Diagram Note — Block Classification & Crystal Structure — Notes

Source: Wikimedia Commons — Electron shell diagram (Lithium, s-block)

Block	Groups	Outer Subshell Filled	No. of Elements	Examples
s-block	1, 2 (+He)	$ns^{1-2}$	14 (including He)	H, Li, Na, K, Be, Mg, Ca
p-block	13–18	$$ns^2,$np^{1-6}$$$	36	B, C, N, O, F, Ne, Al, Si, P, Cl
d-block	3–12	$$(n-1) $d^{1-10}$ ,$ns^{0-2}$$$	40	Fe, Cu, Cr, Mn, Zn, Ni
f-block	Lanthanoids (Ce–Lu), Actinoids (Th–Lr)	$$(n-2)$f^{1-14}$$$	28	Ce, Nd, U, Pu, Th

Observation	Explanation
He is placed in Group 18 (p-block side), not Group 2	Its chemical behavior matches noble gases (inert, complete outer shell = 1 $s^{2}$ )
Zn, Cd, Hg: sometimes excluded from d-block discussion	Their (n-1)d subshell is completely filled ( $d^{10}$ ); no partially filled d-orbitals in ground state or common oxidation states
Cu: [Ar] 3 $d^{10}$ 4 $s^{1}$ (not expected 3 $d^{9}$ 4 $s^{2}$ )	Extra stability of completely filled 3 $d^{10}$
Cr: [Ar] 3 $d^{5}$ 4 $s^{1}$ (not expected 3 $d^{4}$ 4 $s^{2}$ )	Extra stability of half-filled 3 $d^{5}$