You have learnt what covalent bonding is, and how non-metals make covalent bonds. You also learnt how some covalent molecules can be small, or some can be giant covalent structures. In this article, you will learn about small covalent molecules, their properties, and some examples.
- In this article we will talk about simple molecules.
- We will start seeing some examples of simple molecules.
- To finish, we will see the general properties of simple molecules.
Simple molecules
Simple Covalent Molecules are small molecules in which atoms are held together by covalent bonds.
A Covalent Bond is a chemical bond in which elements share electron-pairs with each other. The atoms are held together by the strong electrostatic forces between the nuclei of the atoms and the shared pair of electrons.
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
Note that the dot and cross diagram only shows the electrons in the valence shell.
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.
Alkenes and Alkanes are not the same thing (notice the difference in spelling). Alkanes are compounds of Carbon and Hydrogen (called hydrocarbons) which only have single bonds between Carbon atoms. Alkenes are hydrocarbons which can have 1 or more double bonds between Carbon atoms.
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.
All covalent molecules exist in a definite shape. The position of all atoms in a molecule and their angle with respect to other atoms is fixed. There are several shapes that simple molecules may exist in. These shapes are determined according to the VSEPR theory.
VSEPR theory stands for Valence Shell Electron Pair Repulsion theory.
According to VSEPR theory, the shapes of covalent molecules depend on the number of bonds and the number of unshared electrons of an atom. The unshared electrons of an atom in a molecule hover around the atom in electron clouds, and influence the relative positions of various atoms that form that molecule. It is also possible to determine the bond angles and the length of bonds between any 2 atoms in a molecule.
A v-shaped water molecule. Vaia Originals
VSEPR theory and shapes of molecules is not part of the GCSE curriculum, but if this piques your interest, you can learn more in this article - Shapes of Molecules.
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
| Substance | Melting Point (oC) | Boiling Point (oC) | State at Room Temperature |
| Hydrogen (H2) | -259 | -253 | gas |
| Oxygen (O2) | -218 | -183 | gas |
| Methane (CH4) | -182 | -161 | gas |
| Nitrogen (N2) | -210 | -196 | gas |
| Chlorine (Cl2) | -101 | -35 | gas |
| Water (H2O) | 0 | 100 | liquid |
Water (H2O) is liquid at room temperature (no surprise) because it has a melting point of 0oC, which is higher than most simple covalent substances, but is still considered low.
The weak intermolecular forces a.k.a. van der waals forces increase as the size of the molecules increases. And as the van waals forces increase, more energy is required to overcome them, resulting in higher melting and boiling points.
Take the Halogen gases for example - Chlorine (Cl2), Bromine (Br2), and Iodine (I2). Out of the 3, Cl2 molecule is the smallest, then Br2 is bigger than Cl2, and I2 is the biggest. If what we just learnt about van der waals forces is true, the melting and boiling point should increase as we go from Chlorine to Bromine to Iodine. And indeed, it is true -
| Substance | Melting Point (oC) | Boiling Point (oC) |
| Chlorine (Cl2) | -101 | -35 |
| Bromine (Br2) | -7.2 | 58.8 |
| Iodine (I2) | 113.7 | 184.3 |
Therefore, Melting and Boiling points of simple covalent molecules always increase as we go down a group.
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.
Substances of simple molecules do not conduct electricity. That means water does not conduct electricity.
But have you ever come across a warning to never douse an electrical fire with water? Or that you should stay away from water when using electrical appliances like using a blow-dryer? Do not try to test this at home as it might result in electrocution!
That is because the water that you drink and use everyday does conduct electricity because of the impurities in it. The impurities may be salts or minerals present in it which form ions in the water. Ions in water act as charge carriers and conduct electricity.
Pure H2O, or distilled water would never conduct electricity.
Simple Molecules - Key takeaways
- Simple Covalent Molecules are small molecules in which atoms are held together by covalent bonds.
- A Covalent Bond is a chemical bond in which elements share electron-pairs with each other.
- Examples of simple molecules discussed in this article include - Hydrogen (H2), Chlorine (Cl2), Oxygen (O2), Water (H2O), Ammonia (NH3), Hydrogen Chloride (HCl), Methane (CH4), Ethene (C2H4).
- Simple molecules have low melting and boiling point. This is because molecules are held together by weak intermolecular forces / van der waals forces, which require only little energy to overcome.
- Simple molecules do not conduct electricity. This is because they have no net charge on them, and have no free electrons which can act as charge carriers.
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