Write formulas for (a) ammonium sulfite; (b) ferric oxide; (c) copper(II) hromate; (d) phosphine; (e) calcium bicarbonate; (f) hydrogen cyanidc; (g) lithium bisulfate; (h) selenium tetrafluoride; (i) iron(II) sulfate heptahydrate.

Polyatomic ions are clusters of atoms that are covalently bonded together, but they have an overall positive or negative charge due to the loss or gain of electrons. These ions are treated as single units when they combine with other ions to form ionic compounds. For example, ammonium (\( NH_4^+ \)), sulfite (\( SO_3^{2-} \)), and bicarbonate (\( HCO_3^- \)) are all polyatomic ions. It's important to remember that the charge on a polyatomic ion must be balanced when these ions interact with other cations or anions to form neutral compounds.
When writing chemical formulas involving polyatomic ions, brackets are often used to emphasize the presence of these ions, especially when more than one of a particular polyatomic ion is needed to balance the charge in an ionic compound. For example, in calcium bicarbonate, the formula written is \( Ca(HCO_3)_2 \) to indicate that two bicarbonate ions are necessary to balance the calcium ion's +2 charge.

Understanding oxidation states, often referred to as oxidation numbers, is crucial when working with chemical formulas, especially in the context of ionic compounds. An oxidation state is a number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element in the compound.
The oxidation state helps in determining how elements combine and how electrons are distributed in compounds. For example, in ferric oxide, the term 'ferric' signifies that iron (Fe) has an oxidation state of +3, and oxide means that oxygen (O) has an oxidation state of -2. When combining ions to form ionic compounds, their charges or oxidation states must be balanced to result in a neutral compound. To find the formula for ferric oxide, we combine iron and oxygen in a ratio that balances these charges, resulting in \( Fe_2O_3 \). The key to correct formulas lies in the ability to balance these oxidation states to achieve a compound with no net charge.

Ionic compounds form when positive ions (cations) and negative ions (anions) are bound together by the strong electrostatic forces known as ionic bonds. To write the formula for an ionic compound, it is necessary to ensure that the net charge of the compound is zero. This involves making the total positive charge from the cations equal to the total negative charge from the anions.
An example from the provided exercise is lithium bisulfate. Lithium (Li) carries a +1 charge, and bisulfate (HSO4) has a -1 charge. When these two ions come together, they form lithium bisulfate with the formula \( LiHSO4 \) , where the charges are balanced, and the compound is neutral. In writing formulas for ionic compounds, the use of subscripts indicates the number of each type of ion needed to balance out the charges. Always write the cation first followed by the anion, and simplify the subscripts to the smallest whole number ratio that maintains charge balance.