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Polyatomic Ions

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  • Last Updated : 04 Aug, 2022
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An ion is a chemical entity that has a positive or negative charge of a certain magnitude. The term ‘ion’ can refer to atoms or molecules that have non-zero net charges attached to them. As a result, all ions contain either more protons than electrons in their overall atomic or molecule structure or more electrons than protons in their atomic/molecular structure. Ions with a higher number of protons than electrons are known to have a net positive charge. These ions are typically known as cations. Ions with a greater number of electrons than protons, on the other hand, are known to have a net negative charge. These ions are frequently referred to as anions.

How are Ions formed?

Ions can be prepared in a variety of ways. For example, spontaneous collisions between molecules in a liquid or gaseous fluid can result in one of an atom’s/electrons molecule’s being knocked off. As a result, a positively charged ion and a free electron are formed. Physical ionisation is the most common type of ionisation. The liberated electron could potentially bind to another atom or molecule, resulting in the production of a new negatively charged anion.

Chemical interactions are another major way in which ions can be produced. When an ionic compound, such as salt, is dissolved in a suitable solvent, the atoms that make up the salt dissociate and generate free ions. When common salt, commonly known as sodium chloride, dissolves in water, it dissociates to produce sodium cations and chloride anions. It should be noted that sodium cations are represented by the symbol Na+, whereas chloride anions are represented by the symbol Cl.

Polyatomic Ions

A polyatomic ion, also known as a molecular ion, is a covalently bound set of two or more atoms, or a metal complex, that behaves as a single unit and has a net charge greater than zero. This chemical species is an ion, as opposed to a molecule, which has a net charge of zero. 

The hydroxide ion, which is made up of one oxygen atom and one hydrogen atom and has a net charge of one, is a simple example of a polyatomic ion. Its chemical formula is OH

An ammonium ion, on the other hand, has a charge of +1 and is composed of one nitrogen atom and four hydrogen atoms; its chemical formula is NH4+. In acid-base chemistry and the production of salts, polyatomic ions are frequently used. Polyatomic ions are frequently thought of as the conjugate acid or base of a neutral molecule.

Structure of Polyatomic ions

Polyatomic ions can be thought of in terms of monatomic ions. An atom that has been ionised by gaining or losing electrons is known as a monatomic ion. The ion has a net charge because the total number of electrons in the nucleus is not equal to the total number of protons. As a result, we have additional electrons in the case of a negatively charged anion or not enough electrons in the case of a positively charged cation when compared to the neutral atom. A neutral chlorine atom, for example, has an atomic number of 17, meaning it possesses 17 protons and 17 electrons. The chloride anion is formed when a neutral atom gains an additional electron. The chloride anion now has 17 protons and 18 electrons after gaining an electron. The ion has a net charge of one because there is one extra electron compared to the number of protons.

A polyatomic ion, on the other hand, might be thought of as a molecule that has been ionised by gaining or losing electrons. Because the entire number of electrons in the molecule is not equal to the total number of protons in the molecule, the group of covalently bound atoms in a polyatomic ion has a net charge.

Consider the polyatomic ion OH, which is also known as hydroxide. The dot structure of the hydroxide ion is seen on the left. It is made up of one oxygen atom and one hydrogen atom. The single line connecting them indicates the covalent bond, which is made up of two electrons shared by H and O. The dots around O represent lone electron pairs. The oxygen in hydroxide has three lone pairs of electrons, for a total of six lone pair electrons. The net charge on the hydroxide ion is shown by enclosing the complete dot structure in square brackets and placing the charge on the upper right. We can observe that hydroxide has a 1- charge, which means that the ion contains one more electron than the nucleus of a hydrogen atom plus an oxygen atom.

The Lewis dot structure for the hydroxide ion

Examples of Polyatomic Ions

  • Polyatomic Cations: The majority of frequent polyatomic anions are oxyanions, which are conjugate bases of oxyacids (acids derived from the oxides of non-metallic elements). For example, the sulfate anion, SO4-2, is derived from H2SO4, which can be regarded as SO3 + H2O. 
  • Polyatomic Anions: Ammonium (NH4) is the only polyatomic ion that is a cation.

Formula

Name

NH4+ Ammonium
NO2 Nitrite
NO3 Nitrate
SO32- Sulphite
SO42- Sulphate
OH Hydroxide
CN Cyanide
CO32- Carbonate

Frequently Asked Questions

Question 1: Which is an ionic compound?

Answer:

Ionic compounds are just that: they’re made up of ions. These ions are electron-gaining or electron-losing atoms with a net positive or negative charge. Metals lose electrons and become cations when they have a net positive charge. Non-metals tend to acquire electrons, resulting in anions with a net negative charge.

Question 2: What are the parts of an ionic compound?

Answer:

Ionic compounds are made up of ions, which are charged particles that occur when an atom (or group of atoms) acquires or loses electrons. A cation is a positively charged ion, while an anion is a negatively charged ion.

Question 3: What is a simple ion?

Answer:

Simple ions are ions that are made up of only a single atom.

Question 4: What is a compound ion?

Answer:

Compound ions are ions that are produced by joining together groups of atoms.

Question 5: What is a polyatomic ion?

Answer:

Polyatomic ions can be thought of in terms of monatomic ions. An atom that has been ionised by gaining or losing electrons is known as a monatomic ion. The ion has a net charge because the total number of electrons in the nucleus is not equal to the total number of protons.

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