Write the formulas for these compounds: (a) copper(I) cyanide, (b) strontium chlorite, (c) perchloric acid, (d) hydroiodic acid, (e) disodium ammonium phosphate, (f) lead(II) carbonate, (g) tin(II) fluoride, (h) tetraphosphorus decasulfide, (i) mercury(II) oxide, (j) mercury(I) iodide, (k) cobalt(II) chloride hexahydrate.

Understanding ion charges is essential in chemical formula writing. An ion is an atom or molecule that has gained or lost electrons, giving it a net electrical charge. Positive ions, called cations, lose electrons, while negative ions, or anions, gain electrons. The periodic table is a critical tool to determine these charges, especially for representative elements where the group number can indicate the usual charge for the cation.

For transition metals, it's common to have varying charges, and these are often specified in the compound name, such as copper(I) indicating a +1 charge. Knowledge of common polyatomic ions like ammonium (\( NH_4^+\)) or carbonate (\( CO_3^{2-}\)) is also necessary, as they behave as single charged units in compounds. In writing formulas, it's crucial to balance these charges to ensure the resulting compound is neutral.

The periodic table isn't just a display of elements; it is a comprehensive tool that organizes elements by their chemical properties and is pivotal for writing chemical formulas. Elements are grouped into columns called 'groups' or 'families' and rows known as 'periods.'

The periodicity of chemical properties is due to the arrangement of electrons in the atoms. Elements in the same column have similar charge states, which is meaningful when combining ions to form compounds. For instance, alkaline earth metals in group 2 typically form +2 cations. By using the periodic table effectively, students can predict ion charges and therefore correctly formulate chemical compounds.

Polyatomic ions are charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit. Recognizing and memorizing common polyatomic ions is crucial in chemistry. For example, sulfate (\( SO_4^{2-}\)), nitrate (\( NO_3^{-}\)), and phosphate (\( PO_4^{3-}\)) are all polyatomic ions.

These ions do not alter their composition when they form compounds; instead, they keep their integrity and combine with other ions in a way that overall charge neutrality is maintained. For instance, ammonium (\( NH_4^+\)) will pair with a negative ion such as chloride (\( Cl^-\)) to form ammonium chloride (\( NH_4Cl\)).

In chemistry, hydrates are compounds that contain water molecules within their crystal structure. The water is not merely trapped; it is an integral part of the compound's formula and has a fixed ratio. The presence of water is indicated by the term 'hydrate' and the number of water molecules is prefixed in the compound's name, such as 'hexahydrate' in cobalt(II) chloride hexahydrate.

The proper formula of a hydrate includes the compound plus a dot followed by the number of water molecules, written as \( X \textbf{n}H_2O \) where \( X \) is the ionic compound and \( \textbf{n} \) represents the number of water molecules. When writing chemical formulas for hydrates, it's essential to account for these water molecules to correctly represent the substance.