Is CaO Ionic or Covalent?

Calcium belongs to the family of alkali earth metals, which occupies the second group in the periodic table. Calcium forms numerous compounds of great importance, one of which is calcium oxide (CaO).

Commonly known as “quicklime” or “burnt lime”, calcium oxide, a white crystalline compound, is one of the widely used chemical compounds. It is regarded as one of the oldest compounds known to mankind.

The chemistry of calcium oxide is very interesting. At the same time, calcium oxide often confuses people with some of its properties and nature. One such aspect is the type of chemical bonding.

Is calcium oxide ionic or covalent? Calcium oxide is considered an ionic compound as it comprises a metal (calcium) and a nonmetal (oxygen). Secondly, due to the large difference of electronegativities (∆E=2.4), the calcium atom easily loses two of its outermost shell electrons, which are in turn, gained by the oxygen atom. Since, the loss of electrons by one and gain by another atom form an ionic bond between two elements, calcium oxide (CaO) is also ionic.


Types of Chemical Bonds

When looking at the periodic table, we can note that there are broadly three types of elements in the whole table. These are metals, nonmetals, and metalloids.

Every element wants to acquire stability, for which it performs bonding. Depending upon the types of elements interacting, a specific chemical bond is formed between them. (This notion does bring some exceptions, where electronegativities of elements are almost similar)

Generally, if a metal interacts with metal, or a nonmetal interacts with a nonmetal, or a metalloid interacts with a nonmetal, a covalent bond is formed between them.

On the other hand, when a metal interacts with nonmetals, an ionic bond is formed between the two.

Try to classify the following compounds on the basis of the chemical bond present between them by recognizing the type of elements interacting (exceptions not included):

H2O? It is covalent

NH3? It is covalent

MgCl2? It is ionic

NaCl? It is ionic

HCl? It is covalent

SiO2? It is covalent (as silicon is a metalloid)

Ionic and Covalent Bonds


What is an ionic bond?

Whenever two elements are interacting and there occurs a loss of electrons by one element and gain of the same electron by another, the bond so formed is known as an ionic bond.

This loss and gain of electrons happen due to the large difference in electronegativities of the two elements. Therefore, the element with higher electronegativity is strong enough to attract electrons of an element with lower electronegativity.

The element which is losing electrons is known as “donor”, while an element with higher electron affinity, which is gaining electrons, is known as an “acceptor”.

For a compound to be ionic, the difference between the electronegativities (∆E) of donor and acceptor should be greater than or equal to 2.0

Since, in the case of metals (which are placed on the left side of the periodic table) and nonmetals (which are placed on the right side of the periodic table), the difference of electronegativities is usually more than 2.0.

Hence, they tend to form ionic bonds, where metal acts as donor and nonmetal acts as acceptor element. Some examples of this ionic interaction are NaCl, MgCl2, AlCl3, CaO.


What is a Covalent Bond?

Unlike ionic bonds, covalent bonds are formed due to the “sharing” of electrons between two elements. There is not any loss or gain of electrons.

The shared pair of electrons is generally the electrons present in the outermost shell of atoms. These are known as “bonding pairs of electrons” as they join or bring together atoms of two elements by forming covalent bonds between them.

Sharing of electrons between elements occurs due to small differences in electronegativities. Therefore, neither of the two is strong enough to attract the shared electrons towards itself. Hence, they settle upon sharing electrons with each other.

For a compound to be covalent, the difference between the electronegativities (∆E) of donor and acceptor should be smaller than 2.0. If the difference is less than 1.7, a pure covalent bond is formed.

If the difference between electronegativities is between 1.7 and 2.0, the bond formed is still covalent, rather it is “polar covalent”, as one element is slightly stronger than the other.

Since in cases of interaction between two metals or two nonmetals or a metalloid and nonmetal, the difference of electronegativities is less than 2.0, hence they tend to form covalent bonds.

Some examples of this covalent interaction are CO2, SiO2, NO2, CCl4, H2O.

H2O Covalent bond

Do you know? There are many differences between the physical and chemical properties due to ionic and covalent bondings. Like, the melting points and boiling points of ionic compounds are significantly higher than that of covalent compounds.

Ionic compounds, when dissolved in water, can also conduct electricity due to the presence of free ions, while covalent compounds do not.


Calcium Oxide

Calcium oxide, also known as “lime”, “quicklime” or “burnt lime” is a white, alkaline, crystalline compound at room temperature. At varying temperatures, it can be present in the form of granular powder or gray-white lumps also.

Calcium oxide is an extremely stable compound, as it has a very high melting point of 2572°C. It is insoluble in water because it produces calcium hydroxide in reaction to water. It is soluble in acids.

It is one of the most widely and commonly used compounds since the medieval age. Naturally, it is formed in nature via the decomposition of materials containing calcium carbonate (CaCO3), such as seashells, limestones.

The decomposition reaction produces calcium oxide and evolves carbon dioxide gas.

CaCO3 → CaO + CO2

Do you know? The reaction for the formation of calcium oxide is said to be the first-ever chemical reaction known to the human race. Its evidence dates back to the stone age.

CaO Formation


Ionic Bonding in Calcium Oxide

The calcium oxide (CaO) molecule is composed of one atom of calcium (with two electrons in the outermost shell) and one atom of oxygen (with six electrons in the outermost shell).

Here are the electron configurations of calcium and oxygen in their ground state.

Ca (atomic number = 20): 1s2, 2s2 2p6, 3s2 3p6, 4s2

O (atomic number = 8): 1s2, 2s2 2p4

There is a huge difference in electronegativities of calcium and oxygen: E (Ca) = 1.0 and E (O) = 3.5. The difference, ∆E = 3.5 – 1.0 = 2.5.

As the difference is greater than 2.0, the interaction between calcium and oxygen leads to the formation of ionic bonds, where calcium loses two electrons from its outermost shell and oxygen gains two electrons.

After losing two electrons, calcium acts as the cation (Ca2+)and with the gaining of two electrons, oxygen acts as an anion (O2-). Here are the electronic configurations of calcium and oxygen in an ionic state:

Ca2+ (atomic number 20): 1s2, 2s2 2p6, 3s2 3p6

O2- (atomic number 8): 1s2, 2s2 2p6

Here, you can see both the atoms have acquired stability by completing 8 electrons in their outermost shells and reaching the nearest noble gas configuration.

Calcium has done it by losing two electrons, whereas oxygen has achieved it by gaining two electrons. This is how an ionic bond is formed between them.

Do you know? Calcium oxide should be treated with safety as most of its reactions are extremely vigorous. On exposure to calcium oxide, your eyes and skin can experience irritation and its inhalation can cause sneezing and coughing.



Uses of Calcium Oxide

Calcium oxide serves many important roles in various fields such as:

  1. It is used in the manufacturing of cement, paper, and also in high-grade steel and iron.
    Because of its alkaline nature, it is used in various acid-base reactions, such as in manufacturing caustic soda (NaOH)
  2. It is used in insecticides and pesticides.
  3. It is also used in the medical field.
  4. In laboratories, it is used as a reagent for precipitation reactions and dehydration reactions.
  5. Being alkaline, it is used as a food additive, where it behaves as an acidity regulator.
  6. Calcium oxide reduces soil acidity and improves soil quality and fertility.

Do you know? Calcium oxide is poured into lakes that have become acidic with rainwater containing nitric acid and sulfuric acid. Due to its alkaline nature, it neutralizes these acids.



With this article, we learned about the types of elements and types of bonds formed due to their interaction with each other.

We studied how ionic and covalent bonds are formed and how electronegativity plays a role in determining the type of bonding. Electronegativity differences more than 2.0 correspond to ionic bonds and less than 2.0 correspond to covalent bonds.

Further, we studied in detail the ionic bonding in CaO and uses of the same in various fields.

Happy learning!

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