Chemical Formula

In NH₃, the subscript 3 appears after the symbol H (hydrogen). Subscripts are written below and to the right of the element symbol they refer to.

A subscript tells you the number of atoms of that specific element in one formula unit. Since 3 follows H, there are 3 hydrogen atoms per molecule of NH₃.

The symbol N has no subscript, which means there is exactly 1 nitrogen atom. No subscript always means 1 atom.

Therefore, the subscript 3 in NH₃ means there are 3 hydrogen atoms in one molecule of ammonia. The formula tells us: 1 nitrogen + 3 hydrogen atoms.

The question asks what the subscript 3 represents in NH₃. We need to apply our knowledge of chemical formula notation.

The subscript 3 follows H, not N. Subscripts only apply to the element symbol immediately before them, so this option incorrectly assigns the subscript to nitrogen.

NH₃ contains 1 N + 3 H = 4 total atoms, not 3. This option confuses the subscript value with the total atom count.

Subscripts in a molecular formula describe atom counts, not charges. Charges are shown as superscripts (e.g., NH₄⁺), not subscripts.

The remaining option — 'There are 3 hydrogen atoms in one molecule' — is correct. The subscript 3 after H means 3 hydrogen atoms per formula unit.

Element symbols always have the first letter capitalised and the second letter (if present) in lowercase. This rule is strict and carries chemical meaning.

Cobalt's symbol is Co — capital C followed by lowercase o. Writing CO with both letters capitalised creates an entirely different chemical entity.

CO (capital C + capital O) is interpreted as a compound consisting of one carbon atom (C) and one oxygen atom (O) — this is carbon monoxide, a toxic gas.

The student's error is critical: CO means carbon monoxide, while cobalt must be written as Co. Capitalisation changes the entire meaning of a chemical symbol.

The question asks why writing CO for cobalt is incorrect. We need to apply knowledge of element symbol conventions.

This is factually wrong. CO is the formula for carbon monoxide. Accepting CO as cobalt would confuse a compound with an element.

Metal symbols can have two letters (e.g., Fe for iron, Na for sodium). The number of letters is not the issue here — the capitalisation is.

Element symbols always begin with a capital letter. Writing 'co' entirely in lowercase violates the fundamental naming convention for element symbols.

CO represents carbon monoxide, not cobalt; cobalt is written as Co is correct. The capital O turns the symbol into a two-element compound formula rather than a single-element symbol.