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
Valency in ionic compounds
Valency in covalent compounds
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”.
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 exactly is meant by variable valency in Chemistry?
Variable valency is the property of certain elements, especially transition metals, to exhibit more than one combining capacity (valency) when forming chemical compounds. This means a single element can form different types of ions and bond with other elements in multiple ways. A classic example is iron (Fe), which can have a valency of 2 or 3.
2. Can you give a simple example of variable valency?
A great example is Copper (Cu). It can show two different valencies:
- Valency of 1 (Cuprous): As seen in Copper(I) oxide, Cu₂O.
- Valency of 2 (Cupric): As seen in Copper(II) oxide, CuO.
In each compound, copper combines with oxygen differently because it can lose either one or two electrons.
3. Why do some elements show variable valency while others don't?
The primary reason lies in the element's electron configuration. For transition elements (d-block), the energy difference between the outermost electron shell (ns) and the second-to-last shell ((n-1)d) is very small. This allows electrons from both shells to participate in chemical bonding, leading to different possible valencies. In contrast, elements like sodium only use electrons from their outermost shell, giving them a fixed valency.
4. Which groups of elements in the periodic table are known for showing variable valency?
Variable valency is a hallmark characteristic of the transition elements (d-block elements), such as iron, copper, tin, and lead. Some elements in the p-block, like phosphorus and sulphur, also exhibit this property due to the availability of vacant d-orbitals for expanding their octet.
5. How is variable valency indicated when naming a chemical compound?
There are two common methods:
- Stock Notation: The valency is written as a Roman numeral in parentheses after the element's name. For example, Iron(II) chloride for FeCl₂ and Iron(III) chloride for FeCl₃.
- Older Suffix System: The suffix '-ous' is used for the lower valency (e.g., Ferrous chloride) and '-ic' for the higher valency (e.g., Ferric chloride).
6. Is there a difference between valency and oxidation state?
Yes, they are related but different concepts. Valency is simply the combining power of an element, a pure number without a positive or negative sign. For example, the valency of carbon in CH₄ is 4. The oxidation state, on the other hand, indicates the degree of electron loss or gain and can be positive, negative, or zero. In CH₄, carbon's oxidation state is -4.
7. Why specifically does iron (Fe) show valencies of 2 and 3?
Iron's ability to show +2 and +3 valencies is due to its electronic structure. An iron atom can lose its two outermost 4s electrons to form the Fe²⁺ ion (valency 2). Because the 3d orbital is very close in energy, it can also lose an additional 3d electron to achieve a more stable, half-filled d-orbital, forming the Fe³⁺ ion (valency 3).
