In chemistry and physics, atomic orbitals are a function called the wave orbitals that describes the properties characteristic of no more than two electrons in the vicinity of an atomic nucleus or system of nuclei, as in a molecule. The orbital is often depicted as a three-dimensional region within which there is a 95 percent probability of finding an electron.
Orbitals and Orbits
When a planet moves around the sun, it outlines a path called an orbit. Similarly, an atom can be represented as electrons circling in orbits around a nucleus. In fact, everything is different, and the electrons are in areas of space known as atomic orbitals. Chemistry is content with a simplified atomic model to calculate the Schrödinger wave equation and, accordingly, determine the possible states of an electron.
Orbits and orbitals sound similar, but they have completely different meanings. It is imperative to understand the difference between the two.
Impossibility of image of orbits
To build the trajectory of the movement of something, you need to know exactly where the object is, and be able to establish where it will be in an instant. For an electron, this is impossible.
According to the Heisenberg uncertainty principle, it is impossible to know exactly where the particle is at the moment and where it will be later. (Actually, the principle says that it is impossible to determine simultaneously and with absolute accuracy its moment and momentum).
Therefore, it is impossible to build an orbit of the motion of an electron around a nucleus. Is this a big problem? No. If something is impossible, you should accept it and find ways to get around it.
Hydrogen electron - 1s orbital
Suppose there is one hydrogen atom and at a certain point in time the position of one electron is graphically imprinted. Shortly after this, the procedure is repeated, and the observer discovers that the particle is in a new position. How she got from first place to second is unknown.
If we continue to act in this way, a kind of 3D map of the places where the particle is likely to be formed will gradually form.
In the case of a hydrogen atom, an electron can be anywhere in the spherical space surrounding the nucleus. The diagram shows a cross section of this spherical space.
95% of the time (or any other percentage, since only the size of the Universe can provide 100% certainty) the electron will be within a fairly easily defined region of space, quite close to the core. This area is called the orbital. Atomic orbitals are areas of space in which an electron exists.
What is he doing there? We do not know, we cannot know, and therefore we simply ignore this problem! We can only say that if an electron is on a specific orbital, then it will have a certain energy.
Each orbital has a name.
The space occupied by the hydrogen electron is called the 1s orbital. The unit here means that the particle is at the energy level closest to the nucleus. S talks about the shape of the orbit. S-orbitals are spherically symmetrical with respect to the nucleus - at least as a hollow ball of a rather dense material with a nucleus in its center.
2s
The next orbital is 2s. It is similar to 1s, except that the region of the most likely electron is located farther from the nucleus. This is the orbital of the second energy level.
If you look closely, you can see that closer to the nucleus there is another region of a slightly higher electron density ("density" is another way of indicating the probability that this particle is present in a particular place).
2s electrons (and 3s, 4s, etc.) spend part of their time much closer to the center of the atom than might be expected. The result of this is a slight decrease in their energy in s-orbitals. The closer electrons get closer to the nucleus, the less their energy becomes.
3s, 4s orbitals (etc.) are located farther from the center of the atom.
P-orbitals
Not all electrons inhabit s-orbitals (in fact, very few of them are there). At the first energy level, the only available location for them is 1s, at the second, 2s and 2p are added.
The orbitals of this type are rather like 2 identical balloons, connected to each other on the core. The diagram shows a cross section of a 3-dimensional region of space. Again, the orbital shows only a region with a 95% probability of finding an individual electron.
If we imagine a horizontal plane that passes through the nucleus in such a way that one part of the orbit is above the plane and the other below it, then there is zero probability that the electron will be on this plane. So how does a particle get from one part to another if it can never go through the plane of the nucleus? This is due to its wave nature.
In contrast to the s-, p-orbital has a certain orientation.
At any energy level, you can have three absolutely equivalent p-orbitals located at right angles to each other. They are arbitrarily indicated by the symbols p x , p y and p z . So it is accepted for convenience - what is meant by the directions of X, Y or Z is constantly changing, because the atom randomly moves in space.
P-orbitals at the second energy level are called 2p x , 2p y and 2p z . There are similar orbitals on the subsequent ones - 3p x , 3p y , 3p z , 4p x , 4p y , 4p z and so on.
All levels except the first have p-orbitals. At higher "petals" are longer, with the most likely location of the electron at a greater distance from the nucleus.
d and f orbitals
In addition to the s and p orbitals, there are two other sets of orbitals available for electrons at higher energy levels. On the third, there may exist five d-orbitals (with complex forms and names), as well as 3s and 3p-orbitals (3p x , 3p y , 3p z ). In total, there are 9 of them.
On the fourth, along with 4s and 4p and 4d, 7 additional f-orbitals appear - a total of 16, also available at all higher energy levels.
Placement of electrons in orbitals
An atom can be imagined as a very bizarre house (similar to an inverted pyramid) with a core living on the first floor and various rooms on the upper floors occupied by electrons:
- on the ground floor there is only 1 room (1s);
- on the second room there are already 4 (2s, 2p x , 2p y and 2p z );
- on the third floor there are 9 rooms (one 3s, three 3p and five 3d orbitals) and so on.
But the rooms are not very large. Each of them can contain only 2 electrons.
A convenient way to show the atomic orbitals in which these particles are located is to draw “quantum cells”.
Quantum cells
Atomic orbitals can be represented as squares with electrons in them, depicted as arrows. Often, up and down arrows are used to show that these particles are different from each other.
The need for the presence of different electrons in an atom is a consequence of quantum theory. If they are in different orbitals, this is fine, but if they are located on one, then there must be some subtle difference between them. Quantum theory endows particles with a property called “spin,” which is what it means by the direction of the arrows.
A 1s orbital with two electrons is depicted as a square with two arrows pointing up and down, but it can also be written even faster as 1s 2 . This reads as “one s two”, and not as “one s squared”. The numbers in these notations should not be confused. The first denotes the energy level, and the second - the number of particles in the orbital.
Hybridization
In chemistry, hybridization is the concept of mixing atomic orbitals into new hybrid ones that can pair electrons with the formation of chemical bonds. Sp hybridization explains the chemical bonds of compounds such as alkynes. In this model, the atomic orbitals of carbon 2s and 2p are mixed, forming two sp-orbitals. Acetylene C 2 H 2 consists of the sp-sp-interweaving of two carbon atoms with the formation of a σ-bond and two additional π-bonds.
Atomic carbon orbitals in saturated hydrocarbons have the same hybrid sp 3 orbitals in the form of a dumbbell, one part of which is much larger than the other.
Sp 2 hybridization is similar to the previous ones and is formed by mixing one s and two p-orbitals. For example, three sp 2 - and one p-orbital are formed in an ethylene molecule.
Atomic orbitals: the principle of filling
Imagining the transitions from one atom to another in the periodic table of chemical elements, it is possible to establish the electronic structure of the next atom by placing an additional particle in the next available orbital.
Electrons, before filling higher energy levels, occupy lower, closer to the nucleus. Where there is a choice, they fill the orbitals individually.
This filling order is known as the Hund rule. It is used only when atomic orbitals have equal energies, and also helps minimize repulsion between electrons, which makes an atom more stable.
It should be noted that the energy of the s-orbital is always slightly less than that of p at the same energy level, therefore the former are always filled before the latter.
What is really strange is the position of the 3d orbitals. They are at a higher level than 4s, and therefore the 4s orbitals are filled first, and then all the 3d and 4p orbitals.
The same confusion occurs at higher levels with a large number of interweaving between them. Therefore, for example, the atomic orbitals of 4f are not filled until all places in 6s are occupied.
Knowing the filling order is central to understanding how to describe electronic structures.