Atomic Structure for MPSC State Exam - MPSC Study Notes for Chemistry

Atomic Structure

Historical Background

Maharshi Kanad (600 BC)

• He said that matter is made up of very small particles.
• They called it 'Parmanu'/'परमाणु'.

Democritus

• The theory was proposed in 430 BC.
• He said that all matter is made up of very small particles.
• He called that particle 'atoms'.

John Dalton

• In 1808, he proposed his theory.
• All matter is made up of small particles, called atoms.
• An atom cannot be created or destroyed.

Drawback: According to these three theories, it is not possible to say about the charge and particle in an atom.

J. J. Thomson

• In 1867, he proposed his model of an atom.
• He discovered the inner particle of an atom.
• He compared an atom to a watermelon. Where he considered: 1. The red part of the watermelon as positively charged. 2. Watermelon seeds as negatively charged particles, embedded in them.
• He said that an atom resembles a sphere of positive charge with electrons (negatively charged particles) present inside the sphere.

Drawback: Positive charge is scattered all over the atom and negative charge is scattered in it. Thomson was wrong to say this.

Ernest Rutherford

• In 1911, he proposed his theory.
• He compared an atom to a solar system. In it, the sun is the nucleus and the planet is the election.
• An atom is hollow.
• The midpoint of an atom is called the nucleus.
• All the mass of the atom is contained in the nucleus.
• Electrons move in a circular path outside the nucleus.

Drawback: Rutherford could not say that the electron rotates in a certain orbit.

Atoms

1.     Nucleus

• Discovered by Rutherford.
• Atoms contain protons and neutrons.

2.     Orbit

• Orbits contain electrons and positrons.

Atomic Number

• The number of protons or electrons in an atom is called the atomic number.
• Atomic numbers are denoted by the letter 'Z'.
• g. A carbon atom has 6 protons and 6 electrons. So the atomic number (Z) of carbon is 6.

Atomic mass number

• The complete amount in the protons and neutrons in the nucleus of an atom.
• Atoms contain particles like Protons, Neutrons & Electrons. But since the mass of the electron is very small, only the mass of the protons and the neutrons are taken into account while measuring the mass of the atom.
• The combined number of protons and neutrons in an atom is the atomic mass.
• Molecules are denoted by the letter 'A'.

Atomic Mass (A) = atomic number (Z) + n

Neutron = Atomic Mass (A) - Atomic Number (Z)

• E.g. The atom of carbon has 6 protons and 6 neutrons, so the atomic mass (A) of carbon is 6 + 6 = 12.

Molecular mass

• Molecular mass is the sum of the aggregate masses of atoms in a molecule.
• Example-H_2_O
• Note: The hydrogen atom has 1 proton & 0 neutrons, so H has a molecular mass of 1.

Shell

      Niels Bohr

• He said that electrons move in a fixed orbit, with a certain energy, at a certain speed.
• There is a hollow space between the two chambers, the electron only found in orbit.
• He named these shells K, L, M, N in ascending order of energy level.
• The ability to accommodate electrons in each shell is different and specific.
• Formula to fill the electron in the orbit:2n^2
• Shell =  (n = Orbit number)

K=2X(1)^2 = 2
L=2X(2)^2 = 8
M=2X(3)^2 = 18
N=2X(4)^2 = 38
Subshells

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