Chemistry · Periodic Classification Of Properties · NEET
Because one orbital can hold a maximum of 2 electrons (they must have opposite spins). If a period fills, say, 4 orbitals, those 4 orbitals hold 4 x 2 = 8 electrons. Each new electron means a new element, so 8 electrons = 8 elements. So elements = 2 x orbitals. Always double the orbital count.
Count the subshells that get filled as you cross that period. s = 1 orbital, p = 3 orbitals, d = 5 orbitals, f = 7 orbitals. Period 1 fills only 1s (1 orbital). Period 2 fills 2s + 2p = 1 + 3 = 4 orbitals. Period 4 fills 4s + 3d + 4p = 1 + 5 + 3 = 9 orbitals. Then double the total.
Period 1 fills only the 1s subshell. The s subshell has just 1 orbital. So orbitals = 1, and elements = 2 x 1 = 2 (Hydrogen and Helium). There is no p subshell for n = 1, so no more orbitals are available.
Period 2 fills 2s and 2p. That is 1 + 3 = 4 orbitals. Elements = 2 x 4 = 8 (Li to Ne). The 2d subshell does not exist because for n = 2 the highest allowed subshell is p, so counting stops at 4 orbitals.
In period 4 the order of filling is 4s, then 3d, then 4p. Orbitals = 1 (4s) + 5 (3d) + 3 (4p) = 9. Elements = 2 x 9 = 18. Period 5 is the same shape (5s, 4d, 5p), so it also has 18 elements.
They are related but not identical. 2n^2 gives the maximum electrons a shell can hold. The period length depends on which subshells actually fill in that row (because of the (n-1)d and (n-2)f filling). The safe, always-correct rule for period length is: count the orbitals that fill in that period, then multiply by 2.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Number of elements in a period = 2 x (number of orbitals filled in that period). Since each orbital holds 2 electrons, doubling the orbital count gives the number of electrons added, which equals the number of new elements.
Period 1 has 2, period 2 has 8, period 3 has 8, period 4 has 18, period 5 has 18, period 6 has 32, and period 7 has 32 (when full). This gives the pattern 2, 8, 8, 18, 18, 32, 32.
By the Pauli exclusion principle, two electrons in the same orbital must have opposite spins, and no two electrons can have all four quantum numbers the same. So an orbital is full with exactly 2 electrons.
s has 1 orbital, p has 3, d has 5, and f has 7. So they hold a maximum of 2, 6, 10 and 14 electrons respectively.
NEET asks direct questions on period length, block position, and electronic configuration of elements up to Z = 118. Knowing that elements = 2 x orbitals lets you predict where any element sits without memorizing the whole table.
