Chemistry · Periodic Classification Of Properties · NEET
Yes. The period number equals the number of occupied electron shells, which is the same as the highest principal quantum number n. Sodium (Na) is in Period 3 and has electrons in 3 shells: n=1, n=2, n=3. So Period 3 = 3 shells = highest n is 3. This is the fastest way to place an element in the correct row.
It equals the principal quantum number n, not l, m, or s. The principal quantum number n tells you the main energy level (shell). NCERT states it clearly: 'the period number corresponds to the highest principal quantum number (n) of the elements in the period.' Only n matters for the row; the other quantum numbers do not decide the period.
Look at the largest n value that has electrons. Example: chlorine is 1s2 2s2 2p6 3s2 3p5. The biggest n with electrons is 3, so chlorine is in Period 3. For potassium 1s2 2s2 2p6 3s2 3p6 4s1, the highest n is 4, so it is Period 4. Ignore how many electrons are in that shell; just take the highest n.
Yes, for main-group elements the period number equals the n of the valence (outermost) shell. In magnesium 3s2, the valence shell is n=3, so it is Period 3. Note: for d-block elements the outer s-electron n still gives the period. Scandium [Ar]3d1 4s2 has highest n=4 (the 4s), so it sits in Period 4, even though the 3d subshell has n=3.
It tells you how many electron shells the atom has and which shell is being filled. A higher period number means more shells, so the atom is bigger and its outer electrons are farther from the nucleus. This is why atomic size increases as you go down a group — the period number (n) rises, adding a new shell each time.
Period number counts shells (highest n), not electrons or protons. Neon (Z=10) and sodium (Z=11) have different atomic numbers, but neon is Period 2 (highest n=2) while sodium is Period 3 (highest n=3). The jump to a new period happens when a new shell (new n) starts filling, not at any fixed electron count.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For main-group (s and p block) elements, yes — the period number equals the highest n. For d and f block elements, the period still equals the highest n, which comes from the outer ns electrons, not the inner (n-1)d or (n-2)f electrons.
There are 7 periods. So the principal quantum number n runs from 1 to 7. The 7th period is complete but the shell would theoretically hold up to 32 elements like Period 6.
A new period starts when electrons begin filling a new shell, that is, a new (higher) principal quantum number n. Each new n adds one more electron shell to the atom.
Each period starts with an alkali metal (except Period 1) whose valence electron is in a fresh ns1 shell. For example, Period 3 starts with sodium (3s1), where n=3 begins filling.
