Question

Using the modern method of electronic configuration of elements. How do we find the period and group of element

Answer 1

Short answer — procedure:
1. Write the element’s ground-state electron configuration.
2. Period = the highest principal quantum number n that appears in the configuration (the outermost shell).
3. Block = the type of orbital that receives the last electron (s, p, d, or f). This tells you whether it’s s‑block, p‑block, d‑block (transition metal) or f‑block (lanth./actin.).
4. Group (IUPAC 1–18):
- s‑block: group = number of valence electrons in the outermost ns (ns1 → group 1; ns2 → group 2).
- p‑block: count valence electrons = ns + np. Group = 10 + (ns + np). (Example: C, 2s2 2p2 → 2+2 = 4 → group 14.)
- d‑block (transition metals): group = (ns electrons) + ((n−1)d electrons). That sum gives groups 3–12. Use the actual ground‑state configuration (including common exceptions like Cr, Cu).
- f‑block: these are inner transition metals (lanth./act.). They lie in periods 6 and 7 and are treated separately; they are usually placed below the main table rather than having a distinct 1–18 group number.

Examples
- Oxygen 1s2 2s2 2p4: period = 2; last electron in p → p‑block; valence = 2+4 = 6 → group = 10+6 = 16.
- Iron [Ar] 4s2 3d6: period = 4; last electron in d → d‑block; group = 4s (2) + 3d (6) = 8 → group 8.
- Neon 1s2 2s2 2p6: period = 2; p‑block; valence = 8 → group = 10+8 = 18 (noble gas).

Notes and caveats
- Use the actual ground‑state electronic configuration (some elements like Cr, Cu, Mo, Ag, Au have anomalous s/d occupancies).
- Hydrogen is 1s1 so by this rule it’s period 1, group 1, but chemically it can behave like a nonmetal; place in group 1 is standard on the periodic table.
- For f‑block elements, “group” is not usually given in the simple ns+nd rule; they are classified as lanthanides/actinides.