In the present form of the Periodic Table, what determines the number of elements in a period?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Explanation

🔵 Step 1: What determines the number of elements?

In the modern periodic table, the number of elements in a period is determined by the number of electrons that can be accommodated in the subshells being filled.

Number of elements in a period → Number of electrons accommodated in the subshells being filled

🟢 Step 2: Length of Different Periods

Period	Subshells Filled	Number of Elements
1st	1s	2
2nd	2s, 2p	8
3rd	3s, 3p	8
4th	4s, 3d, 4p	18
5th	5s, 4d, 5p	18
6th	6s, 4f, 5d, 6p	32
7th	7s, 5f, 6d, 7p	32

🎬 3D-Style Period Animation

As more subshells become available for filling, the length of the period increases.

2 8 8 18 18 32

2 → 8 → 8 → 18 → 18 → 32 → 32

🟣 Step 3: Why are the numbers different?

The maximum number of electrons that can be accommodated in a shell is 2n², but the actual length of a period depends on the order in which different subshells are filled.

1st → 2 elements
2nd & 3rd → 8 elements
4th & 5th → 18 elements
6th & 7th → 32 elements

✅ Final Answer

The number of elements in a period is determined by the number of electrons that can be accommodated in the subshells being filled in that period.

Period Length = Number of electrons accommodated in the subshells being filled

📝 MCQ Practice

Q1. The number of elements in a period is mainly determined by:

A) Atomic mass

B) Number of subshells being filled

C) Number of neutrons

D) Atomic mass number

✅ Show Answer

Answer: B) Number of subshells being filled

🎯 Exam Point

Period Length → Subshell Filling

2, 8, 8, 18, 18, 32, 32

Identify the "Law of Octaves" and why it failed for heavier elements.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Law of Octaves — Explanation

🔵 Step 1: Who proposed the Law of Octaves?

The Law of Octaves was proposed by John Newlands in 1865.

He arranged elements in increasing order of their atomic masses.

🎵 Every eighth element had properties similar to the first element.

Newlands compared this repetition of properties with the eight notes of music, hence the name "Law of Octaves."

🟢 Step 2: Statement of the Law

"When elements are arranged in increasing order of atomic masses, the properties of every eighth element are similar to those of the first."

🟠 Step 3: Why did it fail for heavier elements?

The Law of Octaves worked reasonably well only for the lighter elements, mainly up to calcium.

After calcium, the law failed because:

• Newlands forced all elements into groups of eight.
• Elements were not always chemically similar after every eighth element.
• There was no proper provision for newly discovered elements.
• He did not leave gaps for elements that were yet to be discovered.
• Some dissimilar elements were placed in the same group.

⚠️ Main Limitation: The periodic repetition of properties did not continue regularly for heavier elements.

🎬 3D-Style Octaves Animation

The eighth element was expected to show a repetition of properties.

1 2 3 4 5 6 7 8≈1

1st element ↔ 8th element → Similar properties

📝 MCQ Practice

Q1. Who proposed the Law of Octaves?

A) Dmitri Mendeleev

B) John Newlands

C) Henry Moseley

D) Dobereiner

✅ Show Answer

Answer: B) John Newlands

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Q2. Newlands arranged elements mainly according to increasing:

A) Atomic number

B) Atomic volume

C) Atomic mass

D) Valency

✅ Show Answer

Answer: C) Atomic mass

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Q3. According to the Law of Octaves, every ______ element had properties similar to the first.

A) Fifth

B) Sixth

C) Seventh

D) Eighth

✅ Show Answer

Answer: D) Eighth

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Q4. The Law of Octaves was mainly successful up to:

A) Hydrogen

B) Calcium

C) Iron

D) Uranium

✅ Show Answer

Answer: B) Calcium

✅ Final Answer

The Law of Octaves was proposed by John Newlands in 1865. He stated that when elements are arranged in increasing order of atomic mass, every eighth element shows properties similar to the first.

It failed for heavier elements because the periodic repetition of properties was no longer regular after calcium.

🎯 Exam Point

John Newlands → 1865 → Law of Octaves

Basis → Increasing Atomic Mass

Every 8th Element → Similar Properties

Major Limitation → Failed for heavier elements

How does the Modern Periodic Law differ from Mendeleev’s Periodic Law?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Modern Periodic Law vs Mendeleev’s Periodic Law

🔵 Step 1: Mendeleev’s Periodic Law

Mendeleev proposed his Periodic Law in 1869. He arranged elements in increasing order of their atomic masses.

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

🟢 Step 2: Modern Periodic Law

The Modern Periodic Law was proposed by Henry Moseley in 1913. It states that the properties of elements are periodic functions of their atomic numbers.

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

🟠 Step 3: Main Differences

Basis	Mendeleev’s Law	Modern Periodic Law
Proposed by	Dmitri Mendeleev	Henry Moseley
Year	1869	1913
Basis	Atomic mass	Atomic number
Arrangement	Generally increasing atomic mass	Increasing atomic number
Isotopes	Could not be placed satisfactorily	All isotopes occupy the same position
Anomalous pairs	Some anomalous pairs existed	Anomalies are resolved by atomic number

🎬 3D-Style Comparison Animation

Mendeleev → Atomic Mass

Mass Mass Mass

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Modern → Atomic Number

Z=1 Z=2 Z=3

🟣 Step 4: Why was the Modern Law needed?

Atomic number gives the number of protons in the nucleus and determines the electronic configuration of an element. Since chemical properties depend strongly on electronic configuration, atomic number provides a more fundamental basis for periodic classification.

Atomic Number → Electronic Configuration → Chemical Properties

✅ Final Answer

Mendeleev’s Periodic Law is based on atomic mass, whereas the Modern Periodic Law is based on atomic number.

Mendeleev: Properties ∝ Atomic Mass

Modern: Properties ∝ Atomic Number

🎯 Exam Point

Mendeleev → 1869 → Atomic Mass

Moseley → 1913 → Atomic Number