Question

mgx₂
very low density
highly reactive
diatomic
using the periodic table, determine which element these characteristics most likely describe.
a fluorine
b iodine
c oxygen
d sodium

Explanation:

1. First, analyze the compound formula $\text{MgX}_2$: Magnesium has a +2 charge, so each X must have a -1 charge, meaning X is a halogen (group 17 element) or a monatomic anion with -1 charge. This eliminates option C (oxygen, forms -2 charge, would make $\text{MgO}$) and D (sodium, is a cation with +1 charge).
2. Next, use the properties: Iodine (option B) has a high density and is a solid at room temperature, which contradicts "very low density". Fluorine (option A) is a diatomic gas ($\text{F}_2$) with very low density, is the most highly reactive element, and forms $\text{MgF}_2$ with magnesium, matching all given characteristics.

Answer:

A. fluorine