Why Four Blocks
The aufbau principle dictates the order in which subshells are filled, and the electronic configuration that results determines an element's chemistry. NCERT §3.6 turns this directly into a classification scheme: an element is assigned to the s-, p-, d- or f-block according to the type of atomic orbital that receives the last (differentiating) electron as the configuration is built up. The NIOS text states the same rule in different words — if the differentiating electron enters the s-subshell the element is an s-block element, if it enters the p-subshell it belongs to the p-block, and so on for the d- and f-subshells.
This is why the block is a property of atomic number, not atomic mass: the configuration follows from the number of electrons, and the periodic recurrence of properties is itself a periodic function of atomic number. The four blocks also align neatly with two older, broader labels. The s-block and p-block together are the representative or main group elements; the d-block elements are the transition elements; and the f-block elements are the inner-transition elements.
| Block | Last electron enters | Groups | Collective name |
|---|---|---|---|
| s | ns subshell | 1 – 2 | Representative / main group |
| p | np subshell | 13 – 18 | Representative / main group |
| d | (n-1)d subshell | 3 – 12 | Transition elements |
| f | (n-2)f subshell | placed below (3, period 6–7) | Inner-transition elements |
The Block Map
Before examining each block in turn, it helps to fix the geography. The s-block occupies the two columns on the far left, the p-block the six columns on the right, the d-block the ten-column slab wedged into the centre, and the f-block the two long rows detached at the bottom. The thick zig-zag line cutting diagonally through the p-block separates metals from non-metals, with the metalloids straddling it. The schematic below colour-codes the four blocks onto the familiar outline.
The s-Block Elements
The s-block comprises Group 1 (alkali metals) and Group 2 (alkaline earth metals), whose outermost configurations are $\ce{ns^1}$ and $\ce{ns^2}$ respectively. The general valence-shell formula for the block is therefore $\ce{ns^{1-2}}$. Lithium through francium illustrate the Group 1 pattern: each adds a single outer s-electron over a noble-gas core, as in $\ce{Li}$: $\ce{1s^2 2s^1}$ or $\ce{[He]2s^1}$, and $\ce{Na}$: $\ce{[Ne]3s^1}$, $\ce{K}$: $\ce{[Ar]4s^1}$, $\ce{Rb}$: $\ce{[Kr]5s^1}$, $\ce{Cs}$: $\ce{[Xe]6s^1}$ and $\ce{Fr}$: $\ce{[Rn]7s^1}$.
These are all reactive metals with low ionization enthalpies. They lose the outermost electron(s) readily — one to form a $1+$ ion for the alkali metals, two to form a $2+$ ion for the alkaline earth metals. Metallic character and reactivity both increase down each group, and because of this high reactivity the s-block metals are never found pure in nature. Their compounds are predominantly ionic, with lithium and beryllium the noted exceptions on account of their small size and consequently greater covalent (polarising) character.
| Feature | Group 1 (alkali metals) | Group 2 (alkaline earth metals) |
|---|---|---|
| Outer configuration | ns¹ | ns² |
| Common ion | $\ce{M+}$ | $\ce{M^2+}$ |
| Ionization enthalpy | Low (lowest in period) | Low, higher than Group 1 |
| Bonding in compounds | Predominantly ionic (Li exception) | Predominantly ionic (Be exception) |
| Examples | Li, Na, K, Rb, Cs, Fr | Be, Mg, Ca, Sr, Ba, Ra |
The p-Block Elements
The p-block runs from Group 13 to Group 18. Together with the s-block these constitute the representative or main group elements. Across each period the outermost configuration varies from $\ce{ns^2np^1}$ to $\ce{ns^2np^6}$, so the block's general valence formula is $\ce{ns^2np^{1-6}}$. At the end of every period stands a noble gas with the closed valence shell $\ce{ns^2np^6}$; with every valence orbital fully occupied, this arrangement is exceedingly stable and very difficult to disturb by adding or removing electrons, which is why the noble gases show very low chemical reactivity.
Preceding the noble gases are two chemically important non-metal groups. The halogens (Group 17) and the chalcogens (Group 16) both have highly negative electron gain enthalpies and readily accept one or two electrons respectively to reach the stable noble-gas configuration. Across a period non-metallic character increases from left to right, while down a group metallic character increases — so the p-block alone contains metals, non-metals and metalloids, the only block to do so.
Element $Z = 114$ was discovered recently. Predict its family and valence configuration.
Filling beyond radon: $\ce{[Rn]5f^{14}6d^{10}7s^{2}7p^{2}}$. The valence shell is $\ce{ns^2np^2}$ — two p-electrons — so $Z=114$ belongs to the carbon family (Group 14) of the p-block. (NCERT-style reasoning; this matches the NEET 2017 PYQ below.)
Helium is in the p-block group but is s-block by configuration
Helium sits at the top of Group 18 with the noble gases, yet its configuration is $\ce{1s^2}$ — its last electron enters an s-subshell, making it an s-block element by the differentiating-electron rule. It is grouped with the noble gases only because of its chemical inertness and full valence shell, not because it is a p-block element.
Block is decided by the subshell of the last electron; group placement can follow chemistry instead. Helium: s-block by rule, Group 18 by behaviour.
The d-Block Elements
The d-block elements are Groups 3 to 12, occupying the centre of the table. They are characterised by the filling of the inner $(n-1)d$ orbitals, which is why they are called d-block elements, and their general outer electronic configuration is $\ce{(n-1)d^{1-10}ns^{0-2}}$ — palladium being the notable exception with $\ce{4d^{10}5s^0}$. They are all metals, and they typically form coloured ions, exhibit variable oxidation states, show paramagnetism, and are frequently used as catalysts.
The name "transition" reflects their position: they form a bridge between the chemically active s-block metals and the less active p-block elements of Groups 13 and 14. There is, however, an important caveat about which d-block elements are transition metals in the strict sense.
Zn, Cd and Hg are d-block but not typical transition metals
Zinc, cadmium and mercury sit in Group 12 of the d-block, but their configuration is $\ce{(n-1)d^{10}ns^2}$ — a completely filled d-subshell in both the atom and the common $\ce{M^2+}$ ion. With no partly filled d-orbitals available, they do not show most transition-metal properties such as variable valence, coloured ions and catalytic behaviour.
All Group 3–12 elements are d-block by position; Zn, Cd, Hg are excluded from the strict definition of transition metals because of their $\ce{d^{10}}$ configuration.
The block of an element governs how its size, ionization enthalpy and electronegativity change across the table. See Valency and Periodic Trends for the full picture.
The f-Block Elements
The f-block consists of the two rows placed at the bottom of the periodic table: the lanthanoids, $\ce{Ce}$ ($Z = 58$) to $\ce{Lu}$ ($Z = 71$), and the actinoids, $\ce{Th}$ ($Z = 90$) to $\ce{Lr}$ ($Z = 103$). They are characterised by the general outer configuration $\ce{(n-2)f^{1-14}(n-1)d^{0-1}ns^2}$ — the last electron added to each element enters an $f$-orbital. Because this differentiating electron occupies a subshell lying two shells deep, inside even the $(n-1)d$ orbitals of the ordinary transition metals, these elements are called the inner-transition elements.
They are all metals, and within each series the properties of the elements are remarkably similar. The chemistry of the early actinoids is more complicated than that of the corresponding lanthanoids, owing to the larger number of accessible oxidation states. All actinoid elements are radioactive; many have been prepared only in nanogram quantities or less through nuclear reactions, so their chemistry is incompletely studied. The elements beyond uranium are collectively termed the transuranium elements.
General Configurations at a Glance
The single most examinable fact in this subtopic is the general valence-shell configuration of each block, because NEET routinely supplies a configuration and asks for the block, group or family, or vice versa. The table consolidates the four formulae together with the diagnostic properties drawn from NCERT §3.6.
| Block | General valence config. | Groups / members | Characteristic properties | Examples |
|---|---|---|---|---|
| s | $\ce{ns^{1-2}}$ | Groups 1, 2 | Reactive metals; low IE; form $\ce{M+}$/$\ce{M^2+}$; mostly ionic compounds | Na, K, Mg, Ca |
| p | $\ce{ns^2np^{1-6}}$ | Groups 13–18 | Metals, non-metals & metalloids; ends in noble gas $\ce{ns^2np^6}$ | B, C, N, F, Ne |
| d | $\ce{(n-1)d^{1-10}ns^{0-2}}$ | Groups 3–12 | All metals; coloured ions; variable valence; paramagnetism; catalysts | Fe, Cu, Cr, Zn* |
| f | $\ce{(n-2)f^{1-14}(n-1)d^{0-1}ns^2}$ | Lanthanoids (58–71), Actinoids (90–103) | All metals; similar within a series; actinoids radioactive | Ce, U, Th |
*Zn (and Cd, Hg) are d-block by position but, being $\ce{(n-1)d^{10}ns^2}$, are not typical transition metals. Pd is the d-block configuration exception, $\ce{4d^{10}5s^0}$.
Elements $Z = 117$ and $Z = 120$ are not yet (or were not) discovered. Predict their group and configuration.
$Z=117$: $\ce{[Rn]5f^{14}6d^{10}7s^{2}7p^{5}}$ — valence $\ce{ns^2np^5}$, so the halogen family, Group 17 (p-block). $Z=120$: $\ce{[Og]8s^{2}}$ — valence $\ce{ns^2}$, so Group 2 alkaline earth metals (s-block). The block follows directly from the subshell holding the last electron. (NCERT Problem 3.3.)
Metals, Non-metals and Metalloids
Cutting across the block scheme is a second, property-based division of the elements into metals, non-metals and metalloids. Metals make up more than 78% of all known elements and occupy the left side of the table. They are usually solids at room temperature — mercury is the famous liquid exception, while gallium and caesium have very low melting points of 303 K and 302 K respectively. Metals generally have high melting and boiling points, are good conductors of heat and electricity, and are both malleable and ductile.
Non-metals sit at the top right of the table. In any horizontal row the character changes from metallic on the left to non-metallic on the right. Non-metals are usually solids or gases at room temperature with low melting and boiling points (boron and carbon are exceptions), are poor conductors, and most solid non-metals are brittle, neither malleable nor ductile. Metallic character increases down a group and non-metallic character increases left to right across a period.
The change from metallic to non-metallic character is not abrupt: it is marked by the thick zig-zag line of Figure 1. The elements bordering this line and running diagonally across the table show properties of both metals and non-metals and are called metalloids or semi-metals — silicon, germanium, arsenic, antimony and tellurium being the standard examples.
Arrange in increasing order of metallic character: Si, Be, Mg, Na, P.
Metallic character increases down a group and decreases across a period from left to right. Applying both: P < Si < Be < Mg < Na. (NCERT Problem 3.4.)
s, p, d, f-Blocks in one screen
- Block rule: named after the subshell receiving the last (differentiating) electron — s, p, d or f.
- s-block $\ce{ns^{1-2}}$: Groups 1–2, reactive metals, low IE, ionic compounds (Li, Be exceptions).
- p-block $\ce{ns^2np^{1-6}}$: Groups 13–18; contains metals, non-metals, metalloids; ends in noble gas $\ce{ns^2np^6}$.
- d-block $\ce{(n-1)d^{1-10}ns^{0-2}}$: Groups 3–12, transition metals; Zn/Cd/Hg ($\ce{d^{10}}$) are not typical transition metals; Pd is the exception.
- f-block $\ce{(n-2)f^{1-14}(n-1)d^{0-1}ns^2}$: lanthanoids and actinoids, inner-transition metals; actinoids radioactive.
- Property divide: metals (left), non-metals (top right), metalloids along the diagonal zig-zag (Si, Ge, As, Sb, Te).