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Formulas of Ionic Compounds

  • Last Updated : 17 Aug, 2021
Geek Week

The study of the modern periodic table shows all the elements dhows their own kind of nature. Among them, most of the atoms of the elements cannot exist freely in a stable state. Hence, for the purpose of obtaining stability, they share their valence electron with the atoms of other elements. This sharing of electrons forms a bond and they obtain stability by following the octet rule. The present article discusses the same. The study covers ionic compounds, their structure, and the formula writing method.

What are Ionic Compounds?

Ionic compounds are formed when ions with opposing negative and positive charges create ionic bonds and form ionic compounds, which are exactly what they sound like: compounds comprised of ions. Because one atom’s loss or gain matches the loss or gain of the other, one atom basically ‘donates’ an electron to the other atom with whom it couples up.

Following are the characteristics of an ionic compound:

  • Ionic compounds are ion compounds.
  • Ionic compounds have high melting and boiling points. They appear strong and brittle.
  • They are primarily held together by the electrostatic forces between the charge distribution of the bodies.
  • There are ionic compounds that can be described. Each and every compound have their own formulas.

When positive and negative ions share electrons and create an ionic connection, ionic compounds formed. Because of the strong attraction between positive and negative ions, crystalline solids with high melting points are frequently formed. When there is a significant difference in electronegativity between the ions, ionic bonds occur instead of covalent ones. In an ionic compound formula, the positive ion, known as a cation, is stated first, followed by the negative ion, known as an anion. A balanced formula has a zero net charge or neutral electrical charge.

Writing Formulas for Ionic Compounds

Chemical formulae, which reflect the ratios of interacting elements found in an ionic solid or salt, can be used to define ionic compounds. Empirical formulae are commonly used to represent ionic solids. The elements are represented in formula notation by their chemical symbols, which are followed by numeric subscripts that show the relative ratios of the component atoms.

The precise formula for an ionic compound may be found by following two criteria: 

  1. The charge on the component ions can be computed based on the valence electron transfer required to satisfy the octet rule.
  2. The cations and anions are joined in such a way that an electrically neutral compound is formed.

The procedures for writing and evaluating the formula are as follows: 

  • Identify the cation (the part having a positive charge). Cations are mostly metals that are located on the periodic table’s left side. It is the most electropositive (least electronegative) ion. 
  • Identify the anion (the part having a negative charge). It is the ion with the greatest electronegative potential. Halogens and nonmetals are examples of anions. 
  • Keep in mind that hydrogen can have either a positive or negative charge.
  • Write the cation first, then the anion.
  • Adjust the cation and anion subscripts such that the net charge equals 0. To balance the charge, use the lowest whole number ratio between the cation and anion.

Examples of Ionic Compounds

1. Binary Ionic Compounds

Binary ionic compounds are quite simple. Ionic compounds are neutrally charged compounds composed of bonded ions, a cation, and an anion. The cation is positively charged, whereas the anion is negatively charged. When the two unite to form a compound, the compound has no overall charge. 

Example 1: The ionic formula for Sodium chloride.

Sodium chloride contains Na+ and Cl ions:

  • This is one positive charge and one negative charge.
  • The number of charges is already the same.
  • Thus, the formula is NaCl.

Example 2: The ionic formula for Magnesium oxide.

Magnesium oxide contains Mg2+ and O2 ions:

  • These are two positive charges and two negative charges.
  • The number of charges is already the same.
  • Thus the formula is MgO.

Following are some more examples of binary ionic compounds:

 Ionic CompoundFormulaCationAnion
1.Lithium fluorideLiFLi+F
2.Silver chlorideAgClAg+Cl
3.Calcium chlorideCaCl2Ca2+Cl
4.Iron(II) oxideFeOFe2+O2-
5.Aluminum chlorideAlCl3Al3+Cl
6.Iron(III) chlorideFeCl3Fe3+Cl

2. Polyatomic Ionic Compounds

Polyatomic ions are a collection of covalently linked atoms with an overall charge, therefore forming an ion. The hydroxide ion, for example, has the formula OH-1. Hydroxide is a chemical molecule composed of bonded oxygen and hydrogen. Hydroxide received an additional electron from someplace during the process of forming a compound, resulting in OH-1. When making ionic compounds with these polyatomic ions, handle them in the same way that you would with monatomic ions (only one atom).

Example 1: The ionic formula for Calcium hydroxide.

Calcium hydroxide contains Ca2+ and OH ions:

  • This is two positive charges and one negative charge.
  • To make the number of charges the same, we need one Ca2+ ion and two OH ions.
  • Thus, the formula is Ca(OH)2.

Following are some more examples of polyatomic ionic compounds:

 Ionic CompoundFormulaCationAnion
1.Ammonium hydroxideNH4OHNH4+OH
2.Ammonium nitrateNH4NO3NH4+NO3
3.Ammonium carbonate(NH4)2CO3NH4+CO3-2
4.Ammonium sulfate(NH4)2SO4NH4+SO4-2

Sample Problems

Problem 1: Define atoms and what are they made up of?


An atom is the smallest part of an element that can take part in a chemical reaction. Atoms are made up of three particles protons, neutrons, and electrons.

Problem 2: Who discovered an atom? Why it is not possible to see atoms with our naked eyes?


Atom was first introduced by the Greek philosopher Democritus in 450 B.C. We cannot see atoms with our naked eyes because they are too tiny particles i.e. its atomic radii is of the order 10-10 m to 10-9 m.

Problem 3: What is a molecule? What is wrong in saying ‘one mole of nitrogen’?


Molecules are the group of two or more atoms that forms the smallest and are held together by chemical bonds. We should say ‘one mole of nitrogen atoms’ or ‘one mole of nitrogen molecule’ because the above statement does not clarify whether we are discussing atoms or molecules of nitrogen.

Problem 4: What are ionic compounds? Why do ionic compounds have a high melting point?


Ionic compounds are the ions compounds that appear strong and brittle. Ionic compounds have a high melting point because there is a strong electrostatic force of attraction between the oppositely charged ions which needs a large amount of energy to break the strong bonding force between them.

Problem 5: Lithium Flouride is basic in nature? Why?


Lithium Flouride is a salt that is formed from the neutralization of weak hydrofluoric acid and strong base lithium hydroxide. So, they are considered to be basic in nature.

Problem 6: Which ionic compound is necessary for our body and why?


Sodium Chloride is an ionic compound that is essential for our body to maintain blood pressure, absorb and transport nutrients, and maintain the right balance of fluid.

Problem 7: What happens when we swallow sodium fluoride?


When a large number of fluoride gels are swallowed it causes pain, vomiting, or nausea they can also lead to serious problems like lowering our body’s level of calcium.

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