Valency and Oxidation States are important concepts in chemistry that describe the combining capacity of elements and the charge carried by atoms in compounds. Valency represents the number of electrons an atom loses, gains, or shares during chemical bonding, while oxidation state indicates the apparent charge of an atom in a molecule or ion. In the periodic table, valency varies regularly across a period and down a group due to changes in electronic configuration. Understanding the variation of valency and oxidation states helps students explain chemical reactions, bonding, and periodic trends effectively. These concepts are essential for CBSE exams, NEET, JEE, and other competitive examinations.
What is Valency ?
The valency is the most characteristic property of the elements. It has been observed that the chemical properties of elements depend upon the number of electrons present in the outermost shell of the atom. The electrons present in the outermost shell are called valence electrons and these electrons determine the valency of the atom.
In the case of representative elements, the valency is generally equal to either the number of valence electrons or equal to eight minus the number of valence electrons. This is shown in Table.
Table. Valency of elements in different groups.
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
|---|---|---|---|---|---|---|---|---|
| No. of valence electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Valency | 1 | 2 | 3 | 4 | 3, 5 | 2, 6 | 1, 7 | 0, 8 |
This table shows the general valency of representative elements based on the number of valence electrons. However, the transition elements exhibit variable valency.
Periodicity or Variation of Valency in the Periodic Table
(a) Variation in a period
The number of valence electrons increases from 1 to 8 on moving across a period. Consequently, the valency of the elements with respect to hydrogen and chlorine increases from 1 to 4 and then decreases to zero.
This can be illustrated by taking the examples of elements of the second and third periods (see Table). The number in brackets represents the valency of the elements in the compound.
Table. Variation of valency of elements of second and third period.
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 |
|---|---|---|---|---|---|---|---|
| Elements of second period | Li | Be | B | C | N | O | F |
| Valence with respect to H | LiH | BeH₂ | BH₃ or B₂H₆ | CH₄ | NH₃ | H₂O | HF |
| Valency | (1) | (2) | (3) | (4) | (3) | (2) | (1) |
| Valence with respect to Cl | LiCl | BeCl₂ | BCl₃ | CCl₄ | NCl₃ | Cl₂O | ClF |
| Valency | (1) | (2) | (3) | (4) | (3) | (2) | (1) |
| Elements of third period | Na | Mg | Al | Si | P | S | Cl |
| Valence with respect to H | NaH | MgH₂ | AlH₃ | SiH₄ | PH₃ | H₂S | HCl |
| Valency | (1) | (2) | (3) | (4) | (3) | (2) | (1) |
This table clearly shows how valency increases from 1 to 4 and then decreases to 1 (with respect to hydrogen and chlorine) across the period.
“Similar Concept of Atomic Radius Trends : “Variation of Atomic Radius in a Period and Down The Group“
“You may also like to study Ionization Enthalpy Trends : Ionization Enthalpy Trends Along a Period and Down a Group“
(b) Variation in a group
On moving down a group, the number of valence electrons remains the same. Therefore, all the elements in a group exhibit the same valency. For example, all elements of Group 1 have valency one and those of Group 2 have valency two.
“Students should also study Electron Gain Enthalpy Trends : Electron Gain Enthalpy Definition, Units, Factors, Trends“
Periodic Trends in Valency of Representative Elements
The periodic trends in valency of representative elements shown by the formulae of their hydrides and oxides are given in Table below.
Table. Periodic trends in valency of elements as shown by the formulae of their compounds.
| Group → | 1 | 2 | 13 | 14 | 15 | 16 | 17 |
|---|---|---|---|---|---|---|---|
| Period | Formulae of hydrides | ||||||
| 2 | LiH | BeH₂ | B₂H₆ | CH₄ | NH₃ | H₂O | HF |
| 3 | NaH | MgH₂ | AlH₃ | SiH₄ | PH₃ | H₂S | HCl |
| 4 | KH | CaH₂ | GeH₄ | AsH₃ | H₂Se | HBr | |
| 5 | RbH | SnH₄ | SbH₃ | H₂Te | HI | ||
| Formulae of oxides | |||||||
| 2 | Li₂O | BeO | B₂O₃ | CO₂ | N₂O₃, N₂O₅ | ||
| 3 | Na₂O | MgO | Al₂O₃ | SiO₂ | P₄O₆, P₄O₁₀ | SO₂, SO₃ | Cl₂O₇ |
| 4 | K₂O | CaO | Ga₂O₃ | GeO₂ | As₂O₃, As₂O₅ | SeO₂, SeO₃ | |
| 5 | Rb₂O | SrO | In₂O₃ | SnO₂ | Sb₂O₃, Sb₂O₅ | TeO₃ | |
| 6 | Cs₂O | BaO | PbO₂ | Bi₂O₃ |
There are many elements which exhibit variable valency. This is particularly characteristic of transition elements, lanthanoids and actinoids. Transition elements have partially filled d-orbitals. Both (n–1)d and ns electrons can participate in bonding, leading to variable oxidation states.
What is Oxidation State of an Element ?
Nowadays, the term oxidation state is preferred and frequently used instead of valency. Valence is the combining capacity of an atom, based only on electron count in valence shell while oxidation state is the apparent charge of an atom in a compound, assigned considering electronegativity differences. Also oxidation state explains the actual electron transfer or sharing in compounds more accurately, especially in cases of variable valency.
Oxidation state of an element in a particular compound gives the charge acquired by its atoms on the basis of electronegativity consideration from other atoms in the molecule.
Let us consider two oxygen containing compounds : OF₂ and Na₂O. The order of electronegativity of the three atoms involved in these compounds is F > O > Na. Fluorine, has the electronic configuration 2s²2p⁵ and each of the atom of fluorine shares one electron with oxygen in OF₂ molecule. Since fluorine is the highest electronegative element, it is given oxidation state –1. Now there are two fluorine atoms in the molecule, oxygen with outer electronic configuration 2s²2p⁴ shares two electrons with fluorine atoms. Therefore, it shows oxidation state of +2.
However, in Na₂O, oxygen being more electronegative accepts two electrons, one from each of the two sodium atoms and thus exhibits oxidation state of –2. On the other hand, sodium with electronic configuration 3s¹ loses one electron to oxygen and therefore, exhibits oxidation state of +1.
What are the Rules for assigning the Oxidation States
- Oxidation states of elements like O2, S8, H2, P4, Fe, etc are zero.
- Oxygen has an oxidation state of -2. But in its peroxides like Na2O2 and H2O2, it has -1 as its oxidation state
- Similarly, hydrogen has +1. But in Metal Hydrides, such as NaH, LiH, etc, it has -1
- Some elements have the same oxidation states as in their compounds such as
- Halogens have -1 except when they form a compound with one another or Oxygen.
- Alkali Metals such as Na, K, Rb, Li, Cs have +1
- Alkali Earth Metals have +2 such as Mg, Ca, Ba, Be, Sr, etc
The common oxidation states of representative elements are given in Table ahead.
Table. Common oxidation states of representative elements.
| Element | Common Oxidation States | Examples |
|---|---|---|
| H | +1, –1 | HCl (+1), Na⁺H⁻ (–1) |
| Li | +1 | Li⁺Cl⁻ |
| Be | +2 | BeCl₂ |
| B | +3 | BCl₃ |
| C | –4, +4, +2 | CH₄ (–4), CO₂ (+4), CO (+2) |
| N | –3, +2, +3, +4, +5 | NH₃ (–3), NO (+2), N₂O₃ (+3), N₂O₄ (+4), N₂O₅ (+5) |
| O | –2, –1 | H₂O (–2), H₂O₂ (–1) |
| F | –1 | Na⁺F⁻ |
| Na | +1 | Na⁺Cl⁻ |
| Mg | +2 | Mg²⁺(Cl⁻)₂ |
| Al | +3 | Al₂Cl₆ |
| Si | –4, +4 | SiH₄ (–4), SiCl₄ (+4) |
| P | –3, +3, +5 | PH₃ (–3), PCl₃ (+3), PCl₅ (+5) |
| S | –2, +4, +6 | H₂S (–2), SO₂ (+4), SO₃ (+6) |
| Cl | –1, +1, +3, +5, +7 | Na⁺Cl⁻ (–1), HOCl (+1), HClO₂ (+3), HClO₃ (+5), HClO₄ (+7) |
| K | +1 | K⁺Cl⁻ |
| Ca | +2 | Ca²⁺(Cl⁻)₂ |
| Ga | +3 | Ga₂Cl₆ |
| Ge | +4, +2 | GeCl₄ (+4), GeCl₂ (+2) |
| As | –3, +3, +5 | AsH₃ (–3), AsCl₃ (+3), As₄O₁₀ (+5) |
| Se | –2, +4, +6 | H₂Se (–2), SeO₂ (+4), SeO₃ (+6) |
| Br | –1, +1, +5, +7 | Na⁺Br⁻ (–1), HOBr (+1), HBrO₃ (+5), HBrO₄ (+7) |
| Rb | +1 | Rb⁺Cl⁻ |
| Sr | +2 | Sr²⁺(Cl⁻)₂ |
| In | +3 | InCl₃ |
| Sn | +4, +2 | SnCl₄ (+4), SnCl₂ (+2) |
| Sb | –3, +3, +5 | SbH₃ (–3), SbF₃ (+3), SbCl₅ (+5) |
| Te | –2, +4, +6 | H₂Te (–2), TeO₂ (+4), TeO₃ (+6) |
| I | –1, +1, +5, +7 | Na⁺I⁻ (–1), HOI (+1), HIO₃ (+5), HIO₄ (+7) |
| Cs | +1 | Cs⁺Cl⁻ |
| Ba | +2 | Ba²⁺(Cl⁻)₂ |
| Tl | +3, +1 | TlF₃ (+3), Tl⁺Cl⁻ (+1) |
| Pb | +4, +2 | PbO₂ (+4), PbCl₂ (+2) |
| Bi | –3, +3, +5 | BiH₃ (–3), BiF₃ (+3), Bi₂O₅ (+5) |
| Po | +4 | PoO₂ |
It is clear from Table that s-block elements show only one oxidation state either +1 (alkali metals) or +2 (alkaline earth metals). Hence, p-block elements show different oxidation states.
Note: Noble gases (He, Ne, Ar, Kr, Rn) generally show 0 oxidation state. Some compounds like XeF₂ (+2), XeF₄ (+4), XeF₆ (+6), XeO₆⁴⁻ (+8) are exceptions.
Frequently Asked Questions – FAQs Based on Periodicity of valency or oxidation states
What is meant by valence of an element?
Valence is the combining capacity of an atom. It is determined by the number of electrons present in the outermost shell (valence electrons). Valency is also defined as the number of hydrogen atoms which combine directly or indirectly with one atom of an element. Example : One atom of nitrogen combines with three atoms of hydrogen to form ammonia gas. So, the valency of nitrogen is 3.
How is valence related to the number of valence electrons?
For representative elements : Valence = Number of valence electrons, OR Valence = 8 – Number of valence electrons. Example : Oxygen (6 valence electrons) has valence = 2, Carbon (4 valence electrons) has valence = 4.
Why do transition elements show variable valency?
Transition elements have partially filled d-orbitals. Both (n–1)d and ns electrons can participate in bonding, leading to variable oxidation states.
How does valency vary across a period?
The number of valence electrons increases from 1 to 8 across a period. Thus valency first increases from 1 to 4, and then decreases back to 0. Example : In Period 2, Li (valency 1), C (valency 4), Ne (valency 0).
How does valency vary down a group?
Down a group, the number of valence electrons remains constant. Hence, all elements of the group show the same valency. Example : Group 1 (Li, Na, K, etc.) all have valency = 1.
What is the difference between valence and oxidation state?
Valence is the combining capacity of an atom, based only on electron count in valence shell while oxidation state is the apparent charge of an atom in a compound, assigned considering electronegativity differences.
Why is the term oxidation state preferred over valence nowadays?
Because oxidation state explains the actual electron transfer or sharing in compounds more accurately, especially in cases of variable valency.
What is Tetravalency of Carbon?
Carbon has a valency of four, so it is capable of bonding with four other atoms of carbon or atoms of some other monovalent element. This is known as tetravalency of carbon.
Does valency have a charge?
Charge is the value that is attained when an atom loses or gains electrons; this also happens to satisfy the octet rule. Valency is defined for elements whereas charge is defined for loss. So, valency cannot be related to charge.
How do you write the valency of oxygen?
Atomic Number of oxygen is 8. electronic Configuration of oxygen= 2, 6. So the valency is 8-6 = 2.
Important Chapter Links
Valency and Oxidation States are closely related to important topics such as electronic configuration, periodic classification of elements, chemical bonding, ion formation, and periodic properties. Understanding these concepts helps students explain trends in the modern periodic table and predict the chemical behavior of elements. To strengthen conceptual clarity, students should also study related topics like atomic structure, periodic trends and its cause, ionization enthalpy, electron gain enthalpy, and chemical bonding. Regular practice of NCERT questions, exemplar problems, and competitive exam JEE PYQs, NEET PYQs helps improve problem-solving skills and conceptual understanding for CBSE, NEET, and JEE examinations.