Chapter 4: The Structure of Atom

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Atom: The smallest tiny particles of matter is called atom. 

There are three subatomic particles.

(i) Protron

(ii) Neutron

(iii) Electron

The two particles of these particles are charged.  

(i) Protron: It is positively charged particle which is situated inside the nucleus od an atom. It shows all the chemical properties of an atom. Increasing or decreasing the number of protons of an atom caused changing in chemical properties.

• Protons are represented by p⁺ .
• The Mass of a proton is about 2000 times larger than electrons. 
• The mass of a proton is taken 1 unit and it's charge is taken +1.
• It cann't be peeled off easily from nucleus as it located inside the nucleus. if we try to peel off nucleus can break. 

The discovery of Proton was made by E. Goldstein. 

(ii) Electrons: These are negatively charged (-) particles which revolve around the nucleus in well-defined orbits.

• These are represented by e^- .
• The mass of an electron is considered to be negligible and its charge is minus one.
• Electrons can be easily peeled off.  

The discovery of Electrons was made by J. J. Tomson.

(iii) Neutron: Nutrons are elecrically no charge non-atomic particles which are present in nuceus of an atom. They have no any charge. 

The discovery of nutrons : 

In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron.

• Neutrons are present in the nucleus of all atoms, except hydrogen.
• In general, a neutron is represented as ‘n’.
• The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.

Neutral Atom: Generally, all atoms are electrically neutral, because the number of positively charged particles protons are equal to negatively charged particles electrons in any atom. This is the cause to be an atom neutral and net charges on it is zero. 

Example : Oxygen has 8 positively charge particles protons and 8 negatively charged particles electrons. 

• J. J. Tomson is the first scientist who present the model of atom.

Canal Rays : Canal rays are stream of positively charged particles emitted from anode of immersion tube, when electric current is flown through gas at very low pressure. 

 

Chapter 4. Structure of The Atom

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Thomson's Model of An Atom: 

Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.

You can take watermelon as example.

The positive charge in the atom is spread all over like the red edible part of
the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

        Thomson's Model of An Atom

Thomson prposed that: 

(i) An atom consists of a positively charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

The Limitation of Thomson's Model of An Atom: 

(i) The results of experiments carried out by other scientists could not be explained by this model.

The Discovery of Nucleus: 

Rutherford discovered the nucleus of the atom, Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil. The results showed that the atom is empty most of the α -particles passed directly from the gold foil without getting deflected but some alpha particles were deflected by a very small angle. If these particles were deflected, in fact, that was nucleus of the atom or centroid. Thus Rutherford discovered the nucleus.

Results of Alpha Scattering Experiment of E. Rutherford:

The α-particle scattering experiment gave totally unexpected results. The following observations were made:
(i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly one out of every 12000 particles appeared to rebound.

The Features of Atomic Nucleus Model of Rutherford : 

On the basis of his experiment, Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in an atom called the nucleus.

(ii) All the mass of an atom resides in the nucleus.
(iii) The electrons revolve around the nucleus in well-defined orbits.
(iv) The size of the nucleus is very small as compared to the size of the atom.

(v) The Number of electrons are equal to the number of protons in an atom. 

Rutherford's Atomic Model:

Rutherford concluded from the α-particle scattering experiment that–

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

The Limitations of Atomic Model: 

Acording to Rutherford Atomic model : 

The electrons revolve around the nucleus. The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. In this case the atom should be highly unstable and hence matter would not exist in the form. But this is not fact we know atom is stable. 

Neils Bohr's Model of Atom:

Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy. 

Chapter 4. Structure of The Atom

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Energy Levels : There are various and definite orbits around the nucleus of an atom to revolve the electrons. These orbits or shells are called energy levels. 

      Energy Levels in An Atom 

Electronic Configuration: The distribution of electrons in various and discrete shells of an atom is known as electronic configuration.  The maximum numbers of electrons are difinite in each shell. 

The Electronic Structure of Hydrogen:

Hydrogen has only one electron, so that it's outer shell is K and has only one electron. 

The Electronic Structure of Carbon Atom: 

                     

Carbon atom has total 6 electrons. In which it has 2 electrons in its inner most shell K and 4 electrons in outer most shell L.  

These are shells: 

K, L, M, N etc.

Number of maximum electron in various shell: 

Shell K can have maximum 2 electrons. 

Shell L can have maximum 8 electrons. 

Shell M can have maximum 18 electrons and 

Shell N can have maximum 32 electrons. 

There is used a formula to keep maximum electron in each shell : 2n²

On the basis of this formula, the maximum electrons in a shell is found.

Example, For the first shell K : 2(1)² = 2 x 1 = 2 

For the second shell L : 2(2)² = 2 x 4 = 8 

For the third shell M : 2(3)² = 2 x 9 = 18 

For the fourth shell N : 2(4)² = 2 x 16 = 32 

Valency : The number of valence electrons present in outer most shell of an atom is known as valency.

Example: 

Electronic configuration of some elements and its valency: 

(1) Sodium has 11 electrons,

Electronic configuration of sodium:  2, 8, 1 

(2) Oxygen has 8 electrons,

Electronic configuration of Oxygen: 2, 6 

Example: If there is 1 electron in outer most shell of atom of an element, then valency will be 1. If outer most shell has 2, 3 or 4 electron then valency will be 2, 3 or 4 respectively. But if outer most shell has 5, 6 or 7 electrons then valency will be find in such manner as follows. 

When outer most shell has 5 electrons then valency will be 8 - 5 = 3 

When outer most shell has 6 electrons then valency will be 8 - 6 = 2 

When outer most shell has 7 electrons then valency will be 8 - 7 = 1 

When outer most shell has 8 electrons then valency will be 8 - 8 = 0 

Now, See this table: 

Name of Element	Symbol	No. of Atom	No. of protons	No. of Nutrons	No. of Electrons	Electronic configuration  K   L   M   N	Valency
Hydrogen	H	1	1	-	1	1	1
Helium	He	2	2	2	2	2	0
Lithium	Li	3	3	3	3	2   1	1
Berelium	Be	4	4	4	4	2   2	2
Boran	B	5	5	5	5	2   3	3
Carbon	C	6	6	6	6	2   4	4
Nitrogen	N	7	7	7	7	2   5	3
Oxygen	O	8	8	8	8	2   6	2
Flourine	F	9	9	9	9	2   7	1
Neon	Ne	10	10	10	10	2   8	0
Sodium	Na	11	11	11	11	2   8    1	1
Magnesium	Mg	12	12	12	12	2   8    2	2
Aluminium	Al	13	13	13	13	2   8    3	3
Silicon	Si	14	14	14	14	2   8    4	4
Phosphoras	P	15	15	15	15	2   8    5	3, 5
Sulphar	S	16	16	16	16	2   8    6	2
Chlorine	Cl	17	17	17	17	2   8    7	1
Argon	Ar	18	18	18	18	2   8    8	0

Atomic  Number: The total Number of protons present in the nucleus of an atom is known as atomic number.

Example - Let be the total number of protons in the nucleus of an atom is 8 then atomic number will be 8. Similarily if the total number of protons is 11 then atomic number that atom will be 11.  

Atomic number = Number of protons

Atomic Mass: The sum of total number of protons and nutrons present in the nucleus of an atom is known as atomic Mass.   

Example - If an atom has 4 protons and 5 nutrons in its nucleus then atomic mass will be 4 + 5 = 9.  

Similarily if an atom has 7 protons and 7 nutrons the atomic mass will be 7 + 7 = 14. 

Atomic Mass = No. of protons + No. of Electrons 

Note: The number of protons in an atom is equal to the number of electron of an atom. The same number is its atomic number. The number of nutrons may be equal to the number of protons or may not be equal. 

 

Lesson - 4 

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Q1.  Maximum, how many electrons are held in N orbit of an atom?

Ans: 32 electrons.

Q2.  What is the source of Alpha particles of Rutherford experiment?

Ans:  Double charged Helium particle.  

Q3.  Name the non-atomic particle which does not take part in atomic mass.

Ans: Electrons.

Q4.  Name the positive charged atomic particles which presence in nucleus.

Ans: Protons.  

Q5.  Copper has atomic number 29. Then what is its valency?

Ans: 1

Q6. Give the example of isobar?

Ans: Calcium, Argon

Q7.  Name the scientist who proposed that the atom is as chrismas cake.

Ans: J. Thomson.

Q8.  Name the scientist who discovered protons.

Ans: E Gold Steen.

Q9.  

Q10. Name the isotope of hydrogen which has 2 atomic mass number.

Ans: Deuterium

Q11.  Name the non-atomic particle which takes part in atomic mass and having no charge. 

Ans: Neutron

Q12. Name the isotope of element which is used in treatment of goiter. – Iodine

Q13.  Name the scientist who discovered the nucleus.  

Ans: Rutherford

Q14.   

Q15. Name the scientist who discovered the neutron.  

Ans: J. Chadwick

Q16. Name the isotopes of element which is used in atomic reacter as fuel.

Ans:  Uranium