Question

Write a formula for the compound that forms between potassium and each polyatomic ion. $$\begin{array}{ll}{\text { a. carbonate }} & {\text { b. phosphate }} \\\ {\text { c. hydrogen phosphate }} & {\text { d. acetate }}\end{array}$$

Short answer

The formulas for the compounds are: a. K2CO3, b. K3PO4, c. K2HPO4, d. KC2H3O2.

Step-by-step solution

Determine the Charge of Potassium

Potassium is in group 1 of the periodic table, which means it has one valence electron that it wants to give up. This would give potassium a +1 charge. So the ion is represented as K+.

Determine the Charge of Each Polyatomic Ion

Each polyatomic ion has a specific charge that should be known or looked up. Carbonate is CO3^2-. Phosphate is PO4^3-. Hydrogen phosphate is HPO4^2-. Acetate is C2H3O2^-.

Write the Formula for Carbonate Compound

Potassium has a +1 charge and carbonate has a -2 charge. To balance the charges, we need two potassium ions for each carbonate ion. The formula for the compound between potassium and carbonate is K2CO3.

Write the Formula for Phosphate Compound

Since potassium has a +1 charge and phosphate has a -3 charge, three potassium ions are required for each phosphate ion to balance the charges. The formula for the compound is K3PO4.

Write the Formula for Hydrogen Phosphate Compound

To balance the +1 charge of potassium with the -2 charge of hydrogen phosphate, we need two potassium ions. The compound's formula is K2HPO4.

Write the Formula for Acetate Compound

Potassium has a +1 charge and acetate has a -1 charge, so they combine in a one-to-one ratio. The formula for the compound is KC2H3O2.

Key concepts

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms that are covalently bonded together and function as a single ion in a compound. Unlike monatomic ions, which consist of a single atom with a positive or negative charge, polyatomic ions have a specific group of atoms with an overall charge. This charge results because the total number of electrons within the ion does not equal the total number of protons in the nuclei of the atoms involved.

For example, the carbonate ion (CO3^2-) consists of one carbon atom and three oxygen atoms, with an extra two electrons giving the ion a -2 charge. Familiarity with common polyatomic ions, like carbonate, phosphate, and acetate, is crucial because they frequently appear in many chemical compounds.

Here are some examples: sulfate (SO4^2-), nitrate (NO3^-), and ammonium (NH4^+). Remembering their charges and compositions is essential for writing correct chemical formulas and understanding their roles in chemical reactions.

Ionic Charges

The ionic charge of an atom or a group of atoms is determined by the loss or gain of electrons in relation to the number of protons in the nucleus. Atoms seek stability by achieving a full outer shell of electrons, typically resembling the electron configuration of a noble gas. Ions are formed when atoms either lose or gain electrons to achieve this stable state.

For example, potassium (K), which is in group 1 of the periodic table, has a single valence electron that it 'prefers' to lose, resulting in a +1 charge after the loss. On the other hand, atoms or groups of atoms that gain electrons, such as the chloride ion (Cl^-), acquire a negative charge.

When writing chemical formulas, especially for ionic compounds, it's imperative to balance the positive and negative charges to achieve an electrically neutral compound. This is done by adjusting the ratio of cations (positively charged ions) to anions (negatively charged ions) accordingly.

Compound Formation

The formation of compounds, particularly ionic compounds, involves combining cations and anions in a proportion that results in a neutral overall charge. To write the formula for an ionic compound, first, identify the charges of the cations and anions involved. Then, adjust the number of each ion so that the total positive charge equals the total negative charge.

In the provided exercise, potassium ions (K+) combine with various polyatomic ions to form compounds. As potassium has a +1 charge and the polyatomic ions have varying negative charges, multiple potassium ions may be needed to balance out these charges. For instance, to neutralize the -2 charge of the carbonate ion (CO3^2-), two potassium ions are needed, resulting in the formula K2CO3. Mastery of this balancing act is crucial for students to proficiently write chemical formulas.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding and compound formation. The number of valence electrons an atom possesses influences how it will bond with other atoms. Atoms in the same group (vertical column) of the periodic table have the same number of valence electrons, which explains why they often exhibit similar chemical behaviors.

In our exercise, understanding that potassium has one valence electron explains why it forms a K+ ion when it loses that electron, aiming to achieve a noble gas electron configuration. This concept is a fundamental aspect of understanding how atoms bond and is instrumental when predicting the types of ions that elements will form, which is the first step in writing chemical formulas for ionic compounds.