Here's why:
* Covalent bonds: Atoms share electrons to achieve a stable electron configuration.
* Overall charge: The molecule has an imbalance in the number of protons (positive charge) and electrons (negative charge).
Examples of polyatomic ions:
* Ammonium ion (NH₄⁺): Nitrogen and hydrogen atoms are covalently bonded, but the ion has a +1 charge due to the loss of an electron.
* Sulfate ion (SO₄²⁻): Sulfur and oxygen are covalently bonded, but the ion has a -2 charge due to the gain of two electrons.
* Nitrate ion (NO₃⁻): Nitrogen and oxygen are covalently bonded, but the ion has a -1 charge due to the gain of one electron.
Key point: While polyatomic ions have covalent bonds within their structure, they act as a single charged unit when participating in ionic bonding with other ions.