Periodic Table

The periodic table is like a map of all known elements. Just as you can predict climate from a position on a map, you can predict an element's properties from its position in the periodic table.

Structure

Elements are arranged by increasing atomic number.

• Periods (rows): 7 rows. Period number = number of electron shells.
• Groups (columns): 18 columns. Same group = same valence electrons = similar properties.

Important Groups

| Group | Name | Valence Electrons | Examples | |-------|------|-------------------|----------| | 1 | Alkali metals | 1 | Na, K | | 2 | Alkaline earth metals | 2 | Mg, Ca | | 17 | Halogens | 7 | F, Cl, Br | | 18 | Noble gases | 8 (2 for He) | He, Ne, Ar |

Metals, Non-metals, and Metalloids

• Metals (left/center): Conductors, shiny, malleable. Examples: iron, copper.
• Non-metals (right): Poor conductors, brittle. Examples: oxygen, nitrogen.
• Metalloids (along the staircase): Properties of both. Examples: silicon, germanium.

Periodic Trends

Across a period (left to right): Atomic size decreases, electronegativity increases, metallic character decreases.

Down a group (top to bottom): Atomic size increases, electronegativity decreases, metallic character increases.

Valency

Valency is the combining capacity of an element:

• Group 1: valency 1 (Na forms NaCl)
• Group 2: valency 2 (Mg forms MgCl2)
• Group 14: valency 4 (C forms CH4)
• Group 17: valency 1 (Cl forms HCl)
• Group 18: valency 0 (noble gases are stable)

Did You Know? Nepal has deposits of iron, copper, lead, and zinc in various districts. Understanding the periodic table helps geologists identify and classify these mineral resources.

Key Takeaways

• Periodic table is organized by increasing atomic number
• Period = number of shells; Group = number of valence electrons
• Properties follow predictable trends across periods and down groups
• Valency can be determined from the group number