LEARNATHON
III

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### Theory:

Different scientists have suggested various atom models. These have led to a better understanding of atomic structure. Among these models, Rutherford proposed that electrons revolve in well-defined orbits. If that was the case, there is an issue.

The motion of the electrons in Rutherford's model is unstable. As electrons revolve in orbit, they accelerate and lose energy. After that, they fall into the nucleus resulting in the atom which is highly unstable.

To overcome these objections, Neils Bohr proposed a new atomic model.

Postulate of Rutherford's model:
• The electrons revolve around the nucleus in a specific orbit, and these orbits are associated with definite energies called shells or energy levels.
• The electrons do not emit energy when revolving in specific orbits.
These shells or energy levels or orbits are represented by the letters K, L, M, N or by the numbers $$1$$, $$2$$, $$3$$, $$4$$.

Bohr's atom model

Limitations of Bohr's model:

Only hydrogen and hydrogen-like ions were included in this model (example, $$He^+$$, $$Li^{2+}$$, $$Be^{3+}$$ and so on). It was not possible to apply it to multi-electron nuclei.
Distribution of electrons in orbits or shells
Bohr and Bury proposed the distribution of electrons in orbits.
The definite distribution of electrons around the nucleus is called electronic configuration.
To achieve the electronic configuration, it follows a certain set of rules:
• The formula $2{n}^{2}$ defines the total number of electrons in a shell.
Where, n is energy level or orbit number. $$n = 1, 2, 3, 4,$$ etc. Therefore, the maximum number of electrons in different shells are as follows:
 Energy levels Shells Maximum electrons Electron capacity $$1$$ K ${2×\left(1\right)}^{2}=2$ $$2$$ $$2$$ L ${2×\left(2\right)}^{2}=8$ $$8$$ $$3$$ M ${2×\left(3\right)}^{2}=18$ $$18$$ $$4$$ N ${2×\left(4\right)}^{2}=32$ $$32$$

This implies that the first shell (K shell) can have a maximum of two electrons, the second shell (L shell) can have a maximum of eight electrons and so on.

Example:

Sodium atom with energy level ($$2, 8, 1$$) not ($$2, 9$$)
• Unless the inner shells are filled, electrons cannot fill in a given shell. In other words, the shells are gradually filled.
Example:

Incorrect and correct filling of electrons in sodium

According to Bohr, the energy of the shell is proportional to its size. The greater the size, the greater the energy. Since the first shell is the smallest, it has the lowest energy, and it gets filled first.

Hence, the energy level or size of the shells are given by:

K < LM < N

The atomic structure of the first eighteen elements is shown schematically.

Some elements and their electronic configurations
Neils Bohr got the Nobel prize for his work on the structure of the atom in $$1922$$.

Neils Bohr