What’s Chemical Bonding?

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Chemical bonding occurs when two or more atoms join together to form a molecule. It is a general principle of science that all systems will try to reach their lowest energy level, and chemical bonding will only take place when a molecule can be formed that has less energy than its uncombined atoms. The three main bond types are ionic, covalent, and metallic. These all involve electrons moving between atoms in various ways. Another type, much weaker, is the hydrogen bond.

Atomic structure

Atoms consist of a nucleus containing positively charged protons, surrounded by an equal number of negatively charged electrons. Normally, therefore, they are electrically neutral. However, an atom can lose or gain one or more electrons, giving it a positive or negative charge. When one has an electric charge, it is called an ion.

It is the electrons that are involved in the chemical bond. These particles are arranged in shells that can be thought of as existing at increasing distances from the nucleus. Generally, the farther the shells are from the nucleus, the more energy they have. There is a limit to the number of electrons that can occupy a shell. For example, the first, innermost shell has a limit of two, and the next shell has a limit of eight.

In most cases, it is only the electrons in the outer shell that participate in the bond. These are often called valence electrons. As a general rule, the atoms will tend to combine with each other in such a way that they all reach complete outer shells, as these configurations usually have the least energy. A group of elements known as the noble gases – helium, neon, argon, krypton, xenon and radon – already have a complete outer shell and therefore do not normally form chemical bonds. Other elements will generally try to achieve a noble gas structure by giving, accepting, or sharing electrons with other atoms.

Chemical bonds are sometimes represented by something called a Lewis structure, named after American chemist Gilbert N. Lewis. In a Lewis structure, valence electrons are represented by dots just outside the chemical symbols of the elements in a molecule. They clearly show where electrons have moved from one atom to another and where they are shared between atoms.

Ionic bond
This type of chemical bond occurs between metals, which give up electrons easily, and non-metals, which are eager to accept them. The metal gives up the electrons in its incomplete outermost shell to the nonmetal, leaving that shell empty so that the entire underlying shell becomes its new outermost shell. The nonmetal accepts electrons in order to fill its incomplete outer shell. In this way, both atoms have reached complete outer shells. This leaves the metal with a positive charge and the nonmetal with a negative charge, so it’s positive and negative ions that attract each other.
A simple example is sodium fluoride. Sodium has three shells, with a valence electron in the outermost. Fluorine has two shells, with seven electrons in the outermost. Sodium gives its one valence electron to the fluorine atom, so that sodium now has two full shells and one positive charge, while fluorine has two full shells and one negative charge. The resulting molecule – sodium fluoride – has two atoms with complete outer shells bonded together by electrical attraction.

Covalent bond
Non-metal atoms combine with each other by sharing electrons in such a way as to lower their overall energy level. This usually means that when combined, they all have complete outer shells. To take a simple example, hydrogen has only one electron, in its first and only shell, which leaves it one short of a full shell. Two hydrogen atoms can share their electrons to form a molecule where both have a complete outer shell.
It is often possible to predict how atoms will combine with each other by the number of electrons they have. For example, carbon has six, meaning it has a full first shell of two and a full outer shell of four, leaving it four short of a full outer shell. Oxygen has eight, and so it has six in its outer shell: two fewer than a full shell. One carbon atom can combine with two oxygen atoms to form carbon dioxide, where the carbon shares its four electrons, two with each oxygen atom, and the oxygen atoms in turn each share two of their electrons with the carbon atom. Thus, all three atoms have complete outer shells containing eight electrons.

Metallic bonding
In a piece of metal, the valence electrons are more or less free to move around, rather than belong to individual atoms. The metal therefore consists of positively charged ions surrounded by negatively charged mobile electrons. Ions can be moved around relatively easily, but are difficult to detach due to their attraction to electrons. This explains why metals are generally easy to bend but hard to break. The mobility of electrons also explains why metals are good conductors of electricity.
Hydrogen bond
Unlike the examples above, hydrogen bonding involves bonding between, rather than within, molecules. When hydrogen combines with an element that strongly attracts electrons, such as fluorine or oxygen, the electrons are pulled away from the hydrogen. This results in a molecule with an overall positive charge on one side and a negative charge on the other. In a liquid, the positive and negative sides attract, forming bonds between molecules.
While these bonds are much weaker than ionic, covalent, or metallic bonds, they are very strong. Hydrogen bonding occurs in water, a compound containing two hydrogen atoms and one oxygen atom. This means that more energy is required to convert liquid water into a gas than would otherwise be the case. Without hydrogen bonding, water would have a much lower boiling point and could not exist as a liquid on Earth.

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