Reactivity Trends in Group 1 (Alkali Metals) and Group 17 (Halogens)
Why Does Reactivity Increase Down Group 1?
- The alkali metals are renowned for their high reactivity, which increases as you move down the group.
- This trend can be explained by examining ionization energy, the energy required to remove the outermost electron from an atom.
- Ionization energy decreases down the group.
- As you move down Group 1, the atomic radius increases.
- Each successive element has an additional electron shell, increasing the distance between the nucleus and the valence electron.
- This greater distance weakens the electrostatic attraction between the nucleus and the outermost electron.
- Additionally, inner electron shells shield the valence electron from the nucleus’s pull, further reducing ionization energy.
- Lower ionization energy means easier electron loss.
- Alkali metals react by losing their single valence electron to form a positive ion (cation).
- As it becomes easier to lose this electron, the reactivity of the metal increases.
Reaction of Sodium with Water
- The reaction between sodium and water vividly demonstrates alkali metal reactivity:
$$
2\text{Na (s)} + 2\text{H}_2\text{O (l)} \rightarrow 2\text{NaOH (aq)} + \text{H}_2\text{(g)}
$$
Here’s what happens: - Sodium loses its valence electron, forming $ \text{Na}^+ $.
- Water accepts the electron, releasing hydrogen gas ($ \text{H}_2 $).
- Sodium hydroxide ($ \text{NaOH} $) forms, making the solution strongly basic.
- Reactions with water become more vigorous as you move down the group.
- For instance, potassium reacts explosively compared to sodium.
Applications and Implications of Alkali Metal Reactivity
The high reactivity of alkali metals has significant practical applications but also demands careful handling:
- Industrial Use: Sodium and potassium are used in heat transfer systems and the production of compounds like fertilizers.
- Safety Concerns: Alkali metals must be stored under oil or in inert atmospheres to prevent reactions with moisture or oxygen in the air.
- Don’t confuse reactivity with stability.
- While alkali metals become more reactive down the group, they are less stable in their elemental form.
Reactivity of Group 17 Elements (Halogens)
Why Does Reactivity Decrease Down Group 17?
- Halogens are highly reactive non-metals, but their reactivity decreases as you move down the group.
- This trend is tied to their ability to gain an electron to form a negative ion (anion).
- Electron affinity decreases down the group.
- Halogens react by gaining one electron to achieve a stable octet.
- As the atomic radius increases down the group, the added electron experiences weaker attraction to the nucleus due to increased shielding by inner electrons.
- This makes it less energetically favorable for halogens lower in the group (like iodine) to gain an electron compared to those higher up (like fluorine).
