To determine the correct formula for a chromate ion, let's start by analyzing the components and charge of the ion. A chromate ion has:
- One chromium (Cr) atom
- Four oxygen (O) atoms
- A charge of -2, indicating that it has two extra electrons.
Now, let's examine each option provided:
1. Option 1: [tex]\( \text{Cr} \_4 \text{O} \ ^{2-} \)[/tex]
In this option, the notation is incorrect because it implies there are four chromium atoms, which is not the case. Additionally, the placement of the charge is not in a standard format.
2. Option 2: [tex]\( \text{CrO} \_4{ }^{2+} \)[/tex]
This option has the correct elemental count: one chromium (Cr) atom and four oxygen (O) atoms. However, the charge indicated is [tex]\(2+\)[/tex], which is incorrect. The chromate ion has a charge of [tex]\(2-\)[/tex], not [tex]\(2+\)[/tex].
3. Option 3: [tex]\( \text{CrO} \_4{ }^{2-} \)[/tex]
This option correctly represents one chromium (Cr) atom and four oxygen (O) atoms. Moreover, it correctly indicates the [tex]\(2-\)[/tex] charge of the ion.
Given that the chromate ion comprises one chromium atom, four oxygen atoms, and has a [tex]\(2-\)[/tex] charge, the correct chemical formula is represented as:
[tex]\[ \text{CrO}_4^{2-} \][/tex]
Hence, the correct option is:
[tex]\[ \boxed{\text{CrO}_4^{2-}} \][/tex]
