Chemistry · Periodic Classification Of Properties · NEET
A period is one horizontal row. It means one main energy shell is being filled. Period 1 fills shell n=1, period 2 fills shell n=2, and so on. The period number always equals the highest principal quantum number (n) of the outermost electron. So if the last electron of an element goes into n=3, that element is in period 3. This is why period number is so useful in NEET: read the value of n and you instantly know the period.
The count of elements in a period equals twice the number of atomic orbitals being filled in that energy level. Period 1 fills only 1s (1 orbital) so 2 x 1 = 2 elements. Period 2 fills 2s + 2p (1 + 3 = 4 orbitals) so 2 x 4 = 8 elements. Period 4 fills 4s + 3d + 4p (1 + 5 + 3 = 9 orbitals) so 2 x 9 = 18 elements. The d and f subshells add more orbitals, so later periods hold more elements. Remember: 2 electrons fit in every orbital, so element count = 2 x (orbitals filled).
Period 2 fills shell n=2. It starts with lithium: 1s2 2s1 (third electron enters 2s). Beryllium is 1s2 2s2 (2s now full). Then boron starts the 2p subshell: 1s2 2s2 2p1, and carbon, nitrogen, oxygen, fluorine keep adding to 2p. Neon ends the period at 1s2 2s2 2p6 (2p full, shell complete). Once n=2 is full, sodium begins period 3 with a new shell (3s1).
Because 4s has lower energy than 3d, electrons fill 4s first (that is why potassium and calcium start period 4 with 4s). Only after 4s is full does the 3d subshell begin filling, from scandium to zinc. So the '3' in 3d does NOT mean it fills in period 3. The 3d electrons are added during period 4. This is exactly why period 4 has 18 elements: 4s (1) + 3d (5) + 4p (3) = 9 orbitals x 2 = 18. Learn more in the linked concept on why 4s fills before 3d.
Look at which subshell the LAST electron entered. If it ended in ns, it is s-block. If it ended in np, it is p-block. If it ended in (n-1)d, it is d-block (transition). If it ended in (n-2)f, it is f-block. Example: [Ar]3d3 4s2 is vanadium, last-filling electron is in 3d, so it is d-block (a transition metal, not a main-group element). NEET loves testing this.
Because the shell being filled is complete. Neon (1s2 2s2 2p6) has a full n=2 shell, and argon has a stable outer configuration. The next electron has no room in that level, so it must start a brand new shell (higher n). A new shell = a new period. This is why noble gases sit at the far right end of every period.
Yes. An element in period 3 has electrons occupying 3 shells (n=1, 2, 3). The period number equals both the highest n value and the total number of occupied shells. So sodium (period 3) has 3 shells; iron (period 4) has 4 shells. This is a quick trick to count shells for NEET radius and shielding questions.
Which among the following electronic configurations belong to main group elements? A. [Ne]3s1 B. [Ar]3d3 4s2 C. [Kr]4d10 5s2 5p5 D. [Ar]3d10 4s1 E. [Rn]5f0 6d2 7s2
The element Z = 114 has been discovered recently. It will belong to which family/group and electronic configuration?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The period number is always equal to the highest principal quantum number (n) of the electrons in that element. Period 4 means the outermost electrons are in the n=4 shell.
Period 1 fills only the 1s orbital, and one orbital holds a maximum of 2 electrons. So only hydrogen (1s1) and helium (1s2) fit, giving just 2 elements.
Both fill ns and np subshells only. That is 1 + 3 = 4 orbitals, and 2 electrons per orbital gives 8 elements. Period 3 does not fill 3d (that waits for period 4), so it also stops at 8.
Period 7 fills 7s, 5f, 6d and 7p subshells: 1 + 7 + 5 + 3 = 16 orbitals, so a full period would have 32 elements, the same as period 6.
Almost. Electrons fill by increasing energy (Aufbau order, e.g. 4s before 3d), which is what builds the table row by row. This is why the 3d block appears in period 4 even though it is labelled 3d.
