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MHT-CET Chemistry · Modern Periodic Table

Locating an Element: Period, Group and Block from the Configuration

An element's period is the highest shell number in its configuration, its block is the subshell that receives the last electron, and its group follows from how many electrons sit in the outer s, p and (n−1)d subshells.

All Modern Periodic Table notesMHT-CET Chemistry strategy

Why this matters

8 PYQs, all but one EASY. Four name an element from its group and period or give the position of one, four go from a configuration to a group or block. Two cards.

Concept 1 of 2: Group and Period of an Element

Group 1 and group 2 are easy to count down: Li, Na, K, Rb, Cs and Be, Mg, Ca, Sr, Ba, with period 2 at the top. For a d-block element add the (n−1)d and ns electrons: copper, 3d¹⁰4s¹, is period 4 and group 11.

Definition

  • Period = highest n in the configuration.
  • s-block: group = number of s electrons (1 or 2). p-block: group = 10 + (s + p electrons). d-block: group = (n−1)d + ns electrons.
  • Period 5: Rb (group 1), Sr (group 2); Cs and Ba are period 6.
  • Group 17: F, Cl, Br, I, At. Cu: group 11, period 4.
ElementGroupPeriod
Rubidium15Q
Strontium25Q
Copper114Q
Astatine176Q
Caesium, barium1, 26
Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2024 · 12th May Shift 2 · Q98Easy

Example 1 · Modern Periodic Table · Position in Periodic Table and Electronic Configuration

What is the position of copper in long form of periodic table?

Copper in period 3

The d-block starts in period 4 (Sc to Zn). Copper, 3d¹⁰4s¹, has its highest shell n = 4, so it is period 4, group 11.
Drill 3 more on group and period of an element

Concept 2 of 2: From a General Configuration to the Group or Block

Read only the outer shell. ns¹ is group 1, ns²np⁴ group 16. An element that gains two electrons to reach a noble-gas configuration must have been two short — group 16. And the block is wherever the last electron went: (n−1)d means a d-block metal.

Definition

  • ns¹: group 1 (Li … Fr). ns²: group 2.
  • ns²np⁴: group 16 (O, S, Se, Te). ns²np⁵: group 17. ns²np⁶: group 18.
  • Gains 2 e⁻ to reach a noble-gas configuration: group 16.
  • Last electron in (n−1)d: d-block — Ag (4d¹⁰5s¹). Dy, Pu, Pa fill (n−2)f — f-block.
Configuration or clueAnswer
ns¹Group 1 — FrQ
ns²np⁴Group 16 — SeQ
Gains two electrons → noble gasGroup 16Q
Last electron in (n−1)dAgQ
Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q94Moderate

Example 2 · Modern Periodic Table · Position in Periodic Table and Electronic Configuration

Identify the element from following such that the last electron is placed in ( n−1n - 1 )d orbital.

Picking Kr or Xe for ns²np⁴

Noble gases end in np⁶. np⁴ is two short — the chalcogens, group 16.
Drill 3 more on from a general configuration to the group or block

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Group and Period of an Element5 rows
ElementGroupPeriod
Rubidium15Q
Strontium25Q
Copper114Q
Astatine176Q
Caesium, barium1, 26
From a General Configuration to the Group or Block4 rows
Configuration or clueAnswer
ns¹Group 1 — FrQ
ns²np⁴Group 16 — SeQ
Gains two electrons → noble gasGroup 16Q
Last electron in (n−1)dAgQ

Watch out for (2)

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