15. Which of the following is true about polyatomic ions? a. They are several atoms that altogether have a charge. b. They are written in parenthesis. c. They are all negative except for one. d. All of these are true.

## Step 1<br />Polyatomic ions are ions that consist of two or more atoms. These atoms are covalently bonded, meaning they share electrons, and the ion carries an overall charge. This aligns with option (a), which states that polyatomic ions are several atoms that altogether have a charge.<br /><br />## Step 2<br />Polyatomic ions are typically written in parentheses in chemical formulas. This is done to indicate that the atoms within the parentheses are bonded together and act as a single unit. This corresponds to option (b), which states that polyatomic ions are written in parentheses.<br /><br />## Step 3<br />Polyatomic ions can have either a positive or negative charge. This means that not all polyatomic ions are negative. This contradicts option (c), which states that all polyatomic ions are negative except for one.<br /><br />## Step 4<br />Given the above explanations, option (d) is incorrect because not all the statements are true.