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What is a  group of atoms with a positive or negative charge called?  

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A group of atoms that has a positive or negative charge is called a polyatomic ion. The atoms that make up a polyatomic ion are covalently bonded, and the ion tends to behave as a single particle that stays intact during most chemical reactions.

Most polyatomic ions are negatively charged. Some examples are the nitrite ion, `NO_2-` , nitrate ion, `NO_3-` , the sulfate ion, `SO_4^(2-)` , and the carbonate ion, `CO_3^(2-)` . The sulfate and carbonate ion both have two negative charges:

`SO_4^(2-)` and `CO_3^(2-)`

Each ion contains more electrons than the total number of protons in its atoms. For example, the nitrite ion has 18 electrons and a total of 17 protons in the nitrogen and oxygen atoms. 

Ammonium is a common positive polyatomic ion. It has the formula `NH_4+` , and has one less electron than it does protons.

There are many common salts (ionic compounds) that contain polyatomic ions. Here are a few that you might recognize:

`KNO_3` , potassium nitrate, used in gun powder

`NaNO_3` , a food preservative

`CaCO_3`, calcium carbonate, in limestone, chalk and antacids

`NH_4Cl` , ammonium chloride, used in chemical cold packs to treat injuries

`MgSO_4` , magnesium sulfate, also known as Epsoms salts

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