The period and group number of an element in the periodic table can be determined from its electronic configuration using specific rules for s-, p-, d-, and f-block elements. These rules depend on the principal quantum number (n), the distribution of electrons in the outermost orbitals, and the block to which the element belongs. Knowing these rules is essential for solving periodic table questions in Class 11 Chemistry, JEE, and NEET.

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📘 Rules to Find Period and Group Number in the Periodic Table

The modern periodic table is arranged in periods (horizontal rows) and groups (vertical columns) according to increasing atomic number (Z).
For any element, you can determine its period and group by looking at its electronic configuration.

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1️⃣ s-Block Elements

General Electronic Configuration: ns¹⁻²

• Period Number Rule:
  Period = value of n (principal quantum number) of the outermost shell.
• Group Number Rule: Number of ns electrons)
  • If ns¹ → Group 1 (Alkali metals)
  • If ns² → Group 2 (Alkaline earth metals)

Example 1: Sodium (Na), Z = 11

• Configuration: 1s² 2s² 2p⁶ 3s¹
• Outer shell: n = 3 → Period 3
• ns¹ → Group 1

Example 2: Calcium (Ca), Z = 20

• Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
• Outer shell: n = 4 → Period 4
• ns² → Group 2

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2️⃣ p-Block Elements

General Electronic Configuration: ns²np¹⁻⁶

• Period Number Rule:
  Period = value of n in the outermost shell.
• Group Number Rule (for main group elements):
  Group = 10 + (total electrons in ns + np orbitals of outermost shell).

Example 1: Nitrogen (N), Z = 7

• Configuration: 1s² 2s² 2p³
• Outer shell: n = 2 → Period 2
• Group = 10 + (2 + 3) = 15

Example 2: Chlorine (Cl), Z = 17

• Configuration: 1s² 2s² 2p⁶ 3s² 3p⁵
• Outer shell: n = 3 → Period 3
• Group = 10 + (2 + 5) = 17

Example 3: Oxygen (O), Z = 8

• Configuration: 1s² 2s² 2p⁴
• Outer shell: n = 2 → Period 2
• Group = 10 + (2 + 4) = 16

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3️⃣ d-Block Elements (Transition Metals)

General Electronic Configuration: (n−1)d¹⁻¹⁰ns⁰⁻²

• Period Number Rule:
  Period = value of n in the outermost ns orbital.
• Group Number Rule:
  Group = (electrons in (n–1)d) + (electrons in ns).

Example 1: Iron (Fe), Z = 26

• Configuration: [Ar] 3d⁶ 4s²
• n = 4 → Period 4
• Group = 6 (d-electrons) + 2 (s-electrons) = Group 8

Example 2: Copper (Cu), Z = 29

• Configuration: [Ar] 3d¹⁰ 4s¹
• n = 4 → Period 4
• Group = 10 + 1 = Group 11

Example 3: Chromium (Cr), Z = 24 (exception)

• Configuration: [Ar] 3d⁵ 4s¹
• n = 4 → Period 4
• Group = 5 + 1 = Group 6

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4️⃣ f-Block Elements (Inner Transition Metals)

General Electronic Configuration: (n−2)f¹⁻¹⁴(n−1)d⁰⁻²ns²

• Period Number Rule:
  Period = n of the outermost ns orbital.
  • Lanthanides → n = 6 → Period 6
  • Actinides → n = 7 → Period 7
• Group Number Rule:
  All f-block elements are placed in Group 3.

Example 1: Cerium (Ce), Z = 58

• Configuration: [Xe] 4f¹ 5d¹ 6s²
• n = 6 → Period 6
• Group = 3

Example 2: Uranium (U), Z = 92

• Configuration: [Rn] 5f³ 6d¹ 7s²
• n = 7 → Period 7
• Group = 3

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📊 Examples: Finding Period and Group Numbers for Different Block Elements

Block	Element (Symbol, Z)	Electronic Configuration	Step to Find Period	Step to Find Group	Result
s-block	Sodium (Na, 11)	1s² 2s² 2p⁶ 3s¹	Outermost n = 3 → Period 3	ns¹ → Group 1	Period 3, Group 1
s-block	Calcium (Ca, 20)	1s² 2s² 2p⁶ 3s² 3p⁶ 4s²	Outermost n = 4 → Period 4	ns² → Group 2	Period 4, Group 2
p-block	Oxygen (O, 8)	1s² 2s² 2p⁴	Outermost n = 2 → Period 2	Group = 10 + (2+4) = 16	Period 2, Group 16
p-block	Chlorine (Cl, 17)	1s² 2s² 2p⁶ 3s² 3p⁵	Outermost n = 3 → Period 3	Group = 10 + (2+5) = 17	Period 3, Group 17
d-block	Iron (Fe, 26)	[Ar] 3d⁶ 4s²	Outermost n = 4 → Period 4	Group = 6 + 2 = 8	Period 4, Group 8
d-block	Copper (Cu, 29)	[Ar] 3d¹⁰ 4s¹	Outermost n = 4 → Period 4	Group = 10 + 1 = 11	Period 4, Group 11
d-block	Chromium (Cr, 24)	[Ar] 3d⁵ 4s¹	Outermost n = 4 → Period 4	Group = 5 + 1 = 6	Period 4, Group 6
f-block	Cerium (Ce, 58)	[Xe] 4f¹ 5d¹ 6s²	Outermost n = 6 → Period 6	All f-block = Group 3	Period 6, Group 3
f-block	Uranium (U, 92)	[Rn] 5f³ 6d¹ 7s²	Outermost n = 7 → Period 7	All f-block = Group 3	Period 7, Group 3

✅ Tips for Students:

• Always write the full electron configuration first.
• Identify the highest value of n → gives the period number.
• Apply the specific group formula based on the block.
• Remember exceptions in d-block (Cr, Cu, Mo, Ag, etc.) where electrons shift for stability.

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📊 Quick Summary Table

Block	General Configuration	Period Rule	Group Rule
s-block	ns¹–²	n	ns¹ → 1, ns² → 2
p-block	ns² np¹–⁶	n	10 + (s + p electrons)
d-block	(n–1)d¹–¹⁰ ns⁰–²	n	(d electrons + s electrons)
f-block	(n–2)f¹–¹⁴ (n–1)d⁰–² ns²	n	Always Group 3

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FAQs on Rules to Find Period and Group in the Periodic Table

Q1. How do you find the period number of an element?

Answer: The period number is given by the principal quantum number (n) of the outermost shell in the electronic configuration.

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Q2. How do you find the group number of s-block elements?

Answer: For s-block elements, if the outer configuration is ns¹ → Group 1, and if ns² → Group 2.

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Q3. How do you find the group number of p-block elements?

Answer: Group number = 10 + (number of electrons in outermost s and p orbitals).

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Q4. How do you find the group number of d-block elements?

Answer: Group number = (number of electrons in (n–1)d orbital) + (number of electrons in ns orbital).

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Q5. How do you find the group number of f-block elements?

Answer: All f-block elements are placed in Group 3, regardless of their electron count.

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Q6. What is the period number of f-block elements?

Answer: For lanthanides, n = 6 → Period 6; for actinides, n = 7 → Period 7.

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