Studies of molecular structure

 

Studies of molecular structure

The chemical properties of a substance are a function of its structure, and the techniques of X-ray crystallography now enable chemists to determine the precise atomic arrangement of complex molecules. A molecule is an ordered assembly of atoms. Each atom in a molecule is connected to one or more neighbouring atoms by a chemical bond. The length of bonds and the angles between adjacent bonds are all important in describing molecular structure, and a comprehensive theory of chemical bonding is one of the major achievements of modern chemistry. Fundamental to bonding theory is the atomic–molecular concept.

Atoms and elements

As far as general chemistry is concerned, atoms are composed of the three fundamental particles: the proton, the neutron, and the electron. Although the proton and the neutron are themselves composed of smaller units, their substructure has little impact on chemical transformation. As was explained in an earlier section, the proton carries a charge of +1, and the number of protons in an atomic nucleus distinguishes one type of chemical atom from another. The simplest atom of all, hydrogen, has a nucleus composed of a single proton. The neutron has very nearly the same mass as the proton, but it has no charge. Neutrons are contained with protons in the nucleus of all atoms other than hydrogen. The atom with one proton and one neutron in its nucleus is called deuterium. Because it has only one proton, deuterium exhibits the same chemical properties as hydrogen but has a different mass. Hydrogen and deuterium are examples of related atoms called isotopes. The third atomic particle, the electron, has a charge of -1, but its mass is 1,836 times smaller than that of a proton. The electron occupies a region of space outside the nucleus termed an orbital. Some orbitals are spherical with the nucleus at the centre. Because electrons have so little mass and move about at speeds close to half that of light, they exhibit the same wave–particle duality as photons of light. This means that some of the properties of an electron are best described by considering the electron to be a particle, while other properties are consistent with the behaviour of a standing wave. The energy of a standing wave, such as a vibrating string, is distributed over the region of space defined by the two fixed ends and the up-and-down extremes of vibration. Such a wave does not exist in a fixed region of space as does a particle. Early models of atomic structure envisioned the electron as a particle orbiting the nucleus, but electron orbitals are now interpreted as the regions of space occupied by standing waves called wave functions. These wave functions represent the regions of space around the nucleus in which the probability of finding an electron is high. They play an important role in bonding theory, as will be discussed later.

Each proton in an atomic nucleus requires an electron for electrical neutrality. Thus, as the number of protons in a nucleus increases, so too does the number of electrons. The electrons, alone or in pairs, occupy orbitals increasingly distant from the nucleus. Electrons farther from the nucleus are attracted less strongly by the protons in the nucleus, and they can be removed more easily from the atom. The energy required to move an electron from one orbital to another, or from one orbital to free space, gives a measure of the energy level of the orbitals. These energies have been found to have distinct, fixed values; they are said to be quantized. The energy differences between orbitals give rise to the characteristic patterns of light absorption or emission that are unique to each chemical atom.

A new chemical atom—that is, an element—results each time another proton is added to an atomic nucleus. Consecutive addition of protons generates the whole range of elements known to exist in the universe. Compounds are formed when two or more different elements combine through atomic bonding. Such bond formation is a consequence of electron pairing and constitutes the foundation of all structural chemistry.