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Periodic Table & Periodicity revision notes

Every idea in the Periodic Table & Periodicity chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 18 and download the PDF.

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Periodic law and the modern table

Periodic law: Mendeleev to Moseley
4× asked, last 2025
ν=a (Z−b)\sqrt{\nu}=a\,(Z-b)
Moseley: ν\nu = X-ray frequency, a,ba,b constants. ν\sqrt\nu vs ZZ is a straight line, so ZZ, not mass, orders the table.
ZZν\sqrt{\nu} (10⁸ Hz1/2^{1/2})1520253051015Ar (39.95)K (39.10)Co (58.93)Ni (58.69)
Kα X-ray lines (aa, bb from standard data): a straight line in ZZ. By mass, Ar would follow K and Co would follow Ni
  • Modern law: properties repeat with atomic number. Mendeleev used atomic mass and predicted eka-aluminium (Ga) and eka-silicon (Ge) before atomic structure was known. Newlands' octaves and Meyer's curves also used atomic weight.
Periods, groups and blocks
4× asked, last 2025
elements per period=2×orbitals filled: 2, 8, 8, 18, 18, 32, 32\text{elements per period}=2\times\text{orbitals filled}:\ 2,\ 8,\ 8,\ 18,\ 18,\ 32,\ 32
Period = nn of the valence shell. A group shares a similar outer configuration (main groups: same ns/npns/np count, He aside).
HHeLiBeBCNOFNeNaMgAlSiPSClArKCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKrRbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXeCsBaLaCePrNdPmSmEuGdTbDyHoErTmYbLuHfTaWReOsIrPtAuHgTlPbBiPoAtRnFrRaAcThPaUNpPuAmCmBkCfEsFmMdNoLrRfDbSgBhHsMtDsRgCnNhFlMcLvTsOg1182131415161734567891011121234567d-block (transition, groups 3–12)p-blocks-blockCe–LuTh–Lrf-block (inner transition)
Periods of 2, 8, 8, 18, 18, 32, 32 (counted from the layout). He (s-block) sits in group 18; Ce–Lu and Th–Lr are set below
  • Block = subshell getting the last electron: s (groups 1–2), p (13–18), d (3–12), f (lanthanoids ZZ = 58–71, actinoids 90–103), set below so every period keeps one layout. He (1s21s^2) is s-block but sits in group 18 (full shell).
  • s- and p-blocks together are the representative (main-group) elements. s-block metals: reactive, low IE, never found free; compounds mostly ionic, except Li and Be. p-block: metals, metalloids and non-metals.
  • d-block (transition): mostly coloured ions, variable valence, paramagnetism, catalysis; Zn, Cd, Hg ((n−1)d10ns2(n-1)d^{10}ns^2) are atypical. f-block (inner transition): actinoids are radioactive; those after U (Z>92Z>92) are transuranium.
Locating an element from Z
5× asked, last 2025
group={nss-blockns+ndd-block10+ns+npp-block\text{group}=\begin{cases} n_s & \text{s-block}\\ n_s+n_d & \text{d-block}\\ 10+n_s+n_p & \text{p-block}\end{cases}
ns,nd,npn_s, n_d, n_p = nsns, (n−1)d(n-1)d, npnp electrons. Period = highest nn; exceptions: Pd ([Kr]4d10[\ce{Kr}]4d^{10}) period 5, He group 18.
He 21Ne 102Ar 183Kr 36period 4Xe 54period 5Rn 86period 68 past ArZ=26Z=26: 3d64s23d^64s^2 → period 4, group 2+6=82+6=8
Noble gases close the periods; the electrons past one give the group
  • Quick method. Count from the noble gas below ZZ (2, 10, 18, 36, 54, 86). Z=26Z=26 is 8 past Ar: 3d64s23d^64s^2, so period 4, group 8, d-block. The element just above in a group has the same outer set with nn one less.
IUPAC names for Z > 100
3× asked, last 2025
0 nil, 1 un, 2 bi, 3 tri, 4 quad5 pent, 6 hex, 7 sept, 8 oct, 9 enn\begin{gathered}0\ \text{nil},\ 1\ \text{un},\ 2\ \text{bi},\ 3\ \text{tri},\ 4\ \text{quad}\\ 5\ \text{pent},\ 6\ \text{hex},\ 7\ \text{sept},\ 8\ \text{oct},\ 9\ \text{enn}\end{gathered}
Join the digit roots, add -ium; symbol = initials. 120 = unbinilium (Ubn). Drop bi/tri's final i before -ium.
  • For ZZ = 104 to 118 the group is Z−100Z-100. Period 7's p-block is 113 Nh, 114 Fl, 115 Mc, 116 Lv, 117 Ts, 118 Og, filling groups 13 to 18 in turn.

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Practise Periodic Table & Periodicity

14 more concepts in 5 topics

Atomic and ionic radii; Ionisation enthalpy; Electron gain enthalpy; Electronegativity, metallic character and oxides; and Valence, anomalies and diagonal relationship. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Atomic and ionic radii

  • Atomic radius trends14×
  • Ions versus their atoms7×
  • Isoelectronic species14×

Ionisation enthalpy

  • Ionisation enthalpy5×
  • IE across a period and its two dips22×
  • IE down a group: groups 13 and 149×
  • Successive IEs and the big jump9×

Electron gain enthalpy

  • Electron gain enthalpy11×
  • Exceptions: Cl beats F, S beats O14×

Electronegativity, metallic character and oxides

  • Electronegativity (Pauling scale)11×
  • Metallic character and reactivity18×
  • Nature of oxides12×

Valence, anomalies and diagonal relationship

  • Valence and the anomalous first member4×
  • Diagonal relationship5×

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