Chemical bonding happens when two or more atoms join together to form a molecule. It is a general principle in science that all systems will try to reach their lowest energy level, and chemical bonding will only take place when a molecule can form that has less energy than its uncombined atoms. The three main types of bond are ionic, covalent, and metallic. These all involve electrons moving between atoms in various ways. Another, much weaker, type is the hydrogen bond.
Atoms consist of a nucleus containing positively charged protons, which is surrounded by an equal number of negatively charged electrons. Normally, therefore, they are electrically neutral. An atom can, however lose or gain one or more electrons, giving it a positive or negative charge. When one has an electrical charge, it is called an ion.
It is the electrons that are involved in chemical bonding. These particles are arranged into shells that can be thought of as existing at increasing distances from the nucleus. Generally, the further from the nucleus the shells are, 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 a limit of eight.
In most cases, it is only the electrons in the outermost shell that participate in bonding. These are often called the valence electrons. As a general rule, atoms will tend to combine with one another in such a way that they all achieve full outer shells, as these configurations usually have less energy. A group of elements known as the noble gases — helium, neon, argon, krypton, xenon, and radon — already have full outer shells and because of this, they 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 the American chemist Gilbert N. Lewis. In a Lewis structure, the valence electrons are represented by dots just outside the chemical symbols for the elements in a molecule. They show clearly where electrons have moved from one atom to another and where they are shared between atoms.
This type of chemical bonding takes place between metals, which easily give up electrons, and non-metals, which are keen to accept them. The metal gives the electrons in its incomplete outermost shell to the non-metal, leaving that shell empty so that the full shell below becomes its new outermost shell. The non-metal accepts electrons so as to fill up its incomplete outermost shell. In this way, both atoms have achieved full outer shells. This leaves the metal with a positive charge and the non-metal with a negative charge, so they are positive and negative ions that attract one another.
A simple example is sodium fluoride. Sodium has three shells, with one valence electron in the outermost. Fluorine has two shells, with seven electrons in the outermost. The sodium gives its one valence electron to the fluorine atom, so that the sodium now has two complete shells and a positive charge, while the fluorine has two complete shells and a negative charge. The resulting molecule — sodium fluoride — features two atoms with complete outer shells bonded together by electrical attraction.
Atoms of non-metals combine with one another by sharing electrons in such a way that they lower their overall energy level. This usually means that, when combined, they all have full outer shells. To take a simple example, hydrogen has just 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 in which both have a full outer shell.
It is often possible to predict how atoms will combine with one another from the number of electrons they have. For example, carbon has six, which means that it has a full first shell of two and an outermost shell of four, leaving it four short of a full outer shell. Oxygen has eight, and so has six in its outer shell — two short of a full shell. A carbon atom can combine with two oxygen atoms to form carbon dioxide, in which 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. This way, all three atoms have full outer shells containing eight electrons.
In a piece of metal, the valence electrons are more or less free to move around, rather than belonging to individual atoms. The metal therefore consists of positively charged ions surrounded by mobile, negatively charged electrons. The ions can be moved relatively easily, but are difficult to detach, due to their attraction to the electrons. This explains why metals are generally easy to bend but difficult to break. The mobility of the electrons also explains why metals are good conductors of electricity.
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 one another, forming bonds between the molecules.
Although these bonds are much weaker than ionic, covalent, or metallic bonds, they are very important. Hydrogen bonding takes place in water, a compound containing two atoms of hydrogen and one of oxygen. 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 the Earth.