# Chemistry: Atomic Structure and Chemical Bonding: Electromagnetic Radiations and Heisenberg՚s Uncertainty Principle (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Electromagnetic Radiations

- Electromagnetic radiation is a kind of energy, which is transmitted through space in the form of electric and magnetic fields. These do not require any medium to propagate. Visible light, radiant heat, radio waves, X-rays and gamma radiation are some of the examples of electromagnetic radiations.
**Amplitude**: This refers to the maximum height to which the wave oscillates. It equals the height of the crests or depth of the troughs.**Wavelength**: It is the linear distance between two consecutive wave-crests or wave- troughs as. It is represented by a Greek letter lambda (λ) and is expressed in terms of m, cm, nm, or Angstrom ( = ) .**Frequency**: It is defined as the number of wave crests or wave troughs that pass-through a given point per second. It is represented by a Greek letter nu (ν) and is expressed in terms of (second inverse or per second) . It is also called as Hz (Hertz) .**Wave number**: It equals the number of waves per unit length. It is denoted as v (nu bar) and is equal to the reciprocal of the wavelength. The SI unit of v is (meter inverse) . However, sometimes it is also expressed as (centimeter inverse) .

## Heisenberg՚s Uncertainty Principle

- The principle states that for a particle it is impossible to know the exact position and momentum simultaneously.
- Heisenberg՚s principle questioned the validity of Bohr՚s model. It is so because according to Bohr՚s model we can precisely calculate the radius of the orbit (i.e.. , the position of the electron) and the velocity of electron in it. But it is not possible according to Heisenberg՚s principle.

## The Wave Mechanical Model – Orbitals

- Wave Mechanical Model of atom was proposed by Erwin Schrödinger- an Austrian physicist in 1926.
- Each electron circling an atom՚s nucleus occupies a specific orbital and spins a certain direction.
- According to this model, the motion of electron inside an atom could be described in terms of a mathematical function called, wave function, ψ (Greek letter, psi) . The orbital is like a cloud or the wave of energy.
- The wave functions are assumed to contain all the information about the electron and are obtained by solving a differential equation called Schrödinger wave equation (SWE) . The square of the wave function ψ
^{2}is a measure of the probability of finding an electron in a three-dimensional space around the nucleus. - Principle Quantum Number N Azimuthal Quantum Number l, Magnetic quantum number, is the three quantum numbers obtained Schrödinger wave equation (SWE) .

## Quantum Numbers

- A set of values describing the state of an electron including its distance from the nucleus.
- Quantum numbers also include the orientation and type of orbital where it is likely to be found, and its spin.
- There are a total four quantum numbers: the principal quantum number (n) , the orbital angular momentum quantum number (l) , the magnetic quantum number (ml) , and the electron spin quantum number (MS) .
- The principal quantum number, n describes the energy level (or principal shell) of the electron within the atom. Can have only positive non zero integral values (i.e.. , ) . This means that in an atom, the electron can have only certain energies. Thus, we may say that n quantizes energy of the electron. The principal quantum number also determines the mean distance of the electron from the nucleus, i.e.. , its size. Greater the value of n, farther is the electron from the nucleus.

## Types of Orbital (S, P, D, F) : Shapes of S, P and D Orbital

- The four different types of orbitals (s, p, d, and f) have different shapes, and one orbital can hold a maximum of two electrons. The p, d, and f orbitals have different sublevels, thus can hold more electrons.
- The four different types of orbitals (s, p, d, and f) have different shapes, and one orbital can hold a maximum of two electrons. The p, d, and f orbitals have different sublevels, thus can hold more electrons. As stated, the electron configuration of each element is unique to its position on the periodic table.

## Aufbau Principle – Electron Configuration of Atoms

- This principle suggest that Electrons occupy lowest energy orbital available.
- All orbital of a sublevel are of equal energy.
- The sublevels have different energies .
- The orbital within one principal energy level can overlap orbitals of another.

## Pauli՚s Exclusion Principle

- The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
- The Pauli Exclusion Principle states no two electrons (or other fermions) can have the identical quantum mechanical state in the same atom or molecule. In other words, no pair of electrons in an atom can have the same electronic quantum numbers n, l, ml, and Ms.