7.0 PROPERTIES AND TRENDS ACROSS A PERIOD

The Periodic Table is a chemist's roadmap. While elements in the same group have similar properties, elements across a period (horizontal row) show a gradual transition from metallic to non-metallic character. This chapter delves into the fundamental reasons behind these trends by examining how atomic structure – specifically the number of protons and the arrangement of electrons – affects key properties like atomic radius, ionic radius, ionization energy, and electronegativity. Understanding these trends allows you to predict and explain the behaviour of elements without memorizing every fact.

7.1 TRENDS IN ATOMIC AND IONIC RADII

7.1.1 Atomic Radius

The atomic radius is the distance from the nucleus to the outermost electron (the valence shell). It is usually measured in picometers (pm) or angstroms (Å).

Trend across a period (left to right): DECREASES

Explanation: As we move from left to right across a period (e.g., from sodium to argon in Period 3), the following happens:

• The number of protons in the nucleus increases (increasing nuclear charge).
• Electrons are added to the same outer shell. There is no significant increase in shielding because the new electrons join the same principal energy level.
• The increasing positive charge pulls the electron cloud inwards more strongly, making the atomic radius smaller.

Example (Period 3): S