Variable valency

Electrons orbit an atomic nucleus. The greater the number of electron orbits around the nucleus, the longer the distance between the electrons and the nucleus. Atoms in their outer orbit aim to acquire the same stable state as the noble gases by receiving or donating electrons. This is referred to as the atom’s valency. Some items have many valencies. Therefore, they have various kinds of variable valency.

Valency

An atom’s valency is the number of electrons it loses, gains, or shares during a chemical reaction (combination) to achieve a stable configuration. A molecule is made up of the atoms of different elements or radicals that come together in a way that makes them stick together.

The valence orbit of an atom is its outermost shell.

Ionic bonds are formed when valence electrons are exchanged or accepted by the surrounding atoms. For example, a covalent bond is formed when atoms share their valency electrons.

Definition of Variable Valency: 

Certain elements exhibit many valencies; these are referred to as variable valencies—compounds of this kind display different valencies in different combinations. Elements with variable valency include iron, mercury, and copper. Transition elements exhibit various degrees of valency.

Depending upon the type of the reaction, some elements interact with other atoms, accepting, donating, or sharing electrons in varying proportions with them.

For example, when iron reacts with oxygen, it forms ferrous oxide and ferric oxide, among other compounds. This phenomenon is referred to as variable valency. Nickel, copper, tin, and iron are transition metals with variable valency.

Why Are Elements to have Variable Valency?

Suppose you look at the iron example.

Iron is a type of metal (Fe)

The atomic number of iron is 26.

Iron has the electronic configuration 1s2 2s2 2p6 3s2 3p6 4s2 3d6.

Iron is an unstable molecule due to the electrical configuration of 4s2 3d6. The 2s electrons will be removed first. The electrical arrangement of the iron then transforms to 4s0 3d6. 3d6 is not a stable electrical arrangement.

After one electron is removed from the d subshell, the subshell becomes half-filled. This electronic arrangement of 3d5 elements is stable. Consequently, iron is classified as having two and three valencies. These valencies are referred to as iron variable valencies. The valency of the elements must be varied for stability. In addition, the exchange energy of the half-filled subshell is higher, resulting in a further decrease in the energy of the compound.

This makes the compound more stable.

Variable Electrovalency 

Electrons are lost or added to an element’s atom to form ionic bonds. This is called the element’s electrovalency. The s-block elements do not exhibit variable valency. Valency is more variable for P-block elements with larger atomic numbers and transitional inner elements.

There are two primary reasons for variable valency to exist.

• The influence of inert pairs in p-block elements.

• When it comes to inner transition elements, there is very little difference in energy between ns and (n-1) d subshells, as well as ns and (n-2) d subshells.

1. Inert Pair Effect

The majority of the elements in the III-A group have an oxidation state of +3. However, as we move down the group, the oxidation state +1 appears more prominent. Similarly, the elements in the IV A group exhibit a +4 oxidation potential in general, but the +2 state has become more prominent as we descend the group.

When forming bonds, the two electrons in the valence shell (ns2) tend to remain inert. This is referred to as the inert pair effect. Because of this, inert pair effects are the primary cause of p block elements’ variable valency.

2. The Energy Distinction between Orbitals

In transition metals, the oxidation state changes due to the involvement of incompleted-orbitals and electrons from the ns orbital in the fluctuation. Thus, electrons from both energy levels are available for bonding.

In a similar vein, the valency of inner transition elements is varied due to the inadequate filling of f-orbitals. Some f-orbitals can also be used to establish chemical bonds, in addition to ns electrons.

Elements Having Variable Valency:

Nickel, copper, tin, and iron are transition metals with variable valency. Nonmetallic elements like nitrogen and oxygen exhibit varying valencies. When different valency atoms react, the products that come out of the mix have other properties.

Copper (Cu) has two forms: Cuprous (Cu +2) and Cupric (Cu +3)

Iron (Fe) has two forms: Ferrous (Fe +2) and Ferric (Fe +3)

Mercury (Hg) has two forms: Mercurous (Hg +1) and Mercuric (Hg +2)

Silver (Ag) has two forms: Argentous (Ag +1) and Argentic (Ag +2)

Representation of Metals with Variable Valency

When an element has a variable valency, it is customary to use a Roman numeral like a superscript next to its symbol to indicate the valency.

For metals, the lower valency is represented by the suffix ‘ous’, and the higher valency is represented by the suffix ‘ic’, in this order:

For instance, metal like iron has a valency of two and three. Therefore, to represent the lower valency (2), we write ferrous (Fe2+), while to represent the greater valency (3), we write ferric (Fe3+).

On the other hand, the modern method uses Roman numbers to express an element’s variable valency. So we use Fe (II) for ferrous and Fe for ferric (III).

Representation of Nonmetal with Variable Valency

For non-metals, the valency of the element is determined by the number of other atoms attached to it. For example, the elements phosphorus and potassium have a number of valence electrons 3 and 5. When combined with chlorine, it produces two compounds: PCl3 and PCl5. For this, three electrons are shared by phosphorous and three chlorine atoms in the structure of phosphorous trichloride, resulting in the element having valency 3. Pentachloride has five electrons; hence it has a valency of five.

Conclusion

The valency of a given element can change depending on the conditions of a chemical reaction. As a rule, metals contribute electrons from their outer shells to generate positively charged ions. On the other hand, the valence shell of some metals loses electrons. In this case, the element has a variable number of valence electrons, having multiple electropositive valencies.