Final answer:
The formula Ni₂(NH₃)ₘBr⁹ suggests a coordination compound but is missing indications of charge balance. Ammonia (NH3) acts as a ligand in coordination compounds. Correct ionic compounds' formulas don't show physical ion arrangement.
Step-by-step explanation:
The student's question concerns the chemical formula Ni₂(NH₃)ₘBr⁹ and its validity. This formula appears to be incorrect. The mention of NH4 in the provided information should be NH3, which is ammonia. Ammonia (NH3) is typically a ligand in coordination compounds, not an ion like ammonium (NH4+). The formula suggests the presence of both ammonia and bromine and can represent a coordination compound, where nickel ions are likely to be the central metal atoms surrounded by ammonia molecules as ligands. However, the correct notation should clearly indicate the charge of the complex and the charge balance between the metal, ligands, and any counterions, which is missing here.
When discussing ionic compounds, such as those formed with ammonium (NH4+) or nitrate (NO3-), it's important to keep in mind that the formula does not represent the physical arrangement of the ions within, similar to how we shouldn't refer to a sodium chloride (NaCl) 'molecule' since it forms an ionic lattice.
The coordination number of a central metal atom in a coordination compound counts the number of bonds (not the number of distinct ligands) to the central atom. In the case of nickel with ammonia, each ammonia molecule can form a coordinate covalent bond with the nickel atom, thus can contribute to the coordination number, which should be indicated through the complex's formula.