Energy Level: Bohr’s Atomic Model, Aufbau Principle, Hund’s Rule

Energy levels, also known as electron shells, are fixed distances from the nucleus of an atom where the electrons can be found. Electrons are the negatively charged particles in an atom that move around the positive nucleus at the center. Here, we will discuss the topic in detail along with some important questions.

Key Terms: Bohr atomic model, energy level, bohr's theory, hunds rule, aufbau principle, atom, Electron, Nucleus, Electron shell

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Energy Level

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Electrons spinning around a nucleus make up an atom. They are unable to move freely in any direction. Their rotation is limited in specific orbits based on their energy levels. The set distances between electrons and an atom's nucleus are referred to as energy levels. Electron shells are another name for energy levels. An electron can go from one energy level to another, but it cannot move back and forth between them.

• The energy level rises by a predetermined amount. If electrons absorb this fixed energy, they can move from a lower to a higher energy level.
• An electron, on the other hand, emits energy when it jumps from a higher to a lower level. The majority of this energy is emitted in the form of light.
• Emission or absorption of energy occurs when electrons move from one energy level to another.
• The lowest energy level is referred to as the ground state, while the higher energy levels are referred to as excited states.

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Bohr’s Atomic Model

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• The atomic models of Thomson and Rutherford failed to answer any problems about an atom's energy or stability. Niels Bohr presented an atomic model in 1913, characterizing an atom as a small, positively charged nucleus surrounded by electrons that orbit the positively charged nucleus in circular orbits, comparable to planets orbiting the sun in our solar system, with attraction provided by electrostatic forces.
• The Bohr model of an atom is a popular name for this model. An atomic model of a hydrogen atom was proposed by Bohr.
• The stability of electrons revolving in orbits was explained properly by Bohr's model. These orbits were given the term "energy shells" by him.

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Postulates in Bohr’s Atomic Model

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• Around the nucleus, electrons revolve in a fixed circular pattern.
• The orbits remain stationary at all times.
• A certain amount of energy will be contained in each stationary orbit. Orbital shells are another name for these orbits. The electrons won't be able to radiate energy until they continue in their orbits around the nucleus.
• The various energy levels and orbits are represented either numerically (1, 2, 3,...) or alphabetically (K, L, M, N...).
• Integers are used to represent all energy levels, such as n=1, n=2, n=3, and so on. These numbers are quantum numbers. The quantum number's range can vary from the lowest energy level to the highest energy level.
• The first energy level (K) has two electrons, the second has eight, and so on.

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Bohr’s Explanation for Energy Level

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• For an electron, these stationary states/energy levels (different orbit levels) are numbered n = 1, 2, 3,........... The principal quantum numbers are sometimes known as these integers.
• The orbital energy in which an electron is placed is given by: E_n = − R^h 1/ n²

Where, RH is called the Rydberg constant whose value is 2.18 × 10^–18 J.

• When an electron is far from the nucleus' impact, its energy is assumed to be zero. The principal quantum number of an electron in such a stationary condition is denoted by n =. An ionized hydrogen atom is one of several types of hydrogen atoms. The following equation has a negative sign because the stationary state energy is expelled when an electron transitions from one orbit to another, lowering energy.
• An electron is said to be in the ground state when it is placed in the lowest stationary state/energy level conceivable. Electrons at this energy level rotate around the smallest possible orbit. This state has an energy of –13.6 eV.

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Aufbau Principle

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• According to the principle, the orbitals are filled based on their increasing energies in the ground state. 
• Electrons first occupy the position in lower energy, then jump to higher energy levels only when lower levels are filled.

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Pauli’s Exclusion Principle

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• Based on the principle, an orbital can take place a maximum of two electrons having an opposite spin.

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Hund’s Rule of Maximum Multiciplity

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• Hund's rule considers the placement of electrons into decadent orbitals of the same subshells (s, p, d).
• Bonding in s, p, d subshells can not take place until and unless each orbital is engaged by one electron.
• The electrons repel each other as they are negatively charged. The repulsion can be lessened by moving them apart and placing indifferent degenerate subshells.
• All the subshells that have a single electron will spin in the same direction, either clockwise or anticlockwise.

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Things to Remember

• Electrons spinning around a nucleus make up an atom. They are unable to move freely in any direction.
• The Bohr model of an atom is a popular name for this model. An atomic model of a hydrogen atom was proposed by Bohr.
• The stability of electrons revolving in orbits was explained properly by Bohr's model. These orbits were given the term "energy shells" by him.
• One of the postulates of this model is that the various energy levels and orbits are represented either numerically (1, 2, 3,...) or alphabetically (K, L, M, N...).
• According to the Aufbau principle, the orbitals are filled based on their increasing energies in the ground state. 
• Based on Pauli’s exclusion principle, an orbital can take place a maximum of two electrons having an opposite spin.
• Hund's rule considers the placement of electrons into decadent orbitals of the same subshells (s, p, d).

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