In this post, you will get the notes on atomic structure in Chapter 2 class 11. Atomic Structure class 11 notes are written in a simple form you can revise the chapter from the notes easily. The diagrams that are given in the atomic structure are well explained in the notes. The post is based on the 2023-24 batch.
Sub Atomic Particle
Constituent particles present in the atom are known as sub-atomic particles of an atom.
Electrons, protons, and Neutrons are sub-atomic particles in an atom.
| Sub Atomic Particles | Discovery | Charge | Mass |
| Electron | J.J Thomson | -1.60 × 10-19 | 9.1 × 10-31 |
| Proton | Rutherford | 1.60 × 10-19 | 1.67 × 10-27 |
| Neutron | J. Chadwick | 0 | 1.67 × 10-27 |
Mass of proton = Mass of Neutron = 1.amu
Discovery of Electron – J.J. Thomson
- J.J Thomson discovered electrons through a cathode ray discharge tube experiment.
- Thomson placed a glass tube on the cathode and anode with very high voltage.
- The tube field with low pressure and a fluorescent screen on the anode side.
Observation of experiment
When the high voltage is applied to the Discharge tube a specific type of ray known as a cathode ray moves from the cathode to the anode and strick on the ZnS layer. It creates a spark on the layer and the voltmeter shows a negative charge indication with proof of the presence of a negative charge named an electron.
Character stick of Cathode Ray
- It moved from the Cathode to the Anode (Positive terminal)
- It behaves as a negative charge particle in the presence of the electromagnetic field nature of gas fields in a tube.
Charge/Mass ratio of electrons (e/m)
e/m = 1.60 x 10 power -19/ 1.75 x 10 power 11 = 9.1 x 10 power -31
- When high voltage Chatod Rays are passed through the electrical and magnetical field then electrons are affected by the field and deflect from its path and strick on Zns layer.
- Through this experiment charge of mass ratio of electrons was found to be 1.75820 x 10 power 11 kg.
Discovery of charge of the electron
- The charge of electrons was discovered by R. A. Millikan in an oil drop experiment.
- The charge of the electron was found to be -1.60 × 10-19 C.
- With the help of the charge of mass of the electron and the value of the charge of the electron, the mass of the electron was discovered.
Discovery of Proton (Canol Ray)
- Proton was discovered by the modern cathode ray discharge tube experiment.
- In the tube positively charged particles moved from the anode to the cathode which is known as a canal ray. Later these canal rays are called protons.
- The nature of canal ray moves from anode to cathode.
- It behaves as a positively charged particle just like the opposite nature of an electron.
Discovery of Neutron
- The discovery of neutrons was made by James Chadwick.
- Neutron was discovered by bombarding alpha particle into lighter elements.
- The mass of a neutron is similar to the mass of a proton and its charge is 0.
Atomic Models
Thomson’s Plum-Pudding Atomic Model
Thomson suggests that an atom is a positively charged sphere and electrons are distributed into the whole positive sphere. Just as a seed of watermelon or plum pudding.
Rutherford’s Atomic Model
Alpha Particle Scattering Experiment
- Rutherford performs alpha particle scattering experiments.
- In this experiment, he used thin gold foil and placed it in the center of the fluorescent screen.
- He bombarded alpha particles into gold foil and the following observations were found.
Observation of the Experiment
- Most alpha particles pass through the atom without undergoing deflection.
- Only of few alpha particles undergo deflection with small angles.
- One among 10 thousand alpha particles undergo complete deflection and return to its source.
- On the basis of the following observation, Rutherford has given its nuclear atomic model.
Rutherford’s Nuclear Atomic Model
- Most of part of the atom is vacant.
- The tiny heavy body that is positively in charge presents the center of the atom.
- Electrons revolve around the positive charge heavy body named the nucleus.
- Electrons and Nuclei are bonded to each other with electrostatic forces.
Drawback of Rutherford’s Model
- If any charged body like electrons revolves around the nucleus it must fall into the nucleus under the influence of attraction forces but that behavior is never seen in an atom.
- Rutherford does not explain the exact location of an electron around the nucleus.
Development of Bohr’s Model
Two major discoveries E.m radiation and atomic Spectrum helped the development of Bohr’s atomic model.
Electromagnetic Radiation and its wave nature
- Metal Transmit their energy in the form of electromagnetic radiation formed by perpendicular Oscillation of magnetic and electrical fields. Both fields are propagated in a fixed direction.
- It travels with the speed of light 3.10 power 8 milliseconds.
Properties of Electro-Magnetic Radiation
Wave Length(λ) – Distance Between one crust, trough, or one wave. Unit – meter
Frequency(µ) – The number of waves that pass through a point in one second is called frequency. Unit – Hertz
Wave Number – No of wave in one unit length
λ = C/µ where C = speed of light
Electromagnetic Spectrum
Electro Spectrum is the sequence of all kinds of waves on the nature of their wavelength and frequency.
Plank’s Quantum Theory
According to Plank absorption and addition of electromagnetic radiation occur in the form of small packages of energy, not in continuous form. Energy exists in a small packet that is called a quantum.
The energy of Quantum is following-
Energy is proportional to the frequency.
E = hv
h = plank quantum, 6.626 x 10 power -34
Photoelectric effect
When a specific frequency of radiation is strict on electrons, the electrons absorb the energy of radiation and are ejected into a certain direction that phenomenon is called the photoelectric effect. This experiment was done by Max Plank and explained by Einstein.
Observation
- There is no lag in the ejection of electrons and striking of photons.
- The number of ejected electrons is proportional to the intensity of light.
- There is minimum energy is required for the photoelectric effect. That energy is known as threshold energy.
- Energy = Threshold energy + kinetic energy, Energy = total energy of the photon
Emission Spectrum
It is a clerk strip with colorful lines of emitted radiation
Absorption Spectrum
It is a colorful strip with dark lines of the absorption spectrum
Line Spectrum of Hydrogen
When the electron of a hydrogen atom absorbs radiation and jumps into many energy levels and that electron returns to a lower energy state it forms many types of spectrum series which are the following:
Dual Nature of Electromagnetic Radiation
When radiation is transmitted from one place to another place it behaves as a wave but when the radiation strikes on any metal it converts into a particle nature. Therefore electromagnetic radiation has both wave and particle nature.
Neils Bohr Atomic Model
Neils Bohr observed the principle of electromagnetic radiation, Plank’s quantum theory, and the atomic spectrum and proposed a new atomic Model.
This atomic model is only applicable to hydrogen-like atoms. For example helium, lithium and beryllium etc.
Following are the postulates of Bohr’s atomic model:
- Electrons revolve around the nucleus in a fixed circular path in a fixed direction and energy. These circular paths are known as orbits.
- Orbits are the stationary states of any electrons where energy remains constant.
- Electrons can travel from one energy state to another energy state by absorbing and emitting radiation. The energy of electrons during the transmission can be calculated
Delta E = E2 – E1
Delta E = hv
v = E2 – E1/h (Bohr’s frequency law)
Electrons do not Fall into the nucleus while moving because it has a fixed amount of angular momentum.
Limitation of Bohr’s atomic model
- This model is only applicable for single electron atoms.
- It does not explain the spectrum of electric fields and magnetic fields.
- It cannot explain the wave nature of electrons.
- It couldn’t explain the dual nature of matter.
Toward Quantum Mechanical Model
Dual Nature of Matters
According to Broglie, any object that has motion and mass has some momentum value and momentum is the nature of particles. When objects are in motion they exhibit wave nature and create a specific wavelength this shows the wave nature of matter. Therefore, matter has both the wave and particle properties. This is called the dual nature of matter.
λ = h/mv
The principle of Broglie is applicable to microscopic objects. Macroscopic objects have a high value of mass. Therefore, we cannot detect the wave nature of bigger particles.
Heisenberg Uncertainty Rule
A moving object has a certain value of momentum. According to Heisenberg, we cannot determine simultaneously the location and momentum of the electron. There is always an error in location if we find the correct value of momentum and vice versa.
∆X ⋅ ∆p ≥ h/4π
This principle is only applicable to microscopic objects.
Quantum Mechanical Model
- According to Schrodinger electrons revolve around the nucleus in the form of waves which represent the wave nature of an atom.
- The wave nature of the electron is represented by Psi (Ψ).
Orbitals
- The Reason where the finding of electrons is up to 90 percent that are known as orbitals.
- Orbitals are represented by Modules psi Square.
Nodes
- Nods is where the probability of finding electrons is almost zero.
- The energy of the electron is quantized. Electron wave which is near to the nucleus have minimal energy and electron wave from the nucleus have maximum energy.
Quantum Numbers
Quantum Numbers are those numbers that provide us the complete information about electrons, orbits, the number of orbits, the shape of the subshell, and their energy.
Principle Quantum Numbers (n = shell)
- Enter levels/ shell
- value = 1, 2, 3, 4,……nth
- Energy n = 1< n = 2 < n = 3
- Maximum no of orbitals = n2
- Maximum no of electrons = 2n2
Azimuthal Quantum number
- Represent sub-shell
- value(l) = 1
- S = 0, P = 1, D = 2, F = 3
Magnetic Quantum Number
- Number of orbitals in sub-shell
- Orientation of orbitals
- Value(n) = -L to L
Spin Quantum Number
- Represent spin of electrons
- Clockwise spin 1/2 up arrow
- Anti Clockwise -1/2 down arrow
- Spin of period electron 1/2 – 1/2 = 0
Energy of orbitals (n + l)
The energy of orbitals is calculated by their n + l value.
Orbitals are arranged in increasing order of n + l value that is known as Energies of orbitals.
- Shell(n) = n = 1, n = 2, n = 3
- Sub Shell S, P, D, F
Degenerated Orbitals
Orbitals which have equal values of energy are known as Degenerted Orbitals
Shielding/screening Effect
It is the value of repulsion by the core electron over the outermost electron is known as the Screening Effect.
Electronic Configuration
These are the sequences of orbitals that are placed in increasing order of their orbital energy.
1s, 2s, 2p, 3s, 3p, 4s, 3d