To determine which of the given chemicals is a pure element, let's examine each one:
1. [tex]\( \text{HCO}_3^- \)[/tex]:
- This is a bicarbonate ion, which is not a pure element. It consists of hydrogen, carbon, and oxygen atoms.
2. [tex]\( \text{H}_2 \)[/tex]:
- This is molecular hydrogen, composed of two hydrogen atoms. Hydrogen in this form is a pure element.
3. [tex]\( \text{H}_2O \)[/tex]:
- This is water, made up of two hydrogen atoms and one oxygen atom. It is a compound, not a pure element.
4. H:
- This is a single hydrogen atom. It is a pure element.
After examining the options, we see that [tex]\( \text{H}_2 \)[/tex] and [tex]\( H \)[/tex] are pure elements. However, the problem may be asking for the first occurrence of a pure element from the given list.
The order of options given is:
1. [tex]\( \text{HCO}_3^- \)[/tex]
2. [tex]\( \text{H}_2 \)[/tex]
3. [tex]\( \text{H}_2O \)[/tex]
4. [tex]\( H \)[/tex]
Therefore, among the given options, the pure elements [tex]\( H_2 \)[/tex] and [tex]\( H \)[/tex] occur respectively at positions 2 and 4 in the list.
If we are asked specifically for the 1-based index of the pure element among the list, the final answer would be the index of the pure element among these repeated elements.
So, the answer is 4.
