Use adaptive quiz-based learning to study this topic faster and more effectively.

# Covalent bonds

## Covalent bond

In a covalent bond, two atoms share electrons with each other. This distinguishes covalent bonds from ionic bonds, in which electrons are donated by one atom and accepted by the other.

In covalent bonding, both atoms generally share an equal number of electrons. Each covalent bond contains one pair of electrons. Sharing of electrons allows atoms to gain a stable electron configuration.

A molecule is a group of two or more atoms bonded together covalently. Molecules of covalent compounds consist of different atoms while molecules of elements have only one type of atom.

Both water and oxygen are made up of molecules. Water molecules are made from hydrogen and oxygen while oxygen molecules are made from atoms of the same element.

In many cases, an atom does not share all of its valence electrons. These non-bonded electrons usually appear in pairs and are lone pairs of electrons.

In a water molecule, the oxygen atom has two lone pairs of electrons.

## Dot and cross diagrams

The sharing of electrons in covalent bonds can be illustrated using dot and cross diagrams.

The dots and crosses in the diagrams indicate valence electrons. Electrons represented by dots originate from different elements from those represented by crosses.

In reality, electrons are indistinguishable. The dots and crosses are for illustration purposes only.

It is not possible to tell which electron comes from hydrogen or carbon in the $\ce{C-H}$ bond of methane.

An atom usually shares as many valence electrons as needed to achieve a stable electron configuration.

Carbon bonds with four hydrogen atoms, allowing it to fill its valence shell. Each hydrogen also gains a full shell (2 electrons) by sharing electrons from carbon.

## Properties of covalent compounds

Covalent compounds are generally poor conductors of electricity and heat because they do not have free electrons or ions and their atoms are in fixed positions.

Water is a simple covalent molecule. The red sphere represents an oxygen atom while the white spheres represent hydrogen atoms.

The boiling and melting points of covalent compounds vary significantly but tend to be lower than those of ionic compounds.

No chemical bonds have to be broken to melt or boil covalent molecular substances. Energy is required to separate the molecules from each other.

## Giant covalent substances

A giant covalent structure (or a covalent network) has numerous atoms that are covalently bonded in a continuous 3-dimensional network.

Individual molecules cannot be distinguished in this arrangement, and the whole structure can be considered to be one large molecule. However, we can identify an empirical formula based on the repeating units.

The orderly arrangement of silicon and oxygen atoms gives the crystalline appearance of quartz (right).

Quartz is a giant covalent structure that has an empirical formula of $\ce{SiO2}$. Each silicon atom is shared between two oxygen atoms, and this group of atoms repeats throughout the giant covalent structure.

A molecule of $\ce{SiO2}$ can be observed as a single pure quartz crystal.

## Melting points of giant covalent substances

Giant covalent substances are vast structures held together by many strong covalent bonds.

The melting of these compounds therefore involves the breaking of the numerous covalent bonds holding the atoms together.

More energy is required to break bonds in a covalent network than to separate covalent molecules. This accounts for the high melting and boiling points of giant covalent substances.

To melt quartz, all the covalent bonds must break.

Quartz melts at over $1600^{\circ}\text{C}$, while diamond melts at about $4000^{\circ}\text{C}$!

## Diamond

A diamond is a giant covalent molecule made of carbon. Each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral shape as shown below.

The tetrahedral arrangement is repeated throughout the giant covalent structure of diamond. Cut and polished diamonds are shown on the right.

A diamond crystal with no impurities is a giant covalent structure consisting of these carbon atoms.

In diamond, all four valence electrons of the carbon atom is shared with other atoms. This means that there are no free electrons and therefore diamond is not able to conduct electricity.

Diamond is an extremely hard material because of its well-ordered, rigid giant covalent structure. Due to its hardness, diamond can be used as a cutting tool.

Diamond has very high melting and boiling points ($3550^{\circ}\text{C}$ and $4827^{\circ}\text{C}$) because a large amount of energy is required to break all the strong bonds in the giant covalent structure.

## Graphite

Graphite is a giant covalent substance made entirely of carbon but it has very different properties from diamond.

In graphite, each carbon atom is bonded covalently to three other carbon atoms in a planar configuration.

Each carbon atom shares only three of its four valence electrons. The remaining valence electron is delocalised. In other words, it is not associated with a particular covalent bond location.

The planes can slide over each other because there are no covalent bonds between one layer of carbon atoms and the next one. This makes graphite a soft compound.

In graphite, planes of carbon atoms are arranged in layers (left). Graphite is a black solid (right) that is commonly used as pencil lead.

Because of its softness, graphite is often used as a solid lubricant to reduce the friction between two surfaces.

The delocalisation of electrons allows graphite to conduct electricity.

## Comparison between diamond and graphite

Diamond and graphite are both giant covalent substances made entirely of carbon atoms. Both have chemical formula $\ce{C}$, but they do not have a molecular formula.

Diamond and graphite are allotropes of carbon. Allotropes are different forms of the same element due to differences in structure.

Molecular structure

• Diamond: Giant covalent structure, with each carbon covalently bonded to four other carbon atoms in a tetrahedral arrangement to form a rigid structure.
• Graphite: Giant covalent structure, with each carbon covalently bonded to three other carbon atoms in a hexagonal arrangement.

Hardness

• Diamond: Extremely hard. Due to rigid, tetrahedral arrangement of carbon atoms.
• Graphite: Soft. Layers of hexagonally arranged carbon atoms can slide over one another, as the layers are held together by van der Waals forces of attraction.

Melting and boiling points

• Diamond and graphite: Very high. A large amount of energy is required to break numerous, strong covalent bonds between carbon atoms.

Electrical conductivity

• Diamond: Insulator. Mobile electrons are absent. All four valence electrons are used in covalent bonds.
• Graphite: Conductor. Three out of four valence electrons are used for covalent bonding with other carbon atoms. Remaining valence electrons can be delocalised across the planes of carbon atoms.

## Polarity and covalent bonds

Polarity occurs in a molecule when two atoms share electrons unequally along a covalent bond.

Certain elements have a stronger tendency to attract electrons. When atoms of such elements bond with atoms with a weaker tendency to attract electrons, unequal sharing of electrons results.

The electrons will be closer to the atom with the stronger tendency to attract.

This results in a small, relatively negative charge on the atom with stronger attraction to the electrons, and a small relatively positive charge on the other atom.

Atoms of fluorine, oxygen, nitrogen, and chlorine exert stronger pulls, resulting in negative charges around them and positive charges elsewhere in the molecule.

Molecules containing any of these four atoms are thus usually polar.

Water is a polar molecule shown below with red indicating a negative side and blue indicating a positive side.

## Non-polar molecules

Molecules that have an equal distribution of charge are non-polar.

Molecules without fluorine, oxygen, nitrogen, or chlorine are usually non-polar.

Liquid alkanes such as pentane ($\ce{C5H12}$) are non-polar.

Some molecules containing the electronegative elements $(\ce{F},\ce{O},\ce{Cl},\ce{N})$ can be non-polar. This happens when the molecules are structurally balanced and the charge is evenly distributed.

Carbon dioxide ($\ce{CO2}$) is a non-polar molecule but each $\ce{C=O}$ bond is not. The bonds are placed such that the charge on both ends are approximately the same.

The image below shows some non-polar molecules.

From left to right, molecular structures of propane, methane, carbon dioxide and borotrifluoride are shown.

## Polarity and solubility

Solubility depends the polarity of substances being mixed.

Non-polar molecules tend to dissolve only in non-polar molecules. Polar molecules tend to dissolve only in polar molecules.

Non-polar molecules are able to interact with other similar molecules through van der Waals forces of attraction.

Polar molecules interact with each other through dipole-dipole interactions, where the positive end of one molecule interacts with the negative end of another molecule.

Polar and non-polar molecules do not interact strongly with each other due to the difference in the types of interactions they can make.

Polar and non-polar molecules usually do not dissolve in one another. Mixing these two types of molecules usually gives two distinct layers.

These solubility patterns account for why olive oil (non-polar) does not dissolve in water (polar).