As a planet moves around the sun, its precise path, called an orbit, can be traced. A highly simplified view of the atom looks similar, with electrons orbiting the nucleus. However, the truth is different. Electrons actually live in regions of space called orbitals. Orbitals and orbits are words that are similar, but whose concepts are very different and should not be confused.
Bohr’s model
In atomic physics, the Bohr model describes an atom as a small, positively charged nucleus surrounded by electrons. These electrons move in circular orbits around the nucleus; it is a structure similar to that of the solar system, except that it is electrostatic forces, and not gravity, that exert the attraction.
Although useful for explaining the reactivity and chemical bonding of some elements, Bohr’s model of the atom does not accurately reflect how electrons are distributed in the space around the nucleus. This is because the atoms do not revolve around the nucleus like the Earth revolves around the Sun, but instead are in the orbitals of the electrons. These relatively complex shapes are due to the fact that electrons not only behave like particles, but also like waves. The mathematical equations of quantum mechanics, known as wave functions, can predict, with a certain level of probability, where an electron might be at any given time. Thus, the area in which an electron is most likely to be is called its orbit.
atomic orbitals
Atomic orbitals have different shapes but they are all centered on the atomic nucleus. The most common orbitals in elementary quantum chemistry are the orbitals corresponding to the s, p, and d subshells. However, f orbitals are also found in the ground states of the heavier atoms. The order in which electrons fill atomic orbitals and the shape of the orbitals are crucial factors in understanding the chemical behavior of atoms and their reactions.
first electron shell
The orbital closest to the nucleus, called the 1s orbital, can hold up to two electrons. It is called a 1s orbital because it is spherical around the nucleus. The 1s orbital is always filled before any other orbital.
Hydrogen, for example, has one electron. Therefore, only one point in the 1s orbital is occupied. This point is designated as 1s1, where the superscript 1 refers to the electron in the 1s orbital. Helium, on the other hand, has two electrons, so it can completely fill the 1s orbital with its two electrons. This is called 1s2, referring to the two electrons in helium in the 1s orbital.
On the periodic table, hydrogen and helium are the only two elements in the first row (period), because they are the only ones to have electrons only in their first shell, the 1s orbital.
second electron shell
The second electron shell can contain eight electrons. This shell contains another spherical s orbital and three bell-shaped p orbitals, each of which can hold two electrons. Once the 1s orbital is filled, the second electron shell is filled, filling first its 2s orbital and then its three p orbitals. Filling the p orbitals each takes up a single electron; when each p orbital has one electron, a second can be added.
To exemplify we can use lithium (Li), which contains three electrons that occupy the first and second shells. Two electrons fill the 1s orbital and the third electron fills the 2s orbital. Thus, the electronic configuration of lithium is 1s22s1.
Neon (Ne), for its part, has a total of ten electrons: two are in the innermost 1s orbital and eight fill its second shell (two in the 2s orbital and three in the p orbital). Therefore, it is an inert and energetically stable gas, which is why it rarely forms a chemical bond with other atoms.
third electron shell
The larger elements have extra orbitals, which make up the third electron shell. The d and f substrands have more complex shapes and contain five and seven orbitals respectively. The 3n main shell has s subshells, pyd can hold 18 electrons. The 4n main shell has s, p, d and f orbitals and can hold 32 electrons.
As we move further from the nucleus, the number of electrons and orbitals present in the energy levels increases. When moving from one atom to another on the periodic table, the electronic structure can be built by placing one more electron in the next available orbital.
Properties of electrons in orbitals
Electrons display wave-particle duality, which means that they exhibit some properties of particles and some characteristics of waves. Among the properties of particles is, for example, that an electron only has an electric charge of -1 and the movement of electrons in orbitals.
Moreover, electrons do not orbit the nucleus as the Earth does the Sun. The orbit is a standing wave, with energy levels like harmonics on a vibrating string. The lower energy level of an electron is like the fundamental frequency of a vibrating string, while the higher energy levels are like harmonics. Finally, the region that could contain an electron is more like a cloud or atmosphere, except when probability draws a sphere, which only applies when an atom has only one electron.
Sources
- Barradas, F. (2016). Orbitals in chemistry education : an analysis through its graphic representation .
- De Jesus, E. (2003). Orbitals and Chemical Bonds .
