Atoms are the smallest unit, consisting of the positively charged nucleus surrounding the negatively charged electrons. Electron cloud refers to the group of electrons (negatively charged) circulating around the nucleus or a molecule, or electron cloud is the region with a high probability of containing electrons.

Electron Shell is the outermost area of an atom surrounding the atomic nucleus. Electron shells may have one or more electron subshells or sublevels – subshells consist of one or more atomic orbitals. Electron shell orbitals are the region around the space where the probability density of finding an electron is maximum.

Electron cloud model

It states that we cannot know exactly where the electron is at a given time. But they are most likely to be in specified areas. These areas are specified by the orbitals, and orbitals are followed by the shells and sub orbitals.

Electron Shells in Bohr Model 

• The electron can revolve only in certain stable orbits and have fixed energy called energy levels, or stationary states named K, L, M, N, etc.
• The electrons have the same energy as that of the orbit in which it is revolving.
• While revolving in the particular energy level(orbits), electrons do not emit any radiation.
• If an electron absorbs or emits energy, it must move to different energy levels.
• Definition of stable orbits:- it can revolve only on those circular orbits in which the angular momentum of an electron whose energy is an integral multiple of h/2π where h is Planck’s constant.

Quantum Numbers

The set of numbers that helps us to describe the position and energy of electrons in an atom are called quantum numbers. There are four quantum numbers – 

Principle Quantum Number (denoted by n)

It indicates the distance of an electron from the nucleus. As the value of n increases, there is an increase in the distance of the electrons from the nucleus as well as the energy of the electrons. It has values of 1,2,3,4, up to infinity.

Azimuthal Quantum Number (denoted by l)

It indicates the shape of an orbital (not the electrons )and can have values from 0 to (n-1) for a particular value of n.

The value of l indicates the sub-energy level or subshell within a particular shell or energy level.

Magnetic Quantum Numbers (denoted by m)

It indicates the possible orientation of an orbital in space. The value of m for a particular value of l varies from +l to -, l, including zero.

Spin Quantum Numbers (denoted by s)

It not only indicates the electron cloud it also characterises the spin of electrons.

The electron not only moves around the nucleus but also spins about its axis.

Aufbau Principle

This principle states that the electrons fill lower energy atomic orbitals before filling the higher energy orbitals.

Pauli Exclusion Principle

It states that no two electrons in one atom have the same values of all four quantum numbers, or no two electrons in the same orbital must have opposite spin.

Hund’s Rule

It states that in the ground state of an atom, the orbital having the same energy must occupy one electron in each having the same spin before any one of them is doubly occupied.

Nodal planes of different orbits

The nodal plane is defined as the plane which passes through the nucleus, from which the probability of finding electrons present in orbit is zero.

Possible nodal planes for subshells are – 

• The number of nodal planes for s orbit is zero.
• The number of nodal planes for p orbit is one.
• The number of nodal planes for d orbit is two.

Conclusion

The electron shells determine the chemical properties of an atom. Electrons are very small, light, and negatively charged. They are to be gained or lost in chemical reactions. Electrons are arranged in different shells around the nucleus. Each shell can hold up to a certain number of electrons. Within the shells, the electrons are further grouped into subshells, i.e., s,p,d,f in order of increasing energy. Electrons may appear or disappear at random in clouds. The first shell has only the subshell; the second has s and p; the third has s,p,d, and the fourth has s,p,d,f.