Class 11 Chemistry - Bohr’s Model Of The Atom

Class 11 Chemistry - Bohr’s Model Of The Atom

Chemistry notes for class 11: Bohr Atomic Model

• An atom is made up of three particles – electrons, protons, and Electrons have a negative charge and protons have a positive charge whereas neutrons have no charge. They are neutral. Due to the presence of an equal number of negative electrons and positive protons, the atom as a whole is electrically neutral.
• The protons and electrons are located in a small nucleus at the center of the atom. Due to the presence of protons, the nucleus is positively charged.
• The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The ‘energy levels’ or ‘shells’ or ‘orbits’ are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by letters K, L, M, N, O, and The energy levels are counted from the centre outwards.

Arrangement of orbits around the nucleus

• Each energy level is associated with a fixed amount of energy. The energy of an electron in a certain orbit remains constant. As long as it remains in that orbit, it neither emits nor absorbs energy. These are termed stationary states or main energy states.
• The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has maximum energy.
• Bohr proposed that the angular momentum of an electron is quantized. Thus, the motion of an electron is restricted to those orbits where its angular momentum is an integral multiple of h/2π, where h is Planck’s constant.
• Thus we have the relationship mvr = nh/2π, where m is mass of the electron, v is the velocity of electron of said orbit; r is the radius of that orbit, n is a simple integer.
• The stationary states or allowed energy levels are only those where n = 1, 2, 3,…… This is called Bohr quantum condition.
• The energy of an electron changes only when it moves from one orbit to another. An electronic transition from an inner orbit to outer orbit involves absorption of energy. Similarly, when an electron jumps from an outer orbit to inner orbit it releases energy, which is equal to the difference between the two energy levels.
• The energy thus released in the form of a radiation of a certain frequency appears in the form a line in the atomic spectrum. If the energy of an electron in the outer orbit (n2) is E2 and energy of an electron in the inner orbit (n1) is E1 then E2 – E1 = ΔE = hν.

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