To determine which pure substance can be classified as an element, we need to understand the definitions of elements and compounds:
1. Element: A substance that consists of only one type of atom. Elements cannot be broken down into simpler substances by chemical means.
2. Compound: A substance formed when two or more different types of atoms bond together.
Now let's look at each option provided:
1. [tex]\( H_2 \)[/tex]: This substance consists of two hydrogen atoms bonded together. Since there is only one type of atom (hydrogen), [tex]\( H_2 \)[/tex] is classified as an element.
2. [tex]\( NO_3 \)[/tex]: This substance consists of nitrogen and oxygen atoms. Since it contains more than one type of atom, [tex]\( NO_3 \)[/tex] is a compound.
3. [tex]\( NaCl \)[/tex]: This substance consists of sodium and chlorine atoms. Since it contains more than one type of atom, [tex]\( NaCl \)[/tex] is a compound.
4. [tex]\( H_2SO_4 \)[/tex]: This substance consists of hydrogen, sulfur, and oxygen atoms. Since it contains more than one type of atom, [tex]\( H_2SO_4 \)[/tex] is a compound.
From the above analysis, we can conclude that the only pure substance listed that can be classified as an element is [tex]\( H_2 \)[/tex].
Thus, the correct answer is [tex]\( H_2 \)[/tex], which corresponds to choice number [tex]\(\boxed{1}\)[/tex].
