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.
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 ground state whereas the higher energy levels are known as excited states.
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.
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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 orbital s having 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.
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Above shows the representation of 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.
According to the principle, an orbital can occupy a maximum of two electrons having an opposite spin.
Hund's rule deals with the placement of electrons into decadent orbitals of 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.
1. What Is Bohr's Atomic Model?
Considering the limitations, drawbacks of Rutherford’s atomic model, a Danish physicist, Neil Bohr proposed a new model in 1913. Bohr concluded; electrons placed away from the nucleus have more energy than electrons placed near the nucleus.
He proposed a model of hydrogen atom explaining the stability of electrons revolving in orbits around the nucleus. According to Bohr's Model,
All the orbits in which electrons are revolving are known as stationary states.
The energy of an electron which is far away from the nucleus is considered as zero. The principal quantum number of such electrons is taken as infinity.
The electron having the lowest energy level revolves in the orbit having the smallest radii. Such electrons are said to be in the ground state.
The energy of the stationary state is given by the formula En = Rh x (1/n2)
Rh (Rydberg constant) = 2.18 x 10-18 J.
2. List out the postulates in Bohr’s atomic model?
Following are the postulates in Bohr’s atomic model,
Electrons revolve in a fixed circular pattern around the nucleus.
The orbits are always stationary.
Every stationary orbit will contain a fixed amount of energy. These orbits are also called orbital shells. The electrons will not be able to radiate the energy until they continue to move around the nucleus in their obits.
The different energy levels and orbits are represented in either number format (1,2,3,4) or alphabetical format (K, L, M, N, ….). The lowest energy level of the electron is considered as the ground level or ground state.
All the energy levels are denoted by integers e.g. n=1 or 2, 3 and so on. These numbers are quantum numbers. The range of the quantum number can fluctuate between the lowest energy level to the highest energy level.
The first energy level (K) contains 2 electrons, second energy level 8, third energy level 18 and fourth energy level has 32 electrons.