MP Board · Class 10 · Science · Periodic Classification of ElementsDiscuss the various trends observed in the Modern Periodic Table across a period and down a group for the following properties: (a) Valency, (b) Atomic Size, (c) Metallic and Non-metallic character.

Trends in the Modern Periodic Table\nProperties of elements change in a predictable manner as we move across a period or down a group in the Modern Periodic Table. These trends help in understanding the chemical reactivity and nature of elements.

(a) Valency

• Across a Period: As we move from left to right across a period, the number of valence electrons increases from 1 to 8. The valency first increases from 1 to 4 and then decreases to 0 (or 8) for noble gases.
• Down a Group: The valency remains the same down a group because all elements in the same group have the same number of valence electrons in their outermost shell.

(b) Atomic Size (Atomic Radius)

• Across a Period: Atomic radius decreases as we move from left to right across a period. This is because electrons are added to the same shell, increasing the nuclear charge which pulls the electrons closer to the nucleus, reducing the atomic size.
• Down a Group: Atomic size increases down a group. As we go down, a new shell is added at each step, which increases the distance between the outermost electrons and the nucleus, thereby increasing the overall size despite the increased nuclear charge.

(c) Metallic and Non-metallic Character

• Across a Period: Metallic character decreases from left to right across a period, while non-metallic character increases. This happens because the effective nuclear charge increases, making it harder for atoms to lose electrons (metallic property) and easier to gain or share electrons (non-metallic property).
• Down a Group: Metallic character increases down a group, and non-metallic character decreases. As the atomic size increases down a group, valence electrons can be lost more easily due to weaker nuclear attraction on the outermost electrons.