Elements with Variable Valency

Copper (Cu) = Cuprous (Cu²⁺) and cupric (Cu³⁺)

Iron (Fe) = Ferrous (Fe²⁺) and Ferric (Fe³⁺)

Mercury = Mercurous (Hg ⁺¹) and mercuric (Hg ⁺²)

Silver (Ag) = Argentous (Ag⁺¹) and argentic (Ag⁺².)

Stannum (Sn) =  Stannous (Sn⁺²) and Stannic (Sn⁺³)

The element that exhibits lower valency will be suffixed with “ous”. While the element that exhibits higher valency will be suffixed with “ic”.



Electronic Configuration


Sc (Scandium)

3d1 4s2

2 and 3

Ti (Technitium)

3d2 4s2

2, 3, and 4

V (Vanadium)

3d3 4s2

2, 3, 4, and 5.

Cr (Chromium)

3d5 4s1

1, 2, 3, 4, 5, and 6

Mn (Manganese)

3d5 4s2

2, 3, 4, 5, 6, and 7

Fe (Iron)

3d6 4s2

2, 3, 4, 5, 6, and 7

Did you Know? 

The value of valency can never be zero.

The primary valency is the fixed property of elements.

Valency shows the bonding potential of the elements.

FAQs on Variable Valency

1. What is valency?

In chemistry, an element's valency is a measure of its ability to combine with other atoms to form chemical compounds or molecules.

The total number of hydrogen atoms that an atom of a certain element may merge with decides its combining capability. Carbon has a valence of 4 in methane. Nitrogen has a valence of 3 in ammonia. Oxygen has a valence of 2 in water. Chlorine has a valence of 1 in hydrogen chloride. As chlorine has a valence of one, it can be used to replace hydrogen. Click here to learn more.

2. What is the history of valency?

In the second half of the nineteenth century, the notion of valence was created, which helped to explain the molecular structure of inorganic and organic compounds. The more recent theories of chemical bonding, such as the Lewis structures (1916), cubical atom (1902), valence bond theory (1927), valence shell electron pair repulsion theory (1958), molecular orbitals (1928), and all modern methods of quantum chemistry, arose from the search for the underlying causes of valence.

3. What is the relation between valence and electrons?

The Rutherford model of the nuclear atom unveiled the atom's exterior is occupied by the electrons, implying that electrons are responsible for atom contact and chemical bond formation. Gilbert N. Lewis defined valence and chemical bonding in 1916 by stating that atoms have a tendency to form a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding produces octets by sharing electrons, whereas ionic bonding produces octets by transferring electrons from one atom to the next.

The terminology "covalence" was coined by Irving Langmuir in 1919, who noted, "The number of pairs of electrons that any particular atom shares with neighbouring atoms are called the covalence of that atom." As the word co- implies "together," a covalent connection implies that the atoms have the exact valence. Following that, it is now more customary to speak of covalent bonds rather than valence, which has lost favour in the higher-level study due to developments in chemical bonding theory but is still extensively employed in introductory studies as a heuristic introduction to the subject.

4. What happens to valency in a state of oxidation?

Other notations are currently favoured due to the ambiguity of the term valence. Apart from the Stock system of oxidation states (also known as oxidation numbers) and the lambda notation used in the IUPAC nomenclature of inorganic chemistry, the oxidation state is a more precise description of the electronic state of atoms in a molecule.

The number of valence electrons gained or lost by an atom in a molecule is determined by its oxidation state. The oxidation state, unlike the valency number, can be positive (for an electropositive atom) or negative (for an electronegative atom) (for an electronegative atom).

5. What is meant by the highest number of valence bonds?

Frankland was convinced that an element's valence had a single value that matched with the maximum value observed. The number of valencies on atoms that are not used of what is now called p-block elements is often even. Frankland said that the unused valencies saturated each other. For example, nitrogen has a maximum valence of 5, so two valencies are left unconnected when generating ammonia; sulphur has a maximum valence of 6, so four valencies are left unattached when forming hydrogen sulphide.

6. Define Variable Valency?

The capacity of the elements to show different valencies in different compounds is called variable valency.

7. Does Valency have Any Sign?

No, valency does not show any sign. It just shows the number of bond-forming capacities.

8. Name the different Types of Valencies.

The different types of valencies are given below:

  • Electrovalency

  • Co-valency

  • Variable valency