Atomic orbital is the three-dimensional region or space around the nucleus of atom, where the probability of finding an electron is maximum (90 – 95%). In other words, an atomic orbital represents the region in the space where an electron is most likely to be found. It is the most probable space in which the electron spends most of its time in constant motion.
Let’s understand it with a real-life example. Suppose you have the fond of reading books. You spend most of the time in your college library to read books. Here, the college in which you are studying books represents an atom and you as a student representing an electron who is studying in a college and attending the lectures, visiting the library, labs, canteen, etc.
But you spend most of your time in the library for reading books and that may be considered as an orbital because the possibility of finding you (i.e. electron) is maximum there. So, we can say that region or space around the nucleus of an atom where the probability of finding electrons is maximum is called atomic orbital.
Consider another example. Suppose a beehive with bees flying around it. The bees move to different places, such as flowers, trees, and the entrance of the hive. However, most of the time, they are found near the hive because it is their home and the safest place for them. Here, beehive represents an atom while bees represent electrons. The area around the hive where bees are most commonly seen represents an orbital, as it is the region where the probability of finding an electron is highest around the nucleus.
Similarly, an orbital is the most probable space where an electron spends most of its time while in constant motion. An electron in an atom does not stay at a fixed distance from the nucleus. It can move anywhere, but most of the time, it will be found within a small region of space called an orbital. This concept is based on quantum mechanics or the quantum mechanical model of atom, where orbitals represent probability distributions rather than fixed paths. The energy of the electron in an atomic orbital is always the same.
Electron Cloud Representation of Atomic Orbital
Each orbital in an atom is characterized by a definite size, shape, and orientation (i.e. arrangement in space). Although, it is rather difficult to give an exact pictorial representation of an orbital. But the most common method to represent an orbital is a shaded figure in which the intensity of shading represented by dots is proportional to the probability of finding the electron at that location or region. This type of representation is called electron cloud or charge cloud representation of an orbital.
The electron cloud representation is a model used to describe atomic orbitals in the quantum mechanical model of atom. In the quantum mechanical model, an electron does not follow a fixed path around the nucleus of an atom. Its position is described by a probability distribution. The electron cloud represents the area where an electron is most likely to be found at any given time. Below figure shows the electron cloud representation of 1s orbital of hydrogen atom.
The dots shown in the below figure do not represent the number of electrons; rather, they indicate the number of probable instantaneous positions of a single electron. You can see the electron cloud in the figure appears denser at certain points, representing a higher probability of finding the electron there, and less dense at other points, representing a lower probability.
As you move far away from the nucleus, the electron density goes on decreasing and, ultimately, it becomes negligible. So, in the terms of electron cloud representation, the probability of finding an electron in a specific region or space is directly proportional to the density of dots in that location.
Note that the representation of an atomic orbital by an electron cloud does not fix the position of the electron, nor does it describe the path of the electron’s motion. It simply predicts where an electron is most likely to be found at any given time.
This region in space, where the probability of finding an electron is highest, is called an orbital. This concept is based on quantum mechanics, which considers electrons as existing in probability distributions rather than fixed orbits.
Difference between Orbit and Orbital
The main difference between orbit and orbital is given in the tabular form:
| Orbit | Orbital |
|---|---|
| 1. As postulated by Bohr, an orbit is a well defined circular path around the nucleus in which electrons revolve. These are numbered as 1, 2, 3, 4 . . . . or labelled as K, L, M, N . . . . | 1. As postulated by wave nature of an electron, an orbital is the three-dimensional space around the nucleus of an atom within which the probability of finding an electron is maximum. It may be spherical or dumbshell in shape. |
| 2. It represents the two dimensional motion (i.e. planar motion) of an electron around the nucleus of an atom. | 2. It represents the three-dimensional motion of an electron around the nucleus of an atom. |
| 3. The concept of well-defined orbit is not in accordance with the wave character of electrons and Heisenberg’s uncertainty principle. | 3. The concept of an orbital is in accordance with the wave character of electrons and Heisenberg’s uncertainty principle. |
| 4. All orbits are circular and disc like in shape. | 4. Different orbitals have different shapes. For example, s-orbitals are spherically symmetrical, whereas p-orbitals are dumb-bell shaped, and so on. |
| 5. Orbits do not have any directional characteristics, hence they cannot explain shapes of molecules. | 5. All orbitals except s-orbitals have any directional characteristics, hence they can account for shapes of molecules. |
| 6. The maximum number of electrons in any orbit is given by 2n2, where n is the number of orbit. | 6. The maximum number of electrons present in an orbital is two with paired spins. |
