Simple Molecules

Simple molecules

Small covalent molecules or simple covalent molecules are molecules consisting of just a few atoms. They have properties that are very different from other types of covalent molecules. This is the result of how the molecules are formed, and how individual molecules interact with each other. Atoms form covalent bonds to achieve a stable electronic configuration. A stable electronic configuration is when the valence shell (the outermost electron shell) of an atom is completely filled with electrons. You will see how this happens when we discuss the examples of simple molecules.

Simple Molecules: Examples

You have already come across some of the simple molecules before. Let us also illustrate them using a dot and cross diagram. A dot and cross diagram is useful in illustrating covalent bonds between atoms. Dots represent the valence electrons of one atom, while crosses represent the valence electrons in the other atom.

Hydrogen Molecule

Written as H₂. The Hydrogen atom has the electronic configuration of 1s1. It has only 1 electron. The 1s orbital can have a maximum of 2 electrons. Therefore, Hydrogen needs 1 more electron to have a stable electronic configuration. So, 2 Hydrogen atoms share 1 electron with each other so both of them have 2. This is illustrated in the following diagram.

Fig. 1: Hydrogen Molecule | IGCSE Chemistry

Chlorine Molecule

Chlorine has electronic configuration of [Ne] 3s² 3p⁵ . It has 7 electrons in its valence shell (the 3rd shell) and needs just 1 more to complete its octet. So, 2 Chlorine atoms share 1 electron with each other so they both have 1 extra. This is illustrated in the following figure -

Fig. 2: Chlorine Molecule | IGCSE Chemistry

Hydrogen Chloride

Also known as Hydrochloric acid. Both Hydrogen and Chlorine need 1 electron to complete their valence shell as you saw in previous two examples. So, both atoms share 1 electron with each other and form a covalent bond.

Oxygen Molecule

Written as O₂ . Oxygen has the electronic configuration of [He] 2s² 2p⁴ . It has 6 electrons in its valence shell (the 2nd shell) and needs 2 more to complete its octet. So, two oxygen atoms share 2 electrons each, with each other, and form a double covalent bond. This is the first example of double bonds and is illustrated in the diagram below -

Dihydrogen Monoxide (Water)

Don't worry, Dihydrogen Monoxide (H₂O) is just the chemical name for water that you drink! You have already seen that Oxygen needs 2 more electrons to complete its octet, and Hydrogen needs 1 electron to completely fill its valence shell. So, 1 atom of Oxygen forms single covalent bonds with 2 Hydrogen atoms, and everyone is happy! This is shown in the diagram -

Ammonia

Ammonia is NH₃ .Nitrogen forms 3 single bonds with 3 Hydrogen atoms. Nitrogen has the electronic configuration of [He] 2s² 2p³. It has 5 electrons in its valence shell and needs just 3 more to complete its octet, thus sharing 3 electrons with 3 Hydrogen atoms.

Methane

Methane is an Alkane. You will learn more about Alkanes in Organic Chemistry. Methane is written as CH₄. It is a molecule consisting of 1 Carbon atom and 4 Hydrogen atoms. Carbon has the electronic configuration of [He] 2s² 2p² and needs 4 more electrons to complete its octet, so it forms 4 single covalent bonds with 4 different Hydrogen atoms, like so -

Ethene

Ethene is an Alkene. you will learn more about Alkenes is Organic Chemistry.

In the example of Methane, you learnt that Carbon needs 4 more electrons to complete its octet. So, it should be able to make double bonds too, right? Correct. In Ethene, 2 Carbon atoms make double bonds by sharing 2 of their electrons each, with each other. Now, they both need 2 more electrons to complete their octet, which they do by making single bonds with Hydrogen atoms. Ethene is written as C₂H₄.

Simple Molecules: Properties

Substances of simple molecules have some properties which can be seen across all substances made up of such molecules.

Low melting and Boiling Point

Atoms in simple molecules are held together by covalent bonds which are very strong, which take a lot of energy to break. The molecules themselves are held together by weak intermolecular forces, called van der waals forces. To melt or boil a substance, it is only required to break the van der waals forces, which require little energy to break. This is why most substances of simple molecules are gaseous at room temperature.

Fig. 9: Weak intermolecular forces / van der waals forces between water molecules | BBC

Water (H₂O) is liquid at room temperature (no surprise) because it has a melting point of 0^oC, which is higher than most simple covalent substances, but is still considered low.

Non-Conductivity of Electricity

Simple molecules have atoms bonded with covalent bonds. A covalent bond is a shared pair of electrons between atoms. Atoms share electrons to complete their octets in their valence shell and achieve a stable electronic configuration. This process does not ensue in any resultant charge to reside on the molecules, nor result in any free electron or ion which can act as a free charge carrier. Therefore, substances with simple molecules do not conduct electricity.