Energy Level of an Atom

• An atom consists of electrons, revolving around a nucleus. Electrons are small, negatively charged particles that follow a circular path or orbit while moving around the nucleus. 

• They can’t move freely at any random position. Their revolution is restricted in particular orbits according to their energy levels. 

• Energy levels are nothing but the fixed distances of electrons from the nucleus of an atom. The energy levels are also called electron shells. 

• An electron can move in one energy level or to another energy level, but it can not stay in between two energy levels.

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• The figure shows the energy levels of an atom. The first four energy levels are shown here.

• The first energy level is also called level 'K'. The second level is called level L, third energy level as M, and so on.

• The electrons from energy level K contains the least energy whereas the levels that are far from the nucleus contains more energy

• Electrons in the outermost energy level are also called Valence electrons. Various properties of atoms are based on these valence electrons.

Energy State

• The increase in energy takes place by a fixed amount. If electrons absorb this fixed energy, it can jump from lower energy level to a higher level. 

• On the contrary, when an electron jumps from a higher level to a lower level, they emit energy. This emission of energy is generally in the form of light. 

• When electrons transit from one energy level to another, emission or absorption of energy takes place. 

• The lower energy level is called the ground state whereas the higher energy levels are known as excited states.

Energy level Diagrams

• To study the nature of bonding between the electrons, placement of electrons in orbits and to understand the behavior of elements under certain conditions, energy level diagrams are used.

• Energy level diagrams are the representation of placements or arrangements of orbitals (also known as subshells) according to their increasing energy levels.

• Above is the blank energy level diagram which can be used to represent the electrons for any atom under study. Energy level diagrams are known as Grotrian diagrams. It is named after German astronomer Walter Grotrian (from the first half of the 20th century).

• The important observations revealed from these diagrams are,

• The orbitals do not contain the same energies. It can be seen in the above diagram, orbitals 2s and 2p are not placed at the same levels i.e. they do not possess the same energy as each other.

• The orbitals having lower energy are placed nearer to the nucleus. i.e the order s, p, and so on shows that orbitals  have lower energy than that of orbital p. for energy level 3, the arrangement should be 3s<3p<3d.

• For energy level 4, the placement of orbitals is 4s<4p<4d. (orbital s has the lowest energy).

• The outermost orbital of lower energy level has higher energy than the consequent orbital of higher energy level. 4s has lower energy than 3d.

• To fill the vacant energy levels, the Aufbau Principle is used. It is a technique to remember the order of filling the vacant energy levels.

The meaning of the energy level diagram is as follows:

• Thanks to the Grotrian diagram, we can see that light emission and absorption happen at the same wavelengths.

• A molecule or atom travels from a lower energy state to a higher energy state when it absorbs light or collides with another atom or ion that provides sufficient energy.

• In most cases, the emission begins with an atom stimulated to its higher state either by collision or by absorption of light from the environment.

Aufbau Principle

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• Above shows the representation of the Aufbau Principle.

• According to the principle, in the ground state, the orbitals are filled according to their increasing energies. 

• Electrons first occupy the position in lower energy. they jump to higher energy levels only when lower levels are filled.

Pauli’s Exclusion Principle

• According to the principle, an orbital can occupy a maximum of two electrons having an opposite spin.

Hund’s Rule of Maximum Multiplicity

• Hund's rule deals with the placement of electrons into decadent orbitals of the same subshells (s, p, d).

• Bonding in s, p, d subshells can not occur until each orbital is occupied by one electron.

• As the electrons are negatively charged, they repel each other. The repulsion can be minimized by moving them apart and placing indifferent degenerate subshells.

• All the subshells having a single electron will spin in the same direction, either clockwise or anticlockwise.

Summary

Electrons in molecules, like atoms, begin by occupying lower energy levels.

To explain the stability of molecules generated by atoms with more than two electrons, we will only analyze the molecular orbitals formed by their valence shell and not the ones formed by their core orbitals.

Because all of the electrons in the Lewis structure are paired, the bond number obtained using the molecular orbital model differs from the bond number obtained using the Lewis structure because two of the electrons are unpaired.

A diamagnetic atom or molecule is formed when electrons in an atom or molecule are paired. A paramagnetic atom or molecule is formed when electrons in an atom or molecule are not paired.

The atomic orbitals of the atoms in a molecule are combined to form molecular orbitals.

Because electrons are positioned between the two nuclei, the molecular orbital created via constructive interference is a bonding molecular orbital. Because its electrons are placed away from the region between the two nuclei, the molecular orbital generated due to destructive interference is known as an anti-bonding or sigma star molecular orbital, making the molecule less stable.

The energy of molecular orbitals is determined by the energies of the atomic orbitals involved in their production.

For example, the energies of molecular orbitals formed by combination 2s atomic orbitals will be higher than the energies of molecular orbitals formed by combination 1s atomic orbitals. Bonding molecular orbitals have less energy than antibonding orbitals in the same pair of molecular orbitals.

Degenerate orbitals, such as 2px, 2py, and 2p*x, 2p*y, are created by combining 2pz, 2px, and 2py and have the same energies.