Which alkali metal is least reactive and why? Include ionization energies for elements A, B, and C.

Element A: $494 \text{ kJ/mol}$
Element B: $418 \text{ kJ/mol}$
Element C: $519 \text{ kJ/mol}$

Element C has the highest first ionization energy at $519 \text{ kJ/mol}$.

Alkali metals react by losing their outermost electron. A higher ionization energy means more energy is required to remove this electron, making the element less likely to lose it and thus less reactive.

The least reactive metal is C.
Reason: It has the highest first ionization energy, meaning it requires the most energy to lose its outermost electron, making it less likely to react.

Question 4a:
When moist chlorine reacts with hydrogen sulphide, the following reaction occurs:

The observation made is the formation of a yellow solid (sulfur) and the disappearance of the yellowish-green color of chlorine gas.

Observation: A yellow solid is formed, and the yellowish-green color of chlorine fades.