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Naming Polyatomic Ions

The basic unit of any matter is atom which is composed of sub-atomic particles; electrons, protons and neutrons. Electrons are placed in certain energy levels whereas neutrons and protons are placed in the nucleus of the atom. An element consists of same kind of atoms. Atomic symbols are used to denote an atom. The total number of protons in an atom is called as atomic number.  Mass number is defined as total number of protons and neutrons of an atom. Atomic number must be written as sub-script of the atomic symbol and mass number must be written as super-script. An atom is neutral due to balance in positive and negative charge of electrons and protons respectively. If we add or remove any electron from atom, some charge induces over atom and these charged atoms are called as ions. If we add electrons to the atom, it creates negative charge to it and for anions.

Similarly removal of electrons to the atom increases the positive charge and form cation. Ions can be monatomic or polyatomic in nature. Ions with more than one atom are called as polyatomic ions whereas ions with one atom are called as monatomic ions. When an atom is ionized by gaining or losing electrons, it results the formation of monatomic ion. For example; chlorine atom consists of 17 electrons, 17 protons and 17 neutrons. If we add an electron to chlorine atom, it increases the number of electrons; 18 electrons whereas number of protons remain same. Hence one negative charge creates over chlorine atom and form an anion; Cl- which is also called as chloride ion. Here negative charge must be written as superscript to the atomic symbol.

In case of polyatomic ions, more than one atom is bonded together with some charge over all the atoms. So we can say that a polyatomic ion is a group of atoms which carries some charge either positive or negative over it. In these kinds of ions, total number of electrons is not equal to the total number of protons therefore overall charge is equal to the sum of the formal charges on each atom in the ion. For example, in hydroxide ion which is OH-, the negative charge is written as superscript on the left side of the formula. Hydroxide ion contains 1oxygen atom and 1hydrogen atom which are bonded with one covalent bond. It means two electrons are shared between O and H atom. Here oxygen atom contains 3 lone pairs of electrons and net charge on the hydroxide ion is minus one. Hence we can say that hydroxide ion has one extra electron.
Some of the polyatomic ions with their formulae are written below:

Formulas with Polyatomic Ions


How to Naming Polyatomic Ions?

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Ions in which charge is distributed over two or more atoms are called as polyatomic ions. For example phosphate ion is PO43-; in this ion there are four oxygen atoms boned with one center phosphorus atom. Here 3 minus charge is distributed over five atoms.

In other words there are three extra electrons which are shared by all the atoms of polyatomic ion. In phosphate ion, there are 29 valance electrons including P and O and 3 extra electrons which induce negative charge which is written outside the brackets and indicates that the charge is spread on the ion as a whole. 

Phosphate Ion
In polyatomic ions, there is a center atom which is placed in the middle of the ion and attached with other atoms. For example; in phosphate ion, phosphorus is center atom. Similarly in bicarbonate ion, carbon is the center atom.  In the naming or formula writing of polyatomic ions we must know about the number of atoms involve in polyatomic ion and also charge over them. The number of atoms must be written as sub-script and entire formula must be in parenthesis. In the naming of polyatomic ions, some of the names are adopted from their basic compound such as ammonium ion from ammonia, hydronium ion from water etc. Some of the names of polyatomic ions are as per their structure such as dihydrogen phosphate is H2PO3- whereas dihydrogen phosphate is H2PO4- . Some other examples are listed below.

Examples of Polyatomic Ions

Rules for Naming Polyatomic Ions

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Like other compounds we can follow some basic rules for the naming of polyatomic ions. Such as names of ion ends in either -ite or –ate; here –ite represents low oxidation state such as NO2- ion is named as nitrite ion whereas -ate represents high oxidation state such as NO3- ion is named as nitrate ion. The lowest oxidation state of ion indicated by hypo- such as ClO- ion is named as hypochlorite ion. On the contrary prefix per- is used for very highest oxidation state for example $ClO_{4}-$ ion is named as perchlorate ion. Some of the polyatomic ions which once thought to be monatomic ions are ended with suffix –ide such as hydroxide (OH-), cyanide (CN-), and peroxide ($O_2^{2-}$) ions. Some common polyatomic ions with their names are listed below.

Naming Polyatomic Ions

Nomenclature of Polyatomic Ions

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In the nomenclature of polyatomic ions, some common rules of addition of prefix and suffix are used. Many of the polyatomic ions are well known with their common names such as ammonium ion, cyanide ion etc. The negatively charged ions are usually ended with suffix whereas highest oxidation state and lowest oxidation states are better known with their prefix such as pyro-, hypo- etc. 

Examples of Naming Polyatomic Ions

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Examples of Naming Polyatomic Ions

Tips for Naming Polyatomic Ions

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So we can say that for the naming of polyatomic ions, there is no certain rule but we can follow common rules of naming of ionic compounds. Many of the polyatomic ions are known with their common names so we have to learn them as usual.
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