What Is Valency Definition Types Formula and How to Calculate It
Valency is essential in chemistry and helps students understand various practical and theoretical applications related to this topic.
What is Valency in Chemistry?
Valency in chemistry refers to the ability of an atom to combine with other atoms based on the number of electrons it can lose, gain, or share.
The concept of valency helps us predict the number of bonds an element can form, making it important for studying chemical reactions, chemical bonding, and formation of compounds. Valency plays a key role in chapters related to atomic structure, periodic trends, and chemical formulas.
Molecular Formula and Composition
The term “valency” does not have a specific molecular formula. Instead, it is a property or number associated with each element showing how many chemical bonds its atoms can form.
For example, the valency of hydrogen is 1, oxygen is 2, and carbon is 4. This concept falls under the topic of atomic properties and is useful while writing molecular formulas like H2O and CO2.
Preparation and Synthesis Methods
Valency is not something you prepare or synthesize, but rather a number that depends on the arrangement of electrons in an atom's outermost shell. You can determine valency using the periodic table or electron configuration rules.
Vedantu often provides easy tricks and charts to help students find valency quickly in live classes and notes.
Frequent Related Errors
- Confusing valency with the number of valence electrons (they are not always equal).
- Mixing up valency with oxidation number or state.
- Forgetting that some elements have variable valency (such as iron: 2 or 3).
Valency Chart/Table
| Element | Symbol | Valency |
|---|---|---|
| Hydrogen | H | 1 |
| Helium | He | 0 |
| Lithium | Li | 1 |
| Beryllium | Be | 2 |
| Boron | B | 3 |
| Carbon | C | 4 |
| Nitrogen | N | 3 |
| Oxygen | O | 2 |
| Fluorine | F | 1 |
| Neon | Ne | 0 |
| Sodium | Na | 1 |
| Magnesium | Mg | 2 |
| Aluminium | Al | 3 |
| Silicon | Si | 4 |
| Phosphorus | P | 3 |
| Sulfur | S | 2 |
| Chlorine | Cl | 1 |
| Argon | Ar | 0 |
| Potassium | K | 1 |
| Calcium | Ca | 2 |
How to Calculate Valency
Valency is mostly determined by the number of electrons in the outermost shell of an atom. If the outer shell has less than or equal to 4 electrons, valency equals the number of these electrons. If it has more than 4, valency equals 8 minus the number of outer electrons.
Steps to calculate valency:
1. Find the element's group number (main group elements) or note the electron configuration.2. Count the electrons in the outermost shell.
3. If that number is ≤ 4, valency = (number of outer electrons).
4. If it's > 4, valency = 8 minus (number of outer electrons).
Examples:
1. For Oxygen (atomic number 8): Outer shell has 6 electrons. Valency = 8 – 6 = 2.2. For Nitrogen (atomic number 7): Outer shell has 5 electrons. Valency = 8 – 5 = 3.
3. For Sodium (atomic number 11): Outer shell has 1 electron. Valency = 1.
Transition elements and some main-group elements may have variable valency.
Valency Examples (Application)
- Water (H2O): Hydrogen valency 1, Oxygen valency 2 — two hydrogens combine with one oxygen.
- Carbon Dioxide (CO2): Carbon valency 4, Oxygen valency 2 — one carbon combines with two oxygens.
- Ammonia (NH3): Nitrogen valency 3, Hydrogen valency 1 — one nitrogen with three hydrogens.
Uses of Valency in Real Life
Valency is widely used in writing chemical formulas, predicting reactions, designing compounds in medicines, fertilizers, and materials. It helps scientists and students understand how atoms join to form molecules in everyday objects.
Relation with Other Chemistry Concepts
Valency is closely related to atomic structure and oxidation number. It works with the octet rule and helps you understand chemical bonding and types of chemical reactions. Knowing valency makes it easy to predict the formulae and properties of new compounds.
Step-by-Step Reaction Example
1. Take the elements carbon and oxygen.2. Carbon's valency is 4, and oxygen's is 2.
3. To saturate all bonds, two oxygens are required for one carbon (CO2).
4. The chemical equation is: C + O2 → CO2
5. Both carbon and oxygen use their valency to form stable bonds in this molecule.
Lab or Experimental Tips
Remember valency by the rule of “eight” (octet rule) — atoms want to complete their shell with 8 electrons (or 2 for hydrogen and helium). Vedantu educators use tricks like periodic table group numbers to find valency at a glance.
Try This Yourself
- Write the formula for a compound formed by magnesium (Mg) and chlorine (Cl).
- Determine the valency of phosphorus from its electron configuration.
- Give two daily examples where understanding valency is useful.
Final Wrap-Up
We explored valency—its definition, calculation, examples, and connections to other chemistry ideas. Understanding valency helps you write correct formulas, explain reactions, and score higher in chemistry. Keep practicing with Vedantu resources for clarity and mastery of chemistry basics.
FAQs on Valency in Chemistry and How It Determines Bonding
1. What is valency in chemistry?
Valency is the combining capacity of an atom, defined as the number of electrons it loses, gains, or shares to form chemical bonds. It indicates how many bonds an atom can form with other atoms.
- For main-group elements, valency is often related to the number of electrons in the outermost shell.
- For example, hydrogen has valency 1, oxygen has valency 2, and nitrogen has valency 3.
- In H2O, oxygen forms two single covalent bonds, showing its valency is 2.
2. How do you find the valency of an element?
The valency of an element is determined by the number of electrons it needs to lose, gain, or share to achieve a stable outer shell (usually an octet).
- Step 1: Write the electronic configuration.
- Step 2: Count valence electrons in the outermost shell.
- Step 3: If valence electrons ≤ 4, valency equals that number; if > 4, valency = 8 − (valence electrons).
- Example: Sodium (Na) has configuration 2,8,1 → it loses 1 electron → valency = 1.
3. What is the valency of oxygen?
The valency of oxygen is 2 because it needs two electrons to complete its octet. Oxygen has electronic configuration 2,6, meaning it has 6 valence electrons.
- It requires 2 more electrons to reach 8.
- In H2O, oxygen forms two single covalent bonds with hydrogen.
- In oxide ions, it forms O22- or O2- by gaining electrons.
4. What is the difference between valency and oxidation number?
Valency is the combining capacity of an element, while oxidation number is the apparent charge assigned to an atom in a compound. The key differences are:
- Valency is always a whole number and usually has no sign.
- Oxidation number can be positive, negative, or zero and may vary in different compounds.
- Example: In FeCl2, iron has oxidation number +2; in FeCl3, it is +3, showing variable oxidation states.
5. What is variable valency?
Variable valency is the ability of an element, usually a transition metal, to exhibit more than one valency. This happens because electrons from both the outermost s and inner d orbitals can participate in bonding.
- Example: Iron shows valencies 2 and 3.
- In FeO, iron has valency 2.
- In Fe2O3, iron has valency 3.
6. What is the valency of hydrogen?
The valency of hydrogen is 1 because it can lose, gain, or share one electron to achieve a stable duplet configuration. Hydrogen has only one electron in its outer shell.
- In HCl, hydrogen forms one covalent bond.
- In ionic compounds like NaH, it can exist as hydride ion H-.
7. How do you write the chemical formula using valency?
Chemical formulas are written by criss-crossing the valencies of the combining ions or elements and reducing to the simplest whole-number ratio.
- Step 1: Write symbols and their valencies.
- Step 2: Cross the valencies as subscripts.
- Step 3: Simplify if possible.
- Example: Calcium (valency 2) and chlorine (valency 1) → CaCl2.
- Example: Aluminium (3) and oxygen (2) → Al2O3.
8. What is the valency of carbon?
The valency of carbon is 4 because it has four electrons in its outermost shell and needs four more to complete its octet. Carbon’s electronic configuration is 2,4.
- It forms four covalent bonds, as seen in CH4.
- This tetravalency allows carbon to form long chains and rings in organic chemistry.
9. Why is valency important in chemistry?
Valency is important because it determines how atoms combine to form molecules and compounds. It helps predict chemical formulas and bonding patterns.
- Used to write correct chemical formulas like NaCl and MgO.
- Explains formation of covalent and ionic bonds.
- Helps in balancing chemical equations, such as 2H2(g) + O2(g) → 2H2O(l).
10. What is the valency of common elements like Na, Cl, and Mg?
The valency of sodium (Na) is 1, chlorine (Cl) is 1, and magnesium (Mg) is 2 based on their outer shell electrons. Their valencies are determined as follows:
- Na: 2,8,1 → loses 1 electron → valency = 1.
- Cl: 2,8,7 → gains 1 electron → valency = 1.
- Mg: 2,8,2 → loses 2 electrons → valency = 2.
- Thus, magnesium and chlorine form MgCl2.
