General Trends

Group 1 (alkali metals) and Group 2 (alkaline earth metals) reactivity increases down the group. Atomic size increases down a group, leading to increased reactivity.

Metallic Character

Definition: Tendency to lose electrons and form positive ions (cations). Trends:

• Increases down a group (new electronic shells, weaker nuclear attraction)
• Decreases across a period left to right (increased effective nuclear charge)

Example 4.6:

• Higher metallic character: (a) K (over Na), (b) Na (over Mg)

Reactivity

Definition: Capability to react with other elements to form new compounds. Trends:

• Increases down a group (larger atomic size, weaker nuclear attraction)
• Varies across a period (high for Groups 1, 2, 16, and 17)

Density

Generally increases down a group (increased atomic mass and size). Varies across a period (tends to peak in the middle).

Characteristic Properties of Alkali Metals

• Highly reactive (increases down the group)
• Soft with low density (increases down the group)
• Excellent conductors of electricity and heat
• Low melting points
• React with water to form metal hydroxides and hydrogen gas

Prediction of Properties in Group 1

Reactivity increases down the group. General reaction with water: metal + water → metal hydroxide + hydrogen.

Positioning Unknown Elements

Use electronic configuration to determine position and predict properties. Elements in the same group have similar electronic configurations and properties.

Example 4.7:

Unknown element (atomic number 19): Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Position: Alkali metal in the 4th period.

Concept Assessment Exercise 4.8

Lower metallic character:

• (a) Li (compared to K)
• (b) Mg (compared to Ca)
• (c) Compare ionization energy (energy required to remove an electron) and electron affinity (energy released when an atom gains an electron)

Example 4.8:

Softer metal: K (compared to Na)