Question

Write formulas for the compounds formed from the element on the left and each polyatomic ion on the right. (a) \(\mathrm{Rb} \quad \mathrm{NO}_{3}^{-}, \mathrm{SO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) (b) Sr \(\mathrm{NO}_{3}^{-}, \mathrm{SO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) (c) In \(\mathrm{NO}_{3}^{-}, \mathrm{SO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) (Assume In charge is \(3+.\) )

Short answer

The compounds are (a) \text{RbNO}_3, \text{Rb}_2\text{SO}_4, \text{Rb}_3\text{PO}_4; (b) \text{Sr(NO}_3)_2, \text{SrSO}_4, \text{Sr}_3(\text{PO}_4)_2; (c) \text{In(NO}_3)_3, \text{In}_2(\text{SO}_4)_3, \text{InPO}_4.

Step-by-step solution

- Understand Ionic Compound Formation

To write the formulas for the compounds formed between an element and a polyatomic ion, you need to consider the charges on the ions. Each compound must be electrically neutral. Therefore, the total positive charge from the metal cation must balance the total negative charge from the polyatomic anion.

- Writing Formulas for Element Rb

Rubidium (Rb) has a +1 charge. To balance the charges with each polyatomic ion, we will need one Rb for each NO3−, one Rb for each SO42− (with the assumption that the extra negative charge can be balanced out by another Rb), and three Rb atoms for each PO43− to balance the three negative charges.

- Writing Formulas for Element Sr

Strontium (Sr) has a +2 charge. It requires one Sr for each NO3− and one Sr for each SO42− since they both have a -2 charge. For PO43−, we need two Sr atoms to balance the three negative charges, resulting in a formula with six NO3−.

- Writing Formulas for Element In

Indium (In) has a +3 charge. It therefore requires three In atoms to balance the three NO3− ions, two In for three SO42− ions, and one In for each PO43− to ensure that the overall compound is electrically neutral.

Key concepts

Polyatomic Ions

Polyatomic ions are ions consisting of more than one atom, typically a molecule with a net positive or negative charge. Understanding them is crucial for writing chemical formulas because they behave as a single unit during the formation of ionic compounds. For example, the nitrate ion \text(\( \text{NO}_3^- \)) contains one nitrogen atom and three oxygen atoms, and carries a -1 charge. Similarly, the sulfate ion \text(\( \text{SO}_4^{2-} \)) has one sulfur atom and four oxygen atoms with a -2 charge, and the phosphate ion \text(\( \text{PO}_4^{3-} \)) includes one phosphorus atom and four oxygen atoms with a -3 charge.

When these polyatomic ions combine with cations, like Rb or Sr, they form neutral compounds by balancing the charges between them. Recognizing and memorizing common polyatomic ions helps ease the process of writing chemical formulas.

Ionic Compound Formation

Ionic compound formation involves the chemical combination of cations, which are positively charged ions, with anions, which are negatively charged ions or polyatomic ions. In an ionic compound, the total positive charges from the cations must equal the total negative charges from the anions to create a neutral compound. This is achieved through a clear understanding of the rules of ionic bonding and the charges involved.

For instance, when we see Rb, which has a +1 charge, it must pair with a polyatomic ion such as \text(\( \text{NO}_3^- \)) in a one-to-one ratio to form a neutral compound \text(\( \text{RbNO}_3 \)). Similarly, a +2 charged Sr ion will connect with two \text(\( \text{NO}_3^- \)) ions to maintain charge balance, producing \text(\( \text{Sr(NO}_3)_2 \)). Ionic compounds formed in this way have distinct structures and properties, including high melting points and the ability to conduct electricity when melted or dissolved in water.

Charge Balance in Ionic Compounds

Maintaining charge balance in ionic compounds is a key principle and essentially dictates their chemical formulas. Charge balance is the concept that the total number of positive charges must equal the total number of negative charges, ensuring the compound is electrically neutral. We can use simple arithmetic to understand this balance.

As an example, if we have a Sr ion with a +2 charge, we need two nitrate ions to balance it because each \text(\( \text{NO}_3^- \)) has a -1 charge. The resulting compound is \text(\( \text{Sr(NO}_3)_2 \)), where the subscript '2' indicates two nitrate ions are needed. With sulphate ions, \text(\( \text{SO}_4^{2-} \)), only one is needed as it already carries a -2 charge, so the formula becomes \text(\( \text{SrSO}_4 \)). This simplifies the process of writing correct formulas by mere accounting of charges.

Valency and Chemical Bonding

Valency is the combining capacity of an element or ion and plays a significant role in chemical bonding—particularly in the formation of ionic and covalent bonds. In the case of ionic bonding, typically found in ionic compounds, valency refers to the charge of the ion, which is determined by the number of electrons lost (in cations) or gained (in anions).

Indium (In) demonstrates a valency of +3 because it tends to lose three electrons to form a \text(\( \text{In}^{3+} \)) ion. This valency dictates that it will bond with three nitrate ions, leading to the compound \text(\( \text{In(NO}_3)_3 \)). The understanding of valency is crucial as it directly informs the proportions of atoms in a compound. By familiarizing oneself with the common valencies of elements and ions, one can predict and write chemical formulas accurately. Combining atoms with corresponding valencies assures that the resulting compounds are stable and neutral.