To determine which pure substance can be classified as an element, we need to examine the composition of each substance. An element is a pure substance that consists of only one type of atom.
Let's analyze each of the given substances one by one:
1. [tex]$H_2$[/tex] (Hydrogen gas):
- This substance is composed of only hydrogen atoms.
- Since it contains only one type of atom, it is an element.
2. [tex]$NO_3$[/tex] (Nitrate ion):
- This substance consists of nitrogen (N) and oxygen (O) atoms.
- Since it contains more than one type of atom, it is not an element but a compound.
3. [tex]$NaCl$[/tex] (Sodium chloride):
- This substance is made up of sodium (Na) and chlorine (Cl) atoms.
- Since it contains more than one type of atom, it is not an element but a compound.
4. [tex]$H_2SO_4$[/tex] (Sulfuric acid):
- This substance consists of hydrogen (H), sulfur (S), and oxygen (O) atoms.
- Since it contains more than one type of atom, it is not an element but a compound.
Based on this analysis, out of the given substances, [tex]$H_2$[/tex] (Hydrogen gas) is the only one that can be classified as an element because it consists of only one type of atom, hydrogen.
Therefore, the correct answer is:
[tex]\[ \boxed{H_2} \][/tex]
