JEE Mains Chemistry · Classification of Elements and Periodicity

Periodic Law, Blocks and Position in the Table

The properties of elements repeat with atomic number, so an element's period, group and block follow from its electronic configuration, and an element with Z above 100 gets a temporary name built from the digits of Z.

All Classification of Elements and Periodicity notes JEE Mains Chemistry strategy

Why this matters

Fourteen PYQs, twelve of them multiple choice, and one from 2026. Four test the history of the periodic law, from Newlands and Mendeleev to Moseley; four name an element from its atomic number or place it in a block; six locate an element from its configuration, from an ion's electron count or from its lowest oxidation state.

Concept 1 of 3: From atomic weight to atomic number

Every early table sorted elements by atomic weight, and each one broke somewhere. Moseley showed that the real sorting key is the atomic number: the square root of an element's X-ray frequency rises in a straight line with Z. Order by Z and the repeats line up without exceptions.

Definition

Modern periodic law: the properties of elements are a periodic function of their atomic numbers.
A group holds elements with the same outer electronic configuration; a period holds elements with the same highest principal quantum number $n$ .
The number of elements in a period is twice the number of orbitals being filled: period 1 has 2, periods 2 and 3 have 8, periods 4 and 5 have 18, periods 6 and 7 have 32.
The $4f$ and $5f$ series sit in two rows below the table to keep elements with similar properties in one column.
The s-block metals are too reactive to be found free in nature; they occur only as compounds.

Who and when	Sorted by	What they found
Döbereiner, 1829	Atomic weight	Triads such as Li, Na, K: the middle atomic weight is about the mean of the other two
Newlands, 1865	Atomic weight	Law of octaves: every eighth element repeats the first; it worked only up to calcium
Lothar Meyer, 1869	Atomic weight	Plotted atomic volume against atomic weight and saw a repeating curve
Mendeleev, 1869	Atomic weight	Left gaps and predicted eka-aluminium (Ga) and eka-silicon (Ge); reversed some pairs to keep families together
Moseley, 1913	Atomic number	A plot of $\sqrt{\nu}$ against $Z$ is a straight line, so $Z$ is the true basis Only Moseley's work uses atomic number. A statement giving Newlands or Meyer atomic numbers is false.
Modern table	Atomic number	18 groups and 7 periods; blocks s, p, d and f named by the subshell being filled

Everyone before 1913 used atomic weight; the modern law and table use atomic number.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 24 Jan 2025 · Q35Moderate

Example 1 · Classification of Elements and Periodicity · Periodic Law, Blocks and Position in the Table

Which of the following Statements are NOT true about the periodic table? (A) The properties of elements are function of atomic weights. (B) The properties of elements are function of atomic numbers. (C) Elements having similar outer electronic configuration are arranged in same period. (D) An element's location reflects the quantum numbers of the last filled orbital. (E) The number of elements in a period is same as the number of atomic orbitals available in energy level that is being filled. Choose the correct answer from the options given below:

Twice the orbitals, not equal to them

Period 2 fills four orbitals (2s and three 2p) and holds eight elements. A statement that the number of elements equals the number of orbitals is false; each orbital takes two electrons.

Group, not period

Elements with similar outer configurations are stacked in one group. Along a period the outer configuration changes at every step, so a statement placing them in the same period is false.

Drill 3 more on from atomic weight to atomic number

Concept 2 of 3: Names for Z above 100 and the block from Z

A new element gets a temporary name that simply spells out its atomic number, one root per digit, followed by -ium. Its block is the subshell its last electron enters, so writing the configuration tells you the block and the group together.

Definition

Digit roots: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn; add -ium at the end.
Drop a doubled letter: enn + nil gives ennil, and bi or tri before ium gives bium, trium.
Block = subshell of the last electron: s, p, d or f.
Group: s-block = number of $ns$ electrons; d-block = $(n-1)d + ns$ electrons; p-block = $10 + ns + np$ electrons.
Official names, 113 to 118: Nh (group 13), Fl (14), Mc (15, pnictogen), Lv (16, chalcogen), Ts (17, halogen), Og (18, noble gas).

Temporary IUPAC name

\text{name} = \text{root}(d_1) + \text{root}(d_2) + \text{root}(d_3) + \text{ium}

Worked example

Give the temporary IUPAC name, the block and the group of the element with

Z = 116

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 25 July 2022 · Q36Moderate

Example 2 · Classification of Elements and Periodicity · Periodic Law, Blocks and Position in the Table

The IUPAC nomenclature of an element with electronic configuration

\lbrack Rn\rbrack 5f^{14}6{\text{ }d}^{1}7{\text{ }s}^{2}

is:

Count the electrons before naming

When the question gives a configuration, add the core and every electron first.

[\mathrm{Rn}]

is 86, so a configuration ending

5f^{14}6d^{2}7s^{2}

Z = 104

, not a number read off the last subshell.

Drill 3 more on names for z above 100 and the block from z

Concept 3 of 3: Placing an element from its configuration or an ion

The period is the highest shell in use and the group is set by the outer electrons. If a question hides the element behind an ion or an oxidation state, first recover Z, then write the configuration and read off the position.

Definition

Protons $Z$ = electrons + charge of a cation, or electrons − charge of an anion.
Mass number $A = Z + \text{neutrons}$ .
Period = highest $n$ in the configuration.
A p-block non-metal's lowest oxidation state is $-(8 - \text{valence electrons})$ : an oxidation state of −3 means five valence electrons.
The element just above sits one period higher with the same outer $ns\,np$ pattern; the diagonal neighbour is one period down and one group right (B to Si).
Period 2 elements cannot always show their group valency: O never reaches 6 and F never reaches 7, and N's covalency stops at 4.
Pd (46) is in period 5; Os, Ir and Pt (76 to 78) are in period 6.

Recover Z from an ion

Z = e^{-} + q \qquad A = Z + n

qcharge on the ion, with its sign (−3 for an ion X³⁻)
nnumber of neutrons

Worked example

An ion

\mathrm{X^{3-}}

has 18 electrons and 16 neutrons. Find the mass number of X, its period and group, and its physical state at room temperature.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 4 Apr 2026 Shift 1 · Q33Moderate

Example 3 · Classification of Elements and Periodicity · Periodic Law, Blocks and Position in the Table

A monoatomic anion

\left( A^{-} \right)

has 45 neutrons and 36 electrons. Atomic mass, group in the periodic table and physical state at room temperature of the element (A) respectively are :

Adding the charge the wrong way

An anion has MORE electrons than protons. For

\mathrm{X^{2-}}

with 10 electrons,

Z = 8

, not 12. Write

Z = e^{-} + q

with the sign of the charge and the direction takes care of itself.

Drill 5 more on placing an element from its configuration or an ion

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Names for Z above 100 and the block from Z
Temporary IUPAC name
$\text{name} = \text{root}(d_1) + \text{root}(d_2) + \text{root}(d_3) + \text{ium}$
Placing an element from its configuration or an ion
Recover Z from an ion
$Z = e^{-} + q \qquad A = Z + n$

Reference tables (1)

From atomic weight to atomic number6 rows

Who and when	Sorted by	What they found
Döbereiner, 1829	Atomic weight	Triads such as Li, Na, K: the middle atomic weight is about the mean of the other two
Newlands, 1865	Atomic weight	Law of octaves: every eighth element repeats the first; it worked only up to calcium
Lothar Meyer, 1869	Atomic weight	Plotted atomic volume against atomic weight and saw a repeating curve
Mendeleev, 1869	Atomic weight	Left gaps and predicted eka-aluminium (Ga) and eka-silicon (Ge); reversed some pairs to keep families together
Moseley, 1913	Atomic number	A plot of $\sqrt{\nu}$ against $Z$ is a straight line, so $Z$ is the true basis Only Moseley's work uses atomic number. A statement giving Newlands or Meyer atomic numbers is false.
Modern table	Atomic number	18 groups and 7 periods; blocks s, p, d and f named by the subshell being filled

Everyone before 1913 used atomic weight; the modern law and table use atomic number.

Watch out for (4)

Twice the orbitals, not equal to them
→ From atomic weight to atomic number
Group, not period
→ From atomic weight to atomic number
Count the electrons before naming
→ Names for Z above 100 and the block from Z
Adding the charge the wrong way
→ Placing an element from its configuration or an ion

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