Q.The pair of elements which are members of third period are
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Periodic Table Blocks
The Intuition: Why "Blocks" at All?
Imagine you're building a house of cards. Each card has a specific shape and a specific place where it fits. The periodic table is like that house — but instead of cards, we have elements, and instead of shapes, we have electron configurations.
The periodic table is arranged in rows (periods) and columns (groups). But if you look closely, you'll notice that the table isn't a perfect rectangle. There's a detached island of elements (the f-block) floating below, and the main body has a strange "staircase" shape. That shape isn't random — it's dictated by which orbital the last electron enters.
That's the core idea: A block is a set of elements whose last electron enters the same type of orbital (s, p, d, or f).
The Precise Statement
Periodic Table Blocks are regions of the periodic table where elements share the same valence subshell — the subshell being filled as you move across that block.
There are four blocks, named after the four types of atomic orbitals:
| Block | Orbital being filled | Location in the table | Number of groups |
|---|---|---|---|
| s-block | ns | Leftmost 2 columns (Groups 1 & 2) | 2 |
| p-block | np | Rightmost 6 columns (Groups 13–18) | 6 |
| d-block | (n−1)d | Middle 10 columns (Groups 3–12) | 10 |
| f-block | (n−2)f | Two rows below the main table (Lanthanides & Actinides) | 14 |
The "n" in the orbital notation refers to the principal quantum number (the period number). Notice how for d and f blocks, the orbital being filled has a lower n than the period you're in. That's because of the Aufbau principle — orbitals fill in order of increasing energy, and 4s fills before 3d, etc.
How to Read the Blocks
s-block (Groups 1 & 2)
- Last electron enters an s orbital.
- Examples: Hydrogen (1s1), Lithium (2s1), Beryllium (2s2).
- These are highly reactive metals (except H and He). They lose their s electron(s) easily.
p-block (Groups 13–18)
- Last electron enters a p orbital.
- Examples: Carbon (2p2), Oxygen (2p4), Chlorine (3p5).
- This block contains metals, non-metals, and metalloids — the most chemically diverse block.
d-block (Groups 3–12)
- Last electron enters a d orbital — specifically, the (n−1)d subshell.
- Examples: Iron (3d6), Copper (3d10), Zinc (3d10).
- These are transition metals. They often have variable oxidation states and form coloured compounds.
f-block (Lanthanides & Actinides)
- Last electron enters an f orbital — specifically, the (n−2)f subshell.
- Examples: Cerium (4f1), Uranium (5f3).
- These are inner transition metals. They are placed below to keep the table from being absurdly wide.
A common mistake: thinking that the block tells you the group number. It doesn't. The block tells you the orbital type, not the group. For example, both Carbon (Group 14) and Oxygen (Group 16) are in the p-block, but they're in different groups.
Why This Matters
Knowing the block of an element tells you three things instantly:
- Which orbital is being filled — the heart of its electron configuration. …
Elements placed in the same period of the periodic table share the same number of principal electron shells, and period 3 spans atomic numbers 11 to 18. …
Period 3 spans elements with atomic numbers 11 to 18 (Na, Mg, Al, Si, P, S, Cl, Ar). Check each pair for both elements falling in this range.
Check each pair:
- Be (Z=4, period 2), P (Z=15, period 3) -- different periods, mismatch.
- Si (Z=14, period 3), Cl (Z=17, period 3) -- both in period 3. Match.
- N (Z=7, period 2), O (Z=8, period 2) -- both period 2, not period 3. …
Showing the 12 most recent of 47 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.The physical and chemical properties of an element are the periodic function of its(a) Atomic mass(b) Element behaviour(c) No. of protons(d) Atomic number
›Reveal solutionSolution
The MODERN periodic law (after Moseley's correction to Mendeleev's original atomic-mass-based law) states: the physical and chemical properties of elements are periodic functions of their atomic number.
Mendeleev originally arranged elements by increasing atomic mass, which led to a few inconsistencies (e.g. some elements had to be placed out of strict mass order to fit their properties, such as Ar/K and Co/Ni). Henry Moseley later showed, from X-ray spectra, that atomic number (the number of protons, and hence the nuclear charge) is the more fundamental property go …
- CBSE 2026Set ANNUAL1 markMCQQ.The pair of elements which are members of third period are(a) Be, P(b) Si, Cl(c) N, O(d) F, Al
›Reveal solutionSolution
Period 3 spans elements with atomic numbers 11 to 18 (Na, Mg, Al, Si, P, S, Cl, Ar). Check each pair for both elements falling in this range.
Check each pair:
- Be (Z=4, period 2), P (Z=15, period 3) -- different periods, mismatch.
- Si (Z=14, period 3), Cl (Z=17, period 3) -- both in period 3. Match.
- N (Z=7, period 2), O (Z=8, period 2) -- both period 2, not period 3. …
- CBSE 2026Set ANNUAL1 markMCQQ.If the electronic configuration of an element is [Ar]3d^10 4s^2 4p^6, the element belongs to(a) s-block(b) p-block(c) d-block(d) f-block
›Reveal solutionSolution
An element's block is decided by the subshell that receives the last (highest-energy, outermost) electron. Here that is 4p.
The given configuration [Ar]3d^10 4s^2 4p^6 fills, in order: the argon core, then 3d (10 electrons, fully filled), then 4s (2 electrons, fully filled), then 4p (6 electrons, fully filled). This is the configuration of krypton (Z = 36), a noble gas.
Block classification rule:
- s-block: outermost electron enters an s subshell.
- p-block: outermost electron enters a p subshell.
- d-block: outermost electron enters a (n-1)d subshell. …
- CBSE 2026Set ANNUAL1 markMCQQ.Who gave the law of octave ?(a) Dobereiner(b) Newlands(c) Lothar Mayer(d) Mendeleev
›Reveal solutionSolution
The law of octaves was given by Newlands.
John Newlands (1865) arranged elements in order of increasing atomic mass and noticed that every eighth element had properties similar to the first, like the octaves in music — the law of octaves. Do …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is a fundamental property of elements ?(a) Atomic weight(b) Molecular weight(c) Atomic number(d) Density
›Reveal solutionSolution
Atomic number is the fundamental property of an element.
The modern periodic law states that properties of elements are a periodic function of their atomic numbers. The atomic number (number of protons) uniquely identifies an element and determines its chemical behaviour. Atomic and mole …
- CBSE 2025Set ANNUAL1 markMCQQ.Element with atomic number 56 belongs to which block?(a) s(b) p(c) d(d) f
›Reveal solutionSolution
Element Z = 56 is barium, an s-block element.
Atomic number 56 corresponds to barium (Ba), electron configuration [Xe]6s2. Its last electron enters the 6s orbital, so barium …
- CBSE 2025Set ANNUAL1 markMCQQ.To which block is an element having electronic configuration [Ar]4s^1 3d^10 related?(a) s-block(b) p-block(c) d-block(d) f-block
›Reveal solutionSolution
Even though this configuration has 4s1 (only one 4s electron), the element is copper, a d-block element — the block is decided by which subshell was filled LAST in the building-up process (3d here), not by superficial appearance of the outermost shell number.
The configuration [Ar]4s1 3d10 (conventionally written [Ar]3d10 4s1) corresponds to copper, atomic number 29.
Why is it d-block? The 'block' of an element in the periodic table is defined by the subshell that receives the differentiating (last-entering) electron:
- s-block: last electron enters an s subshell (Groups 1, 2)
- p-block: last electron enters a p subshell (Groups 13-18)
- d-block: last electron enters a d subshell (Groups 3-12, transition metals)
- f-block: last electron enters an f subshell (lanthanoids/actinoids) …
- CBSE 2025Set ANNUAL1 markMCQQ.How many groups are present in the modern periodic table?(a) 15(b) 18(c) 10(d) 20
›Reveal solutionSolution
The modern long-form periodic table has 18 groups (columns) and 7 periods (rows).
The modern periodic table is built on the Modern Periodic Law: 'the physical and chemical properties of elements are periodic functions of their atomic numbers.' This is the long form of the periodic table (as opposed to older, shorter forms), and it is organised into:
- 18 groups (vertical columns), numbered 1 to 18 by current IUPAC convention (replacing the older IA-VIIA/IB-VIIB/VIII/0 labelling) …
- CBSE 2025Set ANNUAL1 markQ.Write the general electronic configuration of p-block elements.
›Reveal solutionSolution
p-block elements have the general outer electronic configuration ns2 np(1 to 6), where n is the outermost principal shell.
The p-block consists of Groups 13 to 18, in which the last (differentiating) electron enters the p-orbital of the outermost shell. The general valence-shell electronic configuration is ns2 np1 (Group …
- CBSE 2024Set ANN1 markQ.The element with outer electronic configuration 3s²3p³ belong to which block of the periodic table ?
›Reveal solutionSolution
The last (differentiating) electron enters a 3p orbital, so this element belongs to the p-block.
The block to which an element belongs is decided by the sub-shell that receives the last, or differentiating, electron in its ground-state configuration.
For outer configuration 3s² 3p³:
- The 3s sub-shell is already completely filled (2 electrons).
- The last electrons being added go into the 3p sub-shell (3 electrons). …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following are members of same period?(a) Li, Na, K(b) Li, Mg, Ca(c) Ni, Cu, Zn(d) F, Cl, Br
›Reveal solutionSolution
Ni, Cu, Zn are all period-4 (3d series) elements.
- Li, Na, K: periods 2, 3, 4 respectively — same GROUP (alkali metals), not same period.
- Li, Mg, Ca: periods 2, 3, 4 — a diagonal-relationship style set, not same period. …
- CBSE 2024Set ANNUAL1 markMCQQ.The general outer electronic configuration of d-block elements is(a) (n-1)f^1-14(n-1)^0-1 ns^2(b) ns^1-2(c) (n-1)d^1-10 ns^0-2(d) ns^2 np^1-6
›Reveal solutionSolution
d-block elements are classified by their electrons filling the (n-1)d sub-shell while the outer ns sub-shell holds 0 to 2 electrons.
The four blocks of the periodic table are defined by the general outer electronic configuration of their last-filled sub-shell:
- s-block: ns^1-2 (option b describes only part of this)
- p-block: ns^2 np^1-6 (option d)
- d-block (transition elements): (n-1)d^1-10 ns^0-2 (option c) — the differentiating electron enters the penultimate shell's d sub-shell. …
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