Also called a molecular ion, a polyatomic ion is a charged species that comprises 2 or more atoms joined together by chemical bonds. The “poly” in polyatomic denotes that the structure is composed of many items; in this case, 2 or more atoms.
Formerly, polyatomic ions used to be referred to as radicals and sometimes as radical groups. However, in the modern era, radicals refer to free radicals, which are not always charged species.
The hydroxide ion (OH-) is one good example of a polyatomic ion. It consists of a hydrogen atom and an oxygen atom, and has a negative charge of one (-1). The ammonium ion (NH4+) is another good example, with a nitrogen atom coupled up with 4 hydrogen atoms; it has a positive charge of one (+1).
Polyatomic ions are very important in chemistry, as most ionic compounds contain them. Virtually everything, from water to food to acids and bases, contains polyatomic ions.
Salt formation and acid-base chemistry are some of the areas where polyatomic ions are most useful. We normally consider polyatomic ions the conjugate acid or base of neutral molecules. For instance, in sulfuric acid (H2SO4), the polyatomic bisulfate anion (HSO4-) is the conjugate base.
Now let’s see how you can memorize polyatomic ions.
If you’re a science student, you will likely come across polyatomic ions regardless of whether you’re in high school or college. The problem is that they can be so many that grasping them all becomes a huge challenge. And, you have to remember the name of the ion and the ionic charge associated.
We are going to help you bypass the pain that comes with cramming and assist you in memorizing the polyatomic ions practically and successfully.
Let’s start with the suffixes.
There’s a certain pattern related to the suffixes of polyatomic ion names. For instance, oxygen-related anions (oxyanions) end with the prefixes “ite” or “ate”. To memorize oyanions properly, you need to know the variation between “ite” and “ate”. Those ending with “ate” have an extra oxygen atom whereas those ending with “ite” have 1 less oxygen atom. E.g. the sulfate ion (SO42-) has 4 oxygen atoms while the sulfite ion (SO32-) have 3 oxygen atoms.
Note the subscript after the oxygen (O) atom; it denotes the number of oxygen atoms we have in that polyatomic ion.
Just like the suffixes, the prefixes are used to name polyatomic ions refer to the values of oxygen atoms in the structure. There are 2 most important prefixes – per and hypo. If the prefix is “per”, then the ion contains one oxygen atom over the ion that has “ate” as its suffix. For instance, perchlorate (ClO4-) has 4 oxygen atoms, that is, an oxygen atom over chlorate (ClO3-).
On the other hand, if an ion has “hypo” as its prefix, that means it has an oxygen atom less than the ion with the suffix “ite”. For instance, hypochlorite (ClO-) has just one oxygen atom, that is, one oxygen atom less than chlorite (ClO2-), which has 2.
Hydrogen atoms add a positive charge to polyatomic ions. Therefore, the more the hydrogen atoms in a polyatomic ion, the less its negative charge. If, for example, we’re comparing dihydrogen phosphate with hydrogen phosphate, and we know that HPO4 has a charge of -2, then we know the charge of H2PO4 is -1, as the other -1 is cancelled out by the extra hydrogen atom.
Phosphorus and sulfur are the two elements playing central roles in acidic polyatomic ions. Keep in mind these 2 rules:
Acids whose names contain “or” have oxygen and phosphorus, for instance phosphoric acid (H3PO4).
Acids whose names have “ur” contain sulfur, for example hydrosulfuric acid (H2S).
The table below is meant to help you memorize polyatomic ions properly by applying what we’ve talked about above.
Polyatomic Ions CHERT
Bicarbonate (Hydrogen Carbonate)
Bisulfate (Hydrogen Sulfate)
Bisulfite (Hydrogen Sulfite)
Frequently Asked Question (FAQs)
1. How are polyatomic ions formed?
Polyatomic ions are formed when atoms react with each other and get united through various chemical bonds, such as ionic, metallic or covalent.
2. Are polyatomic ions neutral?
No. They normally have a charge, as the atoms have either gained or lost electron(s). Most of the inorganic ones have a negative charge.
3. What does it mean for a polyatomic ion to have a negative charge?
Positive charges are brought about by protons while negative ones are brought about by electrons. When a polyatomic ion has a negative charge, it means the ion has more electrons in the group than protons.
4. What’s the difference between polyatomic and monatomic ions?
Monoatomic ions have just one ionized atom, like Cl- or Na+ while polyatomic ions have more than one atom in the structure, like NaOH-, which is composed of sodium, oxygen, and hydrogen ions all bonded together.
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