Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 66

Within each energy level, there are distinct areas in space that the electrons occupy. These distinct areas are sub‐shells, which are further arranged into atomic orbitals. A sub‐shell is a group of atomic orbitals that have the same properties, such as shape. The sub‐shells are called s, p, d, and f. Atomic orbitals are regions within a sub-shell that can contain up to two electrons and have a particular orientation in space. For example, the p sub‐shell actually comprises three atomic orbitals, which are called p‐orbitals. To understand about atomic orbitals and sub‐shells, it is useful to spend a little time considering the nature of electrons.

We have seen that electrons are very small, they have a negative charge, and they move very quickly. The early twentieth century was an exciting time for physicists and chemists. Many ground‐breaking experiments were carried out that helped determine the properties of the three main fundamental atomic particles: the proton, the neutron, and the electron. Unfortunately, there isn't time or space to describe these experiments here, but two important experiments gave scientists unexpected and potentially conflicting information about the properties of electrons. These experiments showed that electrons can behave as both particles and waves, and this theory became known as wave‐particle duality. A consequence of wave‐particle duality is Heisenberg's Uncertainty Principle, which states that it is impossible to accurately know both the position and velocity of an electron at the same time. Around the same time, a physicist named Schrödinger produced a mathematical model of the atom, which allowed for the wave‐like behaviour of electrons and produced equations that described the position of an electron in the outer shells of an atom. The equations that Schrödinger derived led to the idea that there are regions in space around the nucleus where there is a high probability of finding an electron of a given energy. These regions in space are called atomic orbitals and can be thought of as sub‐shells within the principal quantum shells.

When considering the occupancy of electrons in atomic orbitals or the behaviour of electrons in chemical reactions, it is often more intuitive to think of electrons as small negatively charged particles – and this is fine. However, it is important to remember that electrons also have wave‐like properties, which prevents us from stating exactly where the electron is at any one time; we can only say where there is a good probability of finding the electron.

As stated earlier electrons in each principal quantum shell of an atom are arranged in orbitals. An orbital is an area in space where there is a high probability of finding an electron of a specific energy. An orbital is actually a mathematical function that describes the wave‐like behaviour of an electron in an atom. When this function is plotted in three dimensions for an electron of a certain energy, a three‐dimensional shape is obtained. The shape represents the space in which there is a high probability of finding an electron with that energy. An atomic orbital can hold a maximum of two electrons.

Any atomic orbital can hold a maximum of two electrons.