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# Electron shells

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 Sub Topics Before a million years ago, there was no life on earth. After cooling of earth environment, the water vapor condensed and formed clouds to form atmosphere which was composed of mainly carbon dioxide gas, water vapor and nitrogen gas. Carbon dioxide gas reacted with water to form carbonic acid which was a weak acid and reacted with rocks to dissolve some of the chemical. This runoff mixed together to form organic molecule which were mainly composed of carbon atom. Remember all the known substances on earth are usually have some content of carbon. There can be two kinds of substances; compounds and elements. Elements are considered as elementary particles which cannot break further in small fragments such oxygen (O), iron (Fe), carbon (C) etc. On the contrary, compounds are composed of two or more elements which can separated by some chemical methods like carbon dioxide. You can observe many chemical reactions in your surroundings infect they also occur in living organisms like digestion, excretion etc. Overall the elementary particles of all the substances in an atom which can combine in various combinations to form different compounds and involve in chemical reactions. A molecule is a final product of combination of atoms like two oxygen atoms combine together to form oxygen molecule. Every atom has its own identity and can be recognize from others due to structural difference. The structure of an atom is not so complicated but arranged in a certain manner.

## Electron Shell Definition

Each of the atom is mainly composed of three sub-atomic particles neutrons, protons and electrons. The center part of atom is a dense area which contains proton and neutrons and known as nucleus. The negatively charged particles; electrons are placed around nucleus in circular paths which are known as electron shell.

Electronic shells are segregated spheres around nucleus in an atom where all the electrons are revolving around positively charged nucleus. It is just like our solar system in which all the planets are revolving around sun. Usually we study five shells around nucleus in an atom which are designated as K, L, M, N, O and so on. Remember there can be infinite electron shells but itâ€™s not necessary that there will be some electron.

The number of electrons in an atom is fixed which is equal to the number of protons; atomic number.
So we can fill the electron shells only as we have electrons in that particular atom. Each shell can accommodate a certain number of electrons only which can determine with the help of principal quantum number of that shell.

## Electron Shell Diagram

1. The arrangement of electrons around nucleus in an atom is known as electron shell diagram or electron shell chart.
2. It represents the number of electrons in each shell and their arrangement in an atom.
3. Remember the electrons first fill in the lower shell than only move to higher one.
4. The number of electrons in a shell can determine with the help of given formula; 2n2 where n stands for the number of shell in that atom.
5. For example; the maximum number of electrons in first shell can be 2 x (1 x 1) = 2 while second shell can accommodate 2 x (2 x 2) = 8 electrons.
6. There can be 2 (3 x 3) = 18 electrons in third shell and 2(4x4)=32 electrons in the fourth shell.
7. The fifth shell can accommodate total 2 x (5 x 5) = 50 electrons.

Lets draw an electron shell model for oxygen atom.
1. There are eight electrons, eight protons and eight neutrons in an oxygen atom.
2. As we know, first shell can accommodate two electrons, therefore remaining six electrons will move to second shell which can accommodate maximum eight electrons but oxygen has only six electrons in its second shell.
3. The electronic shells which are placed close to nucleus are known as inner shells while the outer one is known as outer electron shell or valence electron shell.
4. The valence electron shell always takes part in any of the chemical reaction of the atom and also involve in chemical bonding with other atoms to form molecules.
5. Therefore, there are two electrons in the inner shell of the oxygen atom and six electrons in the outer or valence shell of the oxygen atom.

### Solved Example

Question: Which of the principles explained the order of filling of electrons in various electronic shells?
Solution:

The principles which explains the order of filling of electrons in various electronic shells are
• Pauli-exclusion Principle
• Aufbau principle
• Hund’s rule of multiplicity

## Order of Electron Shells

1. Each shell can further divide into sub-shells which can accommodate a certain number of electrons and arranged in a certain order.
2. The first shell can have one sub-shell (1s) while the second shell (L) can have two sub-shells (2s & 2p).
3. There can be three sub shells in third electron shell (M); 3s, 3p, 3d and four in fourth shell; 4s, 4p, 4d, 4f.
4. After fourth shell, the number of sub shells in an atom remains same.
5. Here s, p, d and f represents sub shells and arranged in a certain order.
6. The number of electrons in a sub shell can determine with the help of 2n+1 formula where â€˜nâ€™ represents the principal quantum number.
7. Hence s-sub shell can accommodate 2 electrons, p can have maximum 6 electrons, d can accommodate ten electrons while fourteen electrons can place on f-sub shell and make the pattern of 2, 6, 10, 14, 18, 22, 26, etc electrons in successive sub-shells.
The filling of electrons in the sub shells is according to their energy levels, from low energy level to higher one and follows Aufbau principle. Therefore the order of filling electrons can write as below.

1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2 4d10 and so on.

The order of sub shell with maximum number of electrons is shown below.

On the basis of order of filling electrons in sub shells; the electronic configuration of iron with 26 electrons can written as; 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6

### Solved Example

Question: How the stability of fulfilled and half filled orbitals changes the order of filling of electrons in orbitals and alters the electronic configuration.
Solution:

The fulfilled and half filled orbitals are highly stable and an atom tends to acquire a stable configuration. Therefore if there is a possibility to get the stable configuration with the transfer of one electron from one orbital to another, element will go with that and acquire the stable configuration. For example; the electronic configuration of chromium
is but if we transfer one electron from 4s to 3d, it will make 4s1, 3d5 valence shell configurations. Here 4s and 3d, both are stable due to half filled hence the electronic configuration of chromium will be; 1s2,2s2,2p6,3s2,3p6,4s1,3d5

## Bohr Model Electron Shells

1. The first successful atomic model for the explanation of structure of atom was Thomson’s atomic model which was also known as plum-pudding or resin-bread model of atoms.
2. According to this model, the negatively charged electrons are randomly distributed in a positively charged cloud of proton.
3. Due to the presence of the equal number of protons and electrons, the atom is neutral in nature. This model could explain the results of gold foil experiment done by Ernst Rutherford sin 1911.
4. His observation did not support the Thomson’s model as the results showed that the maximum part of the atom is empty with a centralized mass body known as nucleus.
5. Therefore, Rutherford purposed the Planetary Model which showed that the centralized mass in an atom is positively charged as composed of protons while electrons are in motion around nucleus and balance due to electrostatic force of attraction.
6. It was just like the solar system in which sun acts as nucleus and all planets can consider as electrons around sun.
7. The main limitation was the stability of an atom which could not explained with the help of this model.
8. As it considered that negatively charged particles revolve around nucleus therefore they must fall in nucleus due to electrostatic force of attraction. But that was not true. We know that atom is an independent and stable entity.
9. Later Bohr Model helped to cover this limitation. According to Bohr’s atomic model, electrons could only occupy certain orbits around nucleus which are related to a certain energy levels.
10. The electrons are arranged in certain electron shell around nucleus which is labelled as K, L, M, N, etc.
11. Each shell corresponds to a certain value of principal quantum number like the value of ‘n’ for K-shell is one, for L-shell it is valued as two.
12. The value of n for M and N shells are nM = 3, nN = 4.
13. The number of maximum electrons in a shell can give as 2n2. As we move away from the nucleus, the energy of electronic shell increases, therefore the transition of electrons from higher electronic shell to lower one release a certain amount of energy in the form of photons.

On the contrary, the transition of electrons from lower to higher electronic level requires some amount of energy. Remember, the energy change during the electronic transition always occurs in the form of distinct packets of energy known as quantum.

The energy of quanta can calculate with the help of the relation between energy and wavelength which is written as below.

$E = h\nu = \frac{hc}{\lambda}$

Where
• h = Planck's constant (6.62517 x 10-27erg.sec)
• $\nu$ = frequency
• c = velocity of light
• $\lambda$ = wavelength

## Periodic Table with Electron Shells

The arrangement of known elements in the periodic table is on the basis of the atomic number of elements. The atomic number of elements is nothing but the number of protons in an atom. Since an atom is neutral in nature therefore, it has an equal number of protons and electrons which makes the atomic number equivalent to the number of electrons. All the electrons in an atom are arranged in a certain manner, in shells, sub shells and orbitals with their maximum capacity.

In the periodic table, usually we write the valence shell electronic configuration of elements which is the outer most electron shell in an atom. Remember the valence shell or outer most shell is mostly studied as it takes part in all the chemical reactions as well as in chemical bonding with other atoms. We can easily find a periodic table with the valence shell electronic configuration of elements written just below to the symbol of the element.

### Solved Example

Question: In the given elements; which one has maximum number of electrons with principle quantum number; n=3;
Ne, Ar, Zn, F, Na, Al

Solution:

Zn has maximum number of electrons in its third shell that is completely filled in this element with 18 electrons.

## Electron Shell Configuration

Let’s discuss how to write the electron shell configuration of the elements. For example; the electron configuration of nitrogen atom which contains totals seven electrons.

Out of seven electrons; two electrons are placed in inner shell while remaining five electrons are in second shell which is valence shell of nitrogen atom. Therefore the electronic configuration can write as;

N = 1s2, 2s2, 2p3

Similarly electronic configuration of silicon with 14-atomic number will be as below.
1s2 2s2 2p6 3s2 3p2

And phosphorus can be written as below;
1s2, 2s2, 2p6, 3s2, 3p1x, 3p1y, 3p1z

### Solved Example

Question: Write the electronic configuration of
1. P
2. S2-
3. Zn3+
4. Br-

Solution:

The electronic configuration is written below.
• P = 1s2, 2s2,2p6,3s2,3p3
• S2- = 1s2, 2s2, 2p6, 3s2, 3p6
• Zn3+ = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d7
• Br- =  1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6

## Electron Shell Capacity

Each electron around the nucleus in an atom can accommodate a certain number of electrons which can determine with the help of principal quantum number related to respective shell. For example, the principal quantum number related to first shell is one, therefore the first electronic shell can accommodate maximum (2(1x1)=2) two electrons.

Similarly eight electrons can occupy in second shell of the atom and eighteen electrons in third electron shell.

### Solved Example

Question: Determine the electron configuration of chromium.
Solution:

Chromium is an element which atomic number 24, i.e. it has 24 protons and 24 electrons. For the filling of electrons in the electronic shells, first electrons fill in lower energy level than move to higher one. Start from 1s orbital which can accommodate maximum two electrons and remains 22 electrons. Now next eight electrons will fill in second shell and remains 14 electrons which can accommodate in 3rd shell. But the energy of 3d is more than 4s, therefore, after 3s and 3p next electron will go in 4s instead of 3d. Hence the electronic configuration of chromium will be; 1s2,2s2,2p6,3s2,3p6,4s2,3d4.