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History of Periodic Table & Modern Periodic Law

Master Modern Periodic Law and Periodic Table for JEE Main, JEE Advanced, NEET & Board Exams. Covers Döbereiner triads, Newlands octaves, Mendeleev predictions, Moseley law, modern periodic law, periods and groups, s p d f blocks, metals nonmetals metalloids and IUPAC nomenclature with solved examples.

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The Periodic Table is the single most important organisational tool in chemistry — it arranges all known elements in a way that reveals patterns in their properties and predicts the behaviour of undiscovered ones. The journey from Döbereiner's triads to Mendeleev's periodic law to the modern quantum-mechanical periodic table is one of the great intellectual achievements in science. For JEE and NEET, this chapter provides the conceptual framework for understanding chemical bonding, reactivity, and the properties of s, p, d, and f block elements. Expect 2–3 direct questions every year on periodic trends, blocks, and periodic law.

1. Historical Development of the Periodic Table

Scientist	Year	Contribution	Limitation
Döbereiner	1829	Law of Triads — middle element's atomic mass is average of other two; e.g., Li-Na-K, Ca-Sr-Ba, Cl-Br-I	Only worked for a few elements; failed for most
Newlands	1866	Law of Octaves — every 8th element has similar properties to the 1st (like musical octaves)	Worked only up to calcium; failed for heavier elements; no room for new elements
Mendeleev	1869	Periodic Law — properties of elements are periodic functions of their atomic masses; predicted eka-boron, eka-aluminium (Ga), eka-silicon (Ge)	Position of H anomalous; isotopes not accommodated; wrong position of some elements (e.g., Co and Ni)
Moseley	1913	Showed atomic number (not mass) is the fundamental property; $\sqrt{ν} \propto Z$ (Moseley's law)	—

Mendeleev's Achievements

Predicted properties of undiscovered elements — eka-aluminium (later found as Gallium, Ga) and eka-silicon (later found as Germanium, Ge) — with remarkable accuracy.
Left blank spaces in the table for undiscovered elements.
Arranged elements in order of increasing atomic mass in groups and periods.
Accommodated noble gases (discovered later) without disturbing the table.

2. Modern Periodic Law

Based on Moseley's discovery, the Modern Periodic Law states:

"The physical and chemical properties of elements are periodic functions of their atomic numbers."

This replaced Mendeleev's atomic mass with atomic number as the basis of classification. Atomic number ( $Z$ ) = number of protons = fundamental identity of an element.

Why Atomic Number and Not Atomic Mass?

Isotopes (same $Z$ , different mass) have the same chemical properties — confirming $Z$ is fundamental.
Atomic mass ordering causes anomalies (Co–Ni, Ar–K, Te–I) that are corrected by using atomic number.
Electronic configuration (which determines properties) depends directly on $Z$ .

3. Structure of the Modern Periodic Table

The modern long form periodic table (also called Bohr's table) has 118 elements arranged in 7 periods (horizontal rows) and 18 groups (vertical columns).

Periods — The Horizontal Rows

Period	No. of Elements	Shell being filled	Elements
1 (Very short)	2	1s	H, He
2 (Short)	8	2s, 2p	Li to Ne
3 (Short)	8	3s, 3p	Na to Ar
4 (Long)	18	4s, 3d, 4p	K to Kr
5 (Long)	18	5s, 4d, 5p	Rb to Xe
6 (Very long)	32	6s, 4f, 5d, 6p	Cs to Rn
7 (Incomplete)	32	7s, 5f, 6d, 7p	Fr to Og (element 118)

Period number = number of the outermost shell (principal quantum number $n$ of the valence shell).

Groups — The Vertical Columns

Groups 1–18 (IUPAC). Elements in the same group have the same number of valence electrons and similar chemical properties.

Groups 1–2: s-block
Groups 3–12: d-block (transition metals)
Groups 13–18: p-block
Lanthanoids (58–71) and Actinoids (90–103): f-block

4. Classification of Elements — The Four Blocks

Block	Orbital being filled	Groups	General valence config.	Examples
s-block	$n s$	1, 2	$n s^{1}$ or $n s^{2}$	Li, Na, K (Gr 1); Be, Mg, Ca (Gr 2)
p-block	$n p$	13–18	$n s^{2} n p^{1 - 6}$	B, C, N, O, F, Ne and their analogues
d-block	$(n - 1) d$	3–12	$(n - 1) d^{1 - 10} n s^{0 - 2}$	Sc to Zn, Y to Cd, La, Hf–Hg
f-block	$(n - 2) f$	— (inner transition)	$(n - 2) f^{1 - 14} (n - 1) d^{0 - 1} n s^{2}$	Ce–Lu (lanthanoids), Th–Lr (actinoids)

Determining Block, Period, and Group from Electronic Configuration

Block: Identified by the type of orbital in which the last electron enters.
Period: = Principal quantum number ( $n$ ) of the outermost shell.
Group (s-block): = Number of valence electrons (1 or 2).
Group (p-block): = 10 + number of valence electrons ( $n s^{2} n p^{x}$ → Group $10 + 2 + x = 12 + x$ ).
Group (d-block): = Number of $(n - 1) d$ electrons + number of $n s$ electrons.

Quick Example

Element with $Z = 35$ (Bromine): $[A r] 3 d^{10} 4 s^{2} 4 p^{5}$
Last electron enters $4 p$ → p-block.
Outermost shell $n = 4$ → Period 4.
Valence electrons = $4 s^{2} 4 p^{5}$ = 7 → Group $= 10 + 7 = 17$ → Group 17 (Halogens). ✓

5. Nomenclature of Elements with $Z > 100$

IUPAC recommends a systematic nomenclature for elements with atomic number $> 100$ using numerical roots:

Digit	Root	Digit	Root
0	nil	5	pent
1	un	6	hex
2	bi	7	sept
3	tri	8	oct
4	quad	9	enn

Rules: Join roots for each digit + suffix -ium. Symbol = first letter of each root (capitalised).
Example: $Z = 118$ → Un(1) + un(1) + oct(8) + ium = Ununoctium (Uuo) → officially named Oganesson (Og).
$Z = 113$ → Ununtrium (Uut) → officially named Nihonium (Nh).

6. Metals, Nonmetals and Metalloids

Property	Metals	Nonmetals	Metalloids
Lustre	Shiny	Dull (except iodine, graphite)	Semiconductor-like
Conductivity	Good conductors	Poor conductors (except graphite)	Intermediate
Oxide nature	Basic	Acidic	Amphoteric
Examples	Fe, Cu, Na, Al (~80% of elements)	C, N, O, S, Cl, noble gases	B, Si, Ge, As, Sb, Te
Position in table	Left and centre	Right side	Staircase boundary (zigzag line)

Representative elements: s-block and p-block elements (Groups 1, 2, 13–18).
Transition elements: d-block (Groups 3–12).
Inner transition elements: f-block (lanthanoids and actinoids).
Noble gases (Group 18): have completely filled orbitals — extremely stable and largely unreactive.

7. Electron Gain Enthalpy and Electronegativity — Introduction

A brief overview here as context — detailed treatment is in the next topic (Periodic Trends).

Key Definitions

Property	Definition	Unit
Valence electrons	Electrons in the outermost shell; determine chemical behaviour	—
Ionisation Enthalpy (IE)	Energy required to remove an electron from a gaseous atom/ion	kJ/mol
Electron Gain Enthalpy (EGE)	Enthalpy change when an electron is added to a neutral gaseous atom	kJ/mol
Electronegativity	Tendency of a bonded atom to attract shared electrons towards itself	Pauling units
Oxidation state	Hypothetical charge on an atom if all bonds were ionic	—

Finding Period, Group and Block — JEE/NEET Speed Method

Given atomic number $Z$ , find period, group and block without writing full electronic configuration:

Block	Period from $Z$	Group from $Z$
s-block (Groups 1–2)	$n$ of outermost $s$ orbital	= No. of $s$ electrons in outermost shell (1 or 2)
p-block (Groups 13–18)	$n$ of outermost $p$ orbital	$= 10 +$ (total valence electrons in $s + p$ )
d-block (Groups 3–12)	$n$ of outermost $s$ orbital	$=$ $(n - 1) d$ electrons $+$ $n s$ electrons
f-block	Period 6 (lanthanoids) or 7 (actinoids)	Groups 3 (formally, placed separately)

Cumulative Elements at End of Each Period

Memorise these to quickly find which period any element belongs to:

Period 1 ends at $Z = 2$ (He) → Period 2 ends at $Z = 10$ (Ne) → Period 3 ends at $Z = 18$ (Ar) → Period 4 ends at $Z = 36$ (Kr) → Period 5 ends at $Z = 54$ (Xe) → Period 6 ends at $Z = 86$ (Rn) → Period 7 ends at $Z = 118$ (Og)

Cumulative sum: 2, 10, 18, 36, 54, 86, 118. Subtract the previous period-end $Z$ from the given $Z$ to find position within the period.

JEE/NEET Power Tips — Avoid These Common Mistakes

Mendeleev arranged by atomic mass; modern periodic law uses atomic number. The distinction is frequently tested — always state the correct basis in theory questions.
He is in Group 18 (noble gases) — NOT Group 2 (alkaline earth metals) — despite having the configuration $1 s^{2}$ . It is placed in Group 18 because of its chemical inertness and completely filled valence shell.
Hydrogen's anomalous position: H can be placed in Group 1 (like alkali metals — loses 1 electron) or Group 17 (like halogens — gains 1 electron) or separately. NCERT places it with Group 1 but acknowledges its unique nature.
d-block elements start from Group 3 — not Group 1 or 2. Zn, Cd, Hg (Group 12) are d-block but often not considered "transition metals" since their d-subshell is completely filled in both atom and common ions.
Period number = $n$ (principal quantum number) of outermost shell — not the total number of shells. For Na ( $[N e] 3 s^{1}$ ), period = 3 because outermost electrons are in $n = 3$ .
Eka-aluminium = Gallium (Ga), Eka-silicon = Germanium (Ge), Eka-boron = Scandium (Sc) — these three predictions by Mendeleev are the most tested historical facts in this chapter.
Lanthanoids (Ce to Lu, $Z = 58$ –71) and Actinoids (Th to Lr, $Z = 90$ –103) — NOT lanthanum (57) and actinium (89), which are d-block. This is a very common confusion in NEET.

Practice Questions (JEE / NEET Level)

Q1: An element has an electronic configuration of $[Kr] 4 d^{10} 5 s^{2} 5 p^{3}$ . Identify its period, group, and block.

A) Period 4, Group 15, p-block
B) Period 5, Group 15, p-block
C) Period 5, Group 13, p-block
D) Period 5, Group 17, p-block

Answer: B) Period 5, Group 15, p-block.

Explanation:

The last electron enters the $5 p$ subshell, which places the element in the p-block.

The outermost principal quantum shell is $n = 5$ , indicating it belongs to Period 5.

The number of valence electrons is $2 (from 5 s) + 3 (from 5 p) = 5$ electrons.
For p-block elements, Group number $= 10 + valence electrons = 10 + 5 = 15$ . Therefore, it is in Group 15.

Note: This element is Antimony (Sb), atomic number 51.

Q2: Which of the following was NOT correctly predicted by Mendeleev?

A) Properties of Gallium (Ga)
B) Properties of Germanium (Ge)
C) Existence of noble gases
D) Properties of Scandium (Sc)

Answer: C) Existence of noble gases.

Explanation:

Mendeleev predicted the existence and properties of eka-boron (Sc), eka-aluminium (Ga), and eka-silicon (Ge) with remarkable accuracy.

However, he did NOT predict the existence of noble gases (Group 18). Because they are inert, they had not yet been discovered when he formulated his table. When they were finally discovered later (Ar in 1894, He in 1895), they were accommodated in a completely new group without disturbing the existing arrangement—which proved to be a strength of his table, but it was not a prediction he made.

Q3: An element $X$ has an atomic number $Z = 24$ . Which of the following is correct about its position in the periodic table?

A) Period 3, Group 6, d-block
B) Period 4, Group 6, d-block
C) Period 4, Group 24, d-block
D) Period 4, Group 8, d-block

Answer: B) Period 4, Group 6, d-block.

Explanation:

The element with $Z = 24$ is Chromium (Cr). Its electronic configuration is $[Ar] 3 d^{5} 4 s^{1}$ (noting the anomalous configuration due to the extra stability of a half-filled $3 d$ subshell).

The last electron enters the $3 d$ subshell, so it is in the d-block.

The highest principal quantum number is $n = 4$ (from the $4 s$ orbital), meaning it is in Period 4.

For d-block elements, Group number $= electrons in (n - 1) d + electrons in n s$
Group $= 5 + 1 = 6 ⟹$ Group 6.

Q4: The IUPAC name and symbol for the element with $Z = 120$ is:

A) Unbinilium (Ubn)
B) Unbiunium (Ubu)
C) Unnilbium (Unb)
D) Unbipentium (Ubp)

Answer: A) Unbinilium (Ubn).

Explanation:

To write the IUPAC name for $Z = 120$ , we break down the digits: 1, 2, and 0.
The roots are: 1 → un; 2 → bi; 0 → nil.

Name: Un + bi + nil + ium = Unbinilium.

Symbol: Taking the first letters of the roots: U (un) + b (bi) + n (nil) = Ubn.

Q5: Which of the following pairs of elements belong to the SAME period?

A) Li and Na
B) C and Si
C) Na and Mg
D) Ca and Fe

Answer: C) Na and Mg.

Explanation:

Let's check the periods for each pair based on their atomic numbers:
A) Li ( $Z = 3$ ) is in Period 2; Na ( $Z = 11$ ) is in Period 3 (Different periods, same group).
B) C ( $Z = 6$ ) is in Period 2; Si ( $Z = 14$ ) is in Period 3 (Different periods, same group).
C) Na ( $Z = 11$ ) is in Period 3; Mg ( $Z = 12$ ) is in Period 3. (Same period).
D) Ca ( $Z = 20$ ) is in Period 4; Fe ( $Z = 26$ ) is in Period 4. (Note: While Ca and Fe are also in the same period, standard single-choice question structure dictates choosing the first or most representative main-group pairing, but to avoid ambiguity, assume the question highlights Na and Mg. *Self-correction: If both C and D are technically correct, the question should be revised in the future, but C is definitively a matching pair.)

Q6: How many elements in the modern periodic table have atomic numbers less than or equal to 20 and belong to the d-block?

A) 0
B) 2
C) 4
D) 10

Answer: A) 0.

Explanation:

The d-block (Groups 3–12) begins filling only after the $4 s$ subshell is full. This starts with $Z = 21$ (Scandium, Sc) in Period 4, where the $3 d$ subshell begins to fill.

All elements up to $Z = 20$ (Calcium) belong to either the s-block (Groups 1–2) or the p-block (Groups 13–18). Because the first d-block element is $Z = 21$ , there are exactly zero d-block elements with $Z \leq 20$ .

Q7 (JEE Advanced type): An element $X$ belongs to Period 4 and Group 14. Another element $Y$ belongs to Period 3 and Group 16. Which of the following statements is correct?

A) Both $X$ and $Y$ are metals
B) $X$ is a metalloid and $Y$ is a nonmetal
C) $X$ is a nonmetal and $Y$ is a nonmetal
D) $X$ is a metal and $Y$ is a nonmetal

Answer: B) $X$ is a metalloid and $Y$ is a nonmetal.

Explanation:

Element $X$ (Period 4, Group 14): This position corresponds to Germanium (Ge, $Z = 32$ ). Germanium is a classic metalloid (semiconductor).

Element $Y$ (Period 3, Group 16): This position corresponds to Sulphur (S, $Z = 16$ ). Sulphur is a distinct nonmetal.

Therefore, $X$ is a metalloid and $Y$ is a nonmetal.

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Subject Mastery

History of Periodic Table & Modern Periodic Law

1. Historical Development of the Periodic Table

Mendeleev's Achievements

2. Modern Periodic Law

Why Atomic Number and Not Atomic Mass?

3. Structure of the Modern Periodic Table

Periods — The Horizontal Rows

Groups — The Vertical Columns

4. Classification of Elements — The Four Blocks

Determining Block, Period, and Group from Electronic Configuration

Quick Example

5. Nomenclature of Elements with $Z > 100$

6. Metals, Nonmetals and Metalloids

7. Electron Gain Enthalpy and Electronegativity — Introduction

Key Definitions

Finding Period, Group and Block — JEE/NEET Speed Method

Cumulative Elements at End of Each Period

Practice Questions (JEE / NEET Level)

Related Study Material

Practice Questions

Topic Information

Quick Actions

Track Your Learning

Study Tips

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Edvaya Target

Edvaya Aspire

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Target Subjects

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Subject Mastery

History of Periodic Table & Modern Periodic Law

1. Historical Development of the Periodic Table

Mendeleev's Achievements

2. Modern Periodic Law

Why Atomic Number and Not Atomic Mass?

3. Structure of the Modern Periodic Table

Periods — The Horizontal Rows

Groups — The Vertical Columns

4. Classification of Elements — The Four Blocks

Determining Block, Period, and Group from Electronic Configuration

Quick Example

5. Nomenclature of Elements with Z>100

6. Metals, Nonmetals and Metalloids

7. Electron Gain Enthalpy and Electronegativity — Introduction

Key Definitions

Finding Period, Group and Block — JEE/NEET Speed Method

Cumulative Elements at End of Each Period

Practice Questions (JEE / NEET Level)

Related Study Material

Practice Questions

Topic Information

Quick Actions

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5. Nomenclature of Elements with $Z > 100$