Ions in chemistry can be a single charged atom, or they can be a group of atoms that act as an ion. These groups of atoms are called polyatomic ions. Polyatomic ions each carry a specific charge, which is determined by their numbers of valence electrons. Many chemistry classes require students to know at least some of the basic polyatomic ions. While there are some ways of figuring out the charges on each ion, as well as tricks to remembering others, there are no solid rules on how they are named and what charges they carry. The only way to be sure of the charges and names of these ions is to memorize them.
Calculate from Oxidation Number
Determine the oxidation number of each atom in the ion. For example, consider the hydroxide ion, which has an oxygen atom and a hydrogen atom. The oxidation number of oxygen is -2, and the oxidation number of hydrogen is +1.
Add together the oxidation numbers of all the atoms in the polyatomic ion. In the example, -2 +1 = -1. This is the charge on the polyatomic ion.
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Write this charge as a superscript to the right of the ion's formula. For a single charge, write - or + instead of 1- or 1+. In the example, the hydroxide ion is expressed as OH^-.
Draw the Lewis Structure of the Ion
Write each atom in the ion with its Lewis dot structure. For example, consider the ammonium ion, which has one nitrogen atom and four hydrogen atoms. The nitrogen atom is expressed with an N surrounded by five dots to represent its valence electrons. Each hydrogen atom is expressed with an H with a single dot next to each one to represent their valence electrons.
Draw the atoms of the ion bonded together with covalent bonds. In the example, the four hydrogen atoms will bond to the nitrogen atom, and each hydrogen atom's lone electron will form a covalent bond with one of nitrogen's electrons.
Replace each bonding pair of electrons with a line to symbolize a covalent bond. In the example, the N would have four bonds, each attached to an H.
If there are any electrons left over after counting eight for each atom or two for each hydrogen, remove them and count those as positive charges on the ion. If there are any electrons needed to have eight for each atom or two for each hydrogen, add those electrons to the structure and count those as negative charges on the ion. In the example, the ammonium ion has a single positive charge because it has an extra electron after bonding with four hydrogen atoms.
Use a mnemonic device to help remember the charges. For example, Mr. P of Papa Podcasts developed the phrase "Nick the Camel ate a Clam for Supper in Phoenix" to remember the polyatomic ions that end in "-ate." The "-ate" is indicated in the sentence itself for easy memory.
Use the first letter (for N, C, S or P) or letters (for Cl) of the nouns in the sentence for the main atom in the polyatomic ion. For example, to write the formula and charge for nitrate, use N as nitrogen for "Nick."
Count the consonants in the word. This is the number of oxygen atoms in the polyatomic ion. For example, there are three consonants in "Nick," so there are three oxygen atoms in nitrate.
Count the vowels in the word. This is the negative charge on the polyatomic ion. For example, there is one vowel in "Nick," so nitrate has a negative one charge.
TL;DR (Too Long; Didn't Read)
Using the oxidation number or valence number of the atoms only works for smaller, simpler polyatomic ions, such as ammonium, hydroxide, cyanide and even acetate. This does not work for polyatomic ions that have atoms that can carry different oxidation numbers, like sulfur or nitrogen. This is why they form "-ite" and "-ate" polyatomic ions with the same charge.