How to draw dot and cross diagram of Covalent Molecules| Step-by-step Explanation [Online Video]

At O levels, students must be able to describe, using dot-and-cross diagrams, the formation of covalent bonds between non-metallic elements.


In this video, we will go through in detail, the drawing of 5 different covalent substances, namely hydrogen, oxygen, water, methane and carbon dioxide. From there, we will determine the structural formula of each substance. Knowing how to write accurate structural formula is particularly important when students deal with Organic Chemistry.

Covalent Bond

Covalent bond is formed between atoms of non-metals. Hence, covalent bond is the bond formed between atoms that share electrons.

After bonding, each atom attains the noble gas electronic structure/configuration.

Do note that only the valence electrons in the outermost shell are involved. Atoms want to take part in chemical reaction because they want to achieve noble gas structure or noble gas configuration.

Atoms of noble gases are unreactive or stable.


Elements and compounds with covalent bonds are called molecules. Hence, a molecule is a group of two or more atoms held together by covalent bonds.


When one pair of electrons is shared, a single bond is formed. For example, molecules like hydrogen, chlorine, fluorine, water, ammonia have single bonds between their atoms.

When two pairs of electrons are shared, a double bond is formed. For example, molecules like oxygen and carbon dioxide have double bonds between their atoms.

We can clearly see the number of bonds in a structural formula of the substance.

Topic: Chemical Bonding, O Level Chemistry, Singapore

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