📘 Chapter 3 – Classification of Elements and Periodicity in Properties

Why is the classification of elements needed?

To group elements with similar properties together for systematic study.

What was significant about Dobereiner's Triads?

Groups of 3 elements with similar properties; the atomic mass of the middle element is approximately the average of the other two.

What limitation did Dobereiner's Triads have?

Not all elements could be categorized into triads.

What was Newlands’ Law of Octaves?

Elements arranged in increasing atomic mass where every 8th element has similar properties.

What was a limitation of Newlands’ Law of Octaves?

It only worked up to Calcium and failed for heavier elements.

What was Mendeleev's major contribution to the periodic table?

He created a periodic table based on atomic mass and left gaps for undiscovered elements.

What is the periodic law as stated by Mendeleev?

Properties of elements are a periodic function of their atomic masses.

How did Moseley's Modern Periodic Law change the classification of elements?

It stated that properties of elements are a periodic function of their atomic number instead of atomic mass.

What defines the structure of the Modern Periodic Table?

It is arranged by atomic number with 18 groups and 7 periods.

What differentiates the blocks in the classification of elements?

They depend on the last electron entry: s-block, p-block, d-block, and f-block.

What trend occurs for atomic radius across a period and down a group?

Atomic radius decreases across a period and increases down a group.

What is the ionic radius behavior for cations and anions?

Cations are smaller than the parent atom, while anions are larger than the parent atom.

What is ionization enthalpy, and how does it change across a period and down a group?

It is the energy required to remove the outermost electron; it increases across a period and decreases down a group.

What is electron gain enthalpy and its trend across periods and down groups?

The energy change when an atom gains an electron; it becomes more negative across a period and less negative down a group.

What is electronegativity and its trend across periods and down groups?

The tendency to attract shared electrons in a bond; it increases across a period and decreases down a group.

What defines the anomalous properties of second-period elements?

They differ from heavier group members due to small size, high electronegativity, and absence of d-orbitals.

What is the trend in metallic and non-metallic character across a period and down a group?

Metallic character decreases and non-metallic character increases across a period; metallic character increases and non-metallic character decreases down a group.