A Level Chemistry Revision: Physical Chemistry – Bonding

Chemical reactions are about breaking and making bonds between atoms, which changes the way atoms share electrons. Bonds can be ionic, covalent, polar, or metallic. Bonding determines the molecular structure and physical properties of a substance. Chemical bonding is determined by various factors, like the number of valence electrons and the ionisation energy.

When you’re revising for A level chemistry, it’s crucial to have a clear understanding of the underlying principles of bonding. You should be able to predict chemical reactions based on how substances pair up with each other, which is determined by bonding or the sharing of electrons. You’ll be able to do this if you know the concepts of orbitals, energy levels, and electronegativity.

What is Chemical Bonding?

Chemical bonding is a long-lasting attraction between atoms, ions or molecules that results in the creation of distinct and stable chemical compounds. Chemical bonds can be formed in various ways, such as through the electrostatic attractive force between ions that have opposite charges, which form ionic bonds and crystalline structures of salt compounds. Bonds can also form through the equal or almost equal sharing of electrons between atoms, which is known as covalent bonding.

Bonding strengths vary depending on the type of bond. For example, ionic bonds are stronger than covalent bonds because of the electronegativity of the atoms. Similarly, primary bonds, such as covalent bonds, are stronger than secondary bonds, such as dipole-dipole interactions and the London dispersion force. Secondary bonds are not lasting or stable bonds.

Chemical bonding involves at least two atoms of either the same or different elements forming a molecule or a compound. For example, nitrogen exists in nature as a molecule consisting of two atoms of the same element, having triple covalent bonds. Electrons are absolutely shared equally in this type of bond. In other covalent bonds, the electrons are slightly nearer to the atoms with stronger electronegativity.

What Are the Basic Principles of Bonding?

When you’re revising the physical chemistry of bonding for A level, it’s necessary to first have a good grasp of the basic principles behind it. For instance, a central principle in chemical bonding is electronegativity. Electronegativity is a way of measuring the strength of an atom in terms of attracting electrons for bonding, relative to other atoms.

Electronegativity

You can easily see the pattern of the way the elements are arranged in the periodic table. Linus Pauling, two-time winner of the Nobel Prize, introduced the concept of electronegativity. He assigned a value of slightly less than 1.0 for alkali metals and a maximum value of 4.0 for fluorine:

As you can see in the illustration, the electronegativity values of the elements increase from  left to right. Meanwhile, the values decrease from top to bottom within a group of elements. The alkali metals and the alkaline earth metals have the tendency to lose electrons, thereby gaining the electron configuration of an inert gas.

Therefore, the elements in groups I and II have the least electronegative atoms. These elements are actually considered as electropositive. On the other side of the periodic table, the halogens, belonging to group VII, have the extreme tendency to gain electrons, and settle into an inert gas configuration.

Electron Configuration

The electron configuration is the distribution of electrons in an atom among its orbitals. This is important in determining the possible chemical bonds.

The electron configurations of elements are usually written based on the groupings and preceding elements in the group. This is necessary to minimise space. For instance, the electron configuration of xenon can be written as the continuation of the electron configuration of krypton:

[Kr] 4d¹⁰5s²5p⁶

Without indicating krypton, the full electron configuration of xenon can be written as:

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶

You can use a mnemonic device, like the illustration below, to write the electron configuration of an element:

Source

The electron configuration of an element is very useful for the following purposes:

• You can determine the valency of an element or how many electrons are in its outer shell. This is crucial in chemical bonding
• You can predict the chemical and physical properties of a group of elements. Elements with similar electron configurations have similar properties. Case in point: the inert noble gases
• You can use the configuration to interpret and analyse the spectra emitted by the elements

The Octet Rule

When it comes to chemical bonding, most of the elements follow the octet rule. This means that in order for an atom of an element to bond with another atom of another element, it must have contact to eight valence electrons. This will fill up the valence shell exactly, resulting in a stable bond.

Filling up the valence shells with eight electrons makes the atom stable and non-reactive. This is the reason why the noble gases are inert. These gases do not need to transfer electrons or receive electrons. In contrast, alkali metals are very reactive because they have only one valence electron in their outer shells.

What Are the Types of Bonding?

There are four types of chemical bonds: ionic bonds, covalent bonds, polar bonds, and hydrogen bonds. 

These types of bonds vary in strength, and the molecular structures that they form also vary. Consequently, the chemical properties and physical characteristics of the compounds they form are largely based on the type of bonds.

• Ionic bond: This tends to be the strongest bond because of the coulombic attraction between the oppositely charged ions. This type of bond is totally asymmetrical because one ion gets all the valence electrons while the other ion is stripped of its valence electrons. The strength of the bond is directly dependent on the charge differences of the ions. Salts and minerals have ionic bonds.

• Covalent bond: This involves the sharing of the valence electrons. However, the sharing is not perfectly symmetrical, except for atoms of the same element that form molecules like nitrogen molecules. Most of the elements follow the octet rule when forming covalent bonds.

• Polar bond: Polar bonds are a type of covalent bond in atoms with significantly different electronegativities. This results in the slightly unbalanced distribution of charges. As a result, parts of the molecule are either slightly positive or slightly negative. One of the simplest and most common substances with polar bonds is water.

• Hydrogen bond: This is an important type of bond that has a special role in organic chemistry and biochemistry. It’s crucial in the various processes of biological organisms, such as defining the shape of DNA molecules. It makes water an excellent solvent for many substances that are essential to life. It’s a type of intermolecular force that forms a special dipole-dipole attraction of a hydrogen atom with a strongly electronegative atom near another electronegative atom that has one pair of electrons.