Anand Classes brings you a detailed explanation of Periodicity of Valence and Oxidation States for JEE, NEET, and CBSE students. Valence, the combining capacity of an atom, depends on the number of electrons present in the outermost shell, and it plays a vital role in determining the chemical properties of elements. Understanding how valency and oxidation states vary across periods and groups in the Periodic Table is essential for mastering competitive exams as well as board exams. Click the print button to download study material and notes.
Periodicity of Valence or Oxidation States
Valence is one of the most characteristic properties 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 their atoms.
- The electrons in the outermost shell are called valence electrons.
- These electrons determine the valence of an atom.
For representative elements:
- The valence is generally equal to the number of valence electrons, or
- Equal to 8 minus the number of valence electrons.
This trend is shown in the following table:
Table. Valence 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 |
| Valence | 1 | 2 | 3 | 4 | 3, 5 | 2, 6 | 1, 7 | 0, 8 |
However, transition elements exhibit variable valency, which makes their behavior different.
Variation of Valency in the Periodic Table
(a) Variation Across a Period
- The number of valence electrons increases from 1 to 8 as we move across a period.
- Consequently, the valency of elements (with respect to hydrogen and chlorine) increases from 1 to 4, and then decreases back to zero.
This can be seen in the elements of the second and third periods:
Table. Variation of Valency of Elements of the Second and Third Periods
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 |
|---|---|---|---|---|---|---|---|
| Elements (2nd period) | Li | Be | B | C | N | O | F |
| Valence with respect to H | LiH (1) | BeH₂ (2) | BH₃ / B₂H₆ (3) | CH₄ (4) | NH₃ (3) | H₂O (2) | HF (1) |
| Valence with respect to Cl | LiCl (1) | BeCl₂ (2) | BCl₃ (3) | CCl₄ (4) | NCl₃ (3) | Cl₂O (2) | ClF (1) |
| Elements (3rd period) | Na | Mg | Al | Si | P | S | Cl |
| Valence with respect to H | NaH (1) | MgH₂ (2) | AlH₃ (3) | SiH₄ (4) | PH₃ (3) | H₂S (2) | HCl (1) |
(b) Variation Down a Group
- On moving down a group, the number of valence electrons remains the same.
- Therefore, all elements in a group exhibit the same valency.
- Example: All elements of Group 1 have valency = 1, and those of Group 2 have valency = 2.
Periodic Trends in Valency
The periodic trends of valence in representative elements can be illustrated by the formulae of their hydrides and oxides.
Table. Periodic Trends in Valence of Elements
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 |
|---|---|---|---|---|---|---|---|
| Hydrides (2nd–5th period) | LiH, NaH, KH, RbH | BeH₂, MgH₂, CaH₂, SrH₂ | B₂H₆, AlH₃, GaH₃ | CH₄, SiH₄, GeH₄, SnH₄ | NH₃, PH₃, AsH₃, SbH₃ | H₂O, H₂S, H₂Se, H₂Te | HF, HCl, HBr, HI |
| Oxides | Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O | BeO, MgO, CaO, SrO, BaO | B₂O₃, Al₂O₃, Ga₂O₃, In₂O₃ | CO₂, SiO₂, GeO₂, SnO₂, PbO₂ | N₂O₃/N₂O₅, P₄O₆/P₄O₁₀, As₂O₃/As₂O₅, Sb₂O₃/Sb₂O₅, Bi₂O₃ | SO₂/SO₃, SeO₂/SeO₃, TeO₃ | Cl₂O₇ |
Variable Valency and Oxidation State
- Many elements (especially transition elements and actinoids) exhibit variable valency.
- Nowadays, the term oxidation state is preferred over valence.
👉 Oxidation state of an element in a compound represents the apparent charge acquired by its atom based on the electronegativity difference between it and the other atoms.
Example:
- In OF₂:
- Fluorine (most electronegative) is assigned –1.
- Oxygen, bonded to two fluorine atoms, shows an oxidation state of +2.
- In Na₂O:
- Oxygen (more electronegative than Na) is –2.
- Sodium, losing one electron each, is +1.
Common Oxidation States of Representative Elements
- s-block elements: Show only one oxidation state → +1 (alkali metals) and +2 (alkaline earth metals).
- p-block elements: Show multiple oxidation states due to variable participation of their valence electrons.
FAQs on Periodicity of Valence and Oxidation States
Q1. What is meant by valence of an element?
Answer:
Valence is the combining capacity of an atom. It is determined by the number of electrons present in the outermost shell (valence electrons).
Q2. How is valence related to the number of valence electrons?
Answer:
For representative elements:
- Valence = Number of valence electrons, OR
- Valence = 8 – Number of valence electrons.
Example:
- Oxygen (6 valence electrons) → Valence = 2.
- Carbon (4 valence electrons) → Valence = 4.
Q3. Why do transition elements show variable valency?
Answer:
Transition elements have partially filled d-orbitals. Both (n–1)d and ns electrons can participate in bonding, leading to variable oxidation states.
Q4. How does valency vary across a period?
Answer:
- The number of valence electrons increases from 1 to 8 across a period.
- 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).
Q5. How does valency vary down a group?
Answer:
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.
Q6. What is the difference between valence and oxidation state?
Answer:
- Valence is the combining capacity of an atom, based only on electron count.
- Oxidation state is the apparent charge of an atom in a compound, assigned considering electronegativity differences.
Q7. Why is the term oxidation state preferred over valence nowadays?
Answer:
Because oxidation state explains the actual electron transfer or sharing in compounds more accurately, especially in cases of variable valency.
Q8. What is the oxidation state of oxygen in OF₂ and Na₂O?
Answer:
- In OF₂, oxygen has oxidation state +2 (since fluorine is more electronegative).
- In Na₂O, oxygen has oxidation state –2 (since it is more electronegative than sodium).
Q9. What are the common oxidation states of s-block elements?
Answer:
- Alkali metals (Group 1) → +1
- Alkaline earth metals (Group 2) → +2
They show only one oxidation state.
Q10. Why do p-block elements show multiple oxidation states?
Answer:
Because of the involvement of both s and p orbitals in bonding, and due to the inert pair effect in heavier elements, p-block elements often exhibit more than one oxidation state.
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