Master the concepts of valency, variable valency, and valence electrons with crystal-clear explanations, diagrams, and practical examples – only on Selftution.com, the #1 trusted educational website for simplified learning!

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In chemistry, we frequently encounter terms like valency, variable valency, valence electrons, and valence shells.  All of these terms explain combining the capacity of an atom of an element with the atoms of other elements to form a molecule.

Topics covered:

• Definition of valency, variable valency, valence shell & valence electrons
• Why do atoms combine to form molecules?
• How do atoms combine?
  • Ionic bond
  • Covalent bond

Valency and Variable Valency, Valence Shell and Electrons of the Iron atom

DEFINITIONS

Definition of Valence Shell

The outermost shell of an atom is known as its valence shell or valance orbit.

Definition of Valence Electrons

Valence electrons are the one which is present in the outermost or valance shell of an atom.

Valence Shell and Electrons of the Iron atom

Definitions of Valency

Valency is the combining capacity of an atom of an element or of ion or a radical with the atoms of other elements or radicals to form molecules.

Or

It is the number of electrons that an atom can lose, gain, or share during a chemical reaction.

Or

Valency of an element or radical is the number of hydrogen atoms, which can combine with or displace one atom of the element or radical, forming a compound.

Definition of Variable Valency

When certain elements exhibit more than one valency, this means they show variable valency.

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Why do atoms combine to form molecules?

All atoms except those of inert gases are chemically unstable. By chemically unstable, we mean that they cannot exist freely in nature. Therefore, they try to stabilize by combining with other atoms of the same or different elements using the octet or duplet rule.

Inert Elements: Electronic Configuration and Distribution

As per the octet rule, to be chemically stable, an atom of an element must have eight electrons in its outermost orbit or shell. Atoms with fewer than eight electrons in their outermost or valence shell are chemically unstable. Such, atoms gain or lose electrons to attain a stable electronic configuration. The electrons present in the valence shell of an atom are called valence electrons. If the atom has only one shell, as in the case of hydrogen and helium, the valence shell can have two electrons. This is called the duplet rule.

Therefore, atoms combine to form molecules to attain chemical stability because they have an incomplete valence shell.

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How do atoms combine?

Atoms combine with two types of bonds – ionic (electrovalent) bonds and covalent bonds.

IONIC BOND: Atoms combine using the ionic bond when one of the atoms participating in molecule formation releases electrons in the valence shell and hands them over to the other atoms. Due to this, the atom that loses electrons becomes positively charged and the other that gains electrons becomes negatively charged. These newly formed positively and negatively charged particles are called ions. These ions bearing opposite charges attract each other with electrostatic force and result in the formation of molecules. This electrostatic force between ions that keeps them together is called an ionic or electrovalent bond.

Electrovalent or Ionic Bond Formation in Sodium Chloride

COVALENT BOND: When atoms do not release or take electrons but share electrons among themselves, they result in the formation of a covalent bond. The number of electrons shared by an atom of an element during the formation of a covalent bond is the valency of that particular element.

Covalent Bond in a Water Molecule

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VALANCE SHELL & ELECTRONS

The outermost shell of an atom is known as its valence shell or valence orbit, and the electrons present in it are called valence electrons. The number of valence electrons varies from 1 to 8 for the atoms of the different elements.

The valence electrons of an atom determine the valency of that element. The knowledge of valence shells and electrons helps in understanding the formation of molecules of elements and compounds.

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VALENCY

The valency of an element is a measure of its combining power with other elements.

Earlier, we used to measure valency in terms of hydrogen atoms. It is the number of hydrogen atoms that will combine with or displace one atom of a particular element or radical. As a hydrogen atom consists of only one valence electron in its outermost shell, it can readily lose or gain an electron to stabilize.

Some examples are,

1. Valency of chlorine in hydrogen chloride: In the compound hydrogen chloride (HCL), one atom of chlorine combines with one atom of hydrogen; hence, the valency of chlorine is 1.
2. Valency of sulfur in hydrogen sulfide: In the compound hydrogen sulfide (H₂S), one atom of sulfur combines with two atoms of hydrogen; hence, the valency of sulfur is 2.
3. Sulfate ion valency in sulfuric acid: In sulfuric acid (H₂SO₄), one ion of sulfate combines with two atoms of hydrogen; hence, the valency of sulfate is 2.

Valency of the First 20 Elements with Electronic Configuration

Modern Definition and Explanation of Valency

As per the modern definition, valency is the number of electrons that an atom can lose, gain, or share during a chemical reaction.

Explanation of valency–

As discussed above, atoms of all elements except those of the inert gases are chemically unstable. To attain stability, atoms have to have eight (octet rule) or two (duplet rule) electrons in their valence shell. Atoms with fewer than eight or two electrons in their valence shell are chemically unstable. Such, atoms gain, lose, or share electrons to attain a stable electronic configuration. The loss, gain, or sharing of electrons by an atom depends upon electrons in the valence shell.

Thus, atoms with one, two, or three electrons in their valence shell prefer to lose or share these electrons to attain stability. Whereas, atoms with seven, six, or five electrons in their valence shell prefer to receive or share one, two, or three electrons, respectively.

It is important to note that we never depict valency with positive and negative symbols.

The atom or a group of atoms that loses electrons forms a positive ion. Whereas those who receive electrons form negative ions. When atoms share electrons then they do not develop electric charges and remain neutral.

Monovalent: The atoms or a group of atoms with a valency of one are called monovalent. It means they gained, lost, or shared one electron to attain a stable electronic configuration of eight or two, as applicable.

Bivalent: The atoms or a group of atoms that gain, lose, or share two electrons are called bivalent.

Trivalent: These are atoms or a group of atoms with a valency of three. It means they gained, lost, or shared three electrons.

Tetravalent: The atoms or a group of atoms with a valency of four are called tetravalent. It means they gained, lost, or shared four electrons.

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VARIABLE VALENCY

An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e., there is a loss of electrons from the penultimate shell. Therefore, such an element is said to exhibit variable valency. Both metals and non-metals can exhibit variable valency. Some of the examples of variable valency are iron, copper, mercury, lead, tin, sulfur, phosphorus, etc.

Representation of variable valency of metals

In the case of metals, to exhibit variable valency, we represent the lower valency by adding the suffix ‘ous’ to the name of the metal; to represent the higher valency, we add the suffix ‘ic’. For example, the metal iron has valencies 2 and 3. For the lower valency (2), we write ferrous (Fe²⁺), and for the higher valency (3), we write ferric (Fe³⁺). Note that the symbol remains the same, but the name changes.

However, in the modern method, we represent the variable valency of an element by Roman numbers. Thus, to represent ferrous, we use Fe(II), and ferric, we use Fe(III).

Variable Valency Examples for Metals

Representation of variable valency of nonmetals

In the case of non-metals, the number of other types of atoms attached to them determines their valency. For example, phosphorus has valencies 3 and 5. With chlorine, it forms two compounds, PCl₃ and PCl₅. Therefore, in the molecule of phosphorus trichloride, phosphorus shares three electrons with three chlorine atoms; thus, its valency is three. Whereas phosphorus pentachloride shares five electrons with five chlorine atoms, thus its valency is five.

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Master the fundamentals of molecules – their structure, types (covalent, ionic, metallic), and everyday examples – with Selftution.com, the #1 trusted educational website for clear, simplified learning.

Welcome to Selftution.com, where science made simple meets in-depth understanding!

So, let’s begin.

Molecules are the tiny particles of any substance that still retain all the properties of that substance.

When you break down a molecule further, it no longer has the same characteristics.

A molecule forms when two or more atoms of the same or different elements join in a fixed ratio.

There are 118 types of atoms, but most do not like to exist alone. They prefer to combine with other atoms to form larger particles called molecules.

This combination of atoms gives each molecule its unique properties and makes up the variety of substances we see around us.

Topics Covered:

• Definition of a molecule
• Difference between atoms and molecules
• Types of molecules
• Importance of the study of molecules
• Types of bonds

Definition of a Molecule

Molecule is the smallest particle of any substance, which is responsible for all physical and chemical properties of that particular substance. It is formed when two or more atoms of the same or different types of elements combine in a fixed ratio.

What are Molecules?

Let us understand with an example.

Sugar Crystals

We all know that sugar tastes sweet. If we break a sugar crystal into two pieces and taste each piece, both pieces will still taste sweet. Now, imagine breaking this crystal into a billion tiny pieces. Each tiny piece will still taste sweet. However, if we break these pieces down to the smallest possible level—the atoms—they won’t taste sweet anymore because atoms have their unique properties. The smallest part of sugar that still tastes sweet is called a sugar molecule.

A molecule of Sugar is the smallest particle of a sugar crystal. It consists of three kinds of atoms – hydrogen, oxygen & carbon

As most of the atoms do not like to stay alone, it is difficult to find a single free atom in nature, except those of inert elements.

A molecule may consist of –

• the same kind of two or more atoms, or
• different kinds of two or more atoms

Molecules with the same types of atoms and with different types of atoms

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Difference between Atoms & Molecules

Atoms are the basic building blocks of all substances around us. The different kinds of atoms combine in a fixed ratio to form in numerous molecules and substances.

The important differences between atoms and molecules are as below:

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Type of molecules

Molecules are of three types: monoatomic, diatomic, and polyatomic

MONOATOMIC MOLECULES

• Monoatomic molecules consist of a single atom. These are typically noble gases, which are stable and rarely form bonds with other atoms. Examples include elements such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These elements are found in the far-right column of the periodic table and are known for their lack of reactivity under normal conditions.

DIATOMIC MOLECULES

• Diatomic molecules consist of two atoms, which can be of the same or different elements. Common examples include hydrogen gas (H2), the most abundant molecule in the universe, and oxygen gas (O2), which is essential for respiration in most life forms. Other examples include chlorine gas (Cl2), used for disinfection, and carbon monoxide (CO), a poisonous gas. Hydrochloric acid (HCl) is another example, which is a strong acid commonly used in industrial processes and found in gastric acid.

POLYATOMIC MOLECULES

• Polyatomic molecules consist of more than two atoms, which can be the same or different elements. These molecules can be quite complex and vary greatly in size and structure. Examples include ozone (O3), which protects life on Earth by absorbing ultraviolet radiation. Phosphorus (P4) is another example, essential for life and used in fertilizers. Sulfur (S8) is found in volcanic areas and is important in industrial applications. Water (H2O) is perhaps the most vital molecule for life, acting as a solvent in biological processes. Carbon dioxide (CO2) is a key greenhouse gas and a product of respiration and combustion. Sodium bicarbonate (NaHCO3), also known as baking soda, is used in baking and as an antacid.

The diversity of polyatomic molecules is immense, with countless compounds yet to be discovered or synthesized. These molecules play crucial roles in various fields, including chemistry, biology, medicine, and industry, highlighting the complexity and richness of molecular science.

Examples of different types of molecules – Monoatomic, Diatomic, and Polyatomic

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Importance of the Study of Molecules

The study of molecules is crucial because the properties of a substance or material depend on the molecules that compose it. Just as bricks build a wall, molecules form substances. The strength and color of a wall depend on the type of bricks used; similarly, the properties of a substance depend on the types of molecules it contains.

For instance, consider the difference between hydrogen gas and water. When two hydrogen atoms (H) bond together, they form a molecule of hydrogen gas (H2), a gas at room temperature. However, when two hydrogen atoms (H) bond with one oxygen atom (O), they form a molecule of water (H2O), which is a liquid at room temperature. Thus, both substances formed had a different set of properties –

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Type of Bonds

Atoms in a molecule are held together by two types of bonds: ionic bonds and covalent bonds.

In an ionic bond, one atom donates electrons from its outermost shell to another atom. This transfer of electrons results in one atom becoming positively charged (the one that loses electrons) and the other becoming negatively charged (the one that gains electrons). The attraction between these oppositely charged ions keeps the atoms together, forming an ionic bond.

Covalent bonds, on the other hand, occur when atoms share electrons instead of transferring them. This sharing allows each atom to achieve a stable electron configuration, and the mutual sharing of electrons holds the atoms together in a covalent bond.

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