In chemistry and physics, atomic orbitals are a function called a wave function 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. An orbital is often depicted as a three-dimensional region within which there is a 95 percent chance of finding an electron.
Orbitals and orbits
When a planet moves around the Sun, it traces a path called an orbit. Similarly, an atom can be represented as electrons circling in orbits around the nucleus. In fact, things are different, and the electrons are in regions of space known as atomic orbitals. Chemistry is content with a simplified model of the atom to calculate the Schrödinger wave equation and, accordingly, determine the possible states of the electron.
Orbits and orbitals sound similar, but they have completely different meanings. It is extremely important to understand the difference between them.
Impossible to display orbits
To plot the trajectory of something, you need to know exactly where the object isis located, and be able to establish where it will be in a moment. This is impossible for an electron.
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. (In fact, the principle says that it is impossible to determine simultaneously and with absolute accuracy its momentum and momentum).
Therefore, it is impossible to build an orbit of the electron around the nucleus. Is this a big problem? No. If something is not possible, it should be accepted and ways around it should be found.
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 thereafter, the procedure is repeated and the observer finds that the particle is in a new position. How she got from the first place to the second is unknown.
If you keep doing this, you will gradually form a kind of 3D map of where the particle is likely to be.
In the case of the hydrogen atom, the electron can be anywhere within 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 one hundred percent certainty) the electron will be within a fairly easily defined region of space, close enough to the nucleus. Such a region is called an orbital. Atomic orbitals areregions of space where an electron exists.
What is he doing there? We don't know, we can't know, and therefore we simply ignore this problem! We can only say that if an electron is in a particular 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 tells about the shape of the orbit. S-orbitals are spherically symmetrical about the nucleus - at least like a hollow ball of fairly dense material with a nucleus at its center.
2s
The next orbital is 2s. It is similar to 1s, except that the electron's most likely location is farther from the nucleus. This is an orbital of the second energy level.
If you look closely, you will notice that closer to the nucleus there is another region of slightly higher electron density ("density" is another way of indicating the probability that this particle is present in a certain place).
2s electrons (and 3s, 4s, etc.) spend some of their time much closer to the center of the atom than one might expect. The result of this is a slight decrease in their energy in s-orbitals. The closer the electrons get to the nucleus, the lower their energy becomes.
3s-, 4s-orbitals (and so on) are getting further from the center of the atom.
P-orbitals
Not all electrons live in s orbitals (in fact, very few of them do). On the first energy level, the only available location for them is 1s, on the second, 2s and 2p are added.
Orbitals of this type are more like 2 identical balloons, connected to each other at the core. The diagram shows a cross section of a 3-dimensional region of space. Again, the orbital only shows the area with a 95 percent chance of finding a single electron.
If we imagine a horizontal plane that passes through the nucleus in such a way that one part of the orbit will be above the plane and the other below it, then there is a zero probability of finding an electron on this plane. So how does a particle get from one part to another if it can never pass through the plane of the nucleus? This is due to its wave nature.
Unlike the s-, p-orbital has a certain directionality.
At any energy level, you can have three absolutely equivalent p-orbitals located at right angles to each other. They are arbitrarily denoted by the symbols px, py and pz. This is accepted for convenience - what is meant by the X, Y or Z directions is constantly changing, as the atom moves randomly in space.
P-orbitals at the second energy level are called 2px, 2py and 2pz. There are similar orbitals on subsequent ones - 3px, 3py, 3pz, 4p x, 4py,4pz and so on.
All levels, except for the first one, have p-orbitals. At higher levels, the "petals" are more elongated, 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 to electrons at higher energy levels. On the third, there may be five d-orbitals (with complex shapes and names), as well as 3s- and 3p-orbitals (3px, 3py, 3pz). There are a total of 9 here.
On the fourth, along with 4s and 4p and 4d, 7 additional f-orbitals appear - 16 in total, also available at all higher energy levels.
Placement of electrons in orbitals
An atom can be thought of as a very fancy house (like an inverted pyramid) with a nucleus living on the ground floor and various rooms on the upper floors occupied by electrons:
- there is only 1 room on the first floor (1s);
- on the second room there are already 4 (2s, 2px, 2py and 2pz);
- 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 big. Each of them can only hold 2 electrons.
A convenient way to show the atomic orbitals that these particles are in is to draw "quantum cells".
Quantum cells
NuclearOrbitals can be represented as squares with the electrons in them shown as arrows. Often, up and down arrows are used to show that these particles are different.
The need for different electrons in an atom is a consequence of quantum theory. If they're in different orbitals, that's fine, but if they're in the same orbit, then there must be some subtle difference between them. Quantum theory endows particles with a property called "spin", which is what the direction of the arrows refers to.
The
1s orbital with two electrons is shown as a square with two arrows pointing up and down, but it can also be written even faster as 1s2. It reads "one s two", not "one s squared". The numbers in these notations should not be confused. The first is the energy level, and the second is the number of particles per orbital.
Hybridization
In chemistry, hybridization is the concept of mixing atomic orbitals into new hybrid orbitals capable of pairing electrons to form chemical bonds. Sp hybridization explains the chemical bonds of compounds such as alkynes. In this model, the 2s and 2p carbon atomic orbitals mix to form two sp orbitals. Acetylene C2H2 consists of an sp-sp entanglement of two carbon atoms with the formation of a σ-bond and two additional π-bonds.
Atomic orbitals of carbon in saturated hydrocarbons haveidentical hybrid sp3-orbitals, shaped like a dumbbell, one part of which is much larger than the other.
Sp2-hybridization is similar to the previous ones and is formed by mixing one s and two p-orbitals. For example, in an ethylene molecule, three sp2- and one p-orbital are formed.
Atomic orbitals: filling principle
Imagining transitions from one atom to another in the periodic table of chemical elements, one can establish the electronic structure of the next atom by placing an additional particle in the next available orbit.
Electrons, before filling the higher energy levels, occupy the lower ones located closer to the nucleus. Where there is a choice, they fill the orbitals individually.
This filling order is known as Hund's rule. It only applies when the atomic orbitals have equal energies, and also helps to minimize repulsion between electrons, making the atom more stable.
Note that the s-orbital always has slightly less energy than the p orbital at the same energy level, so the former always fill up before the latter.
What's really weird is the position of the 3d orbitals. They are at a higher level than the 4s, and so the 4s orbitals fill up first, followed by all the 3d and 4p orbitals.
The same confusion occurs at the higher levels with more weaves in between. Therefore, for example, the 4f atomic orbitals are not filled until all the places on the6s.
Knowing the fill order is central to understanding how to describe electronic structures.
