Chapter 4th Structure of Atom Class 09 (Science)

 

       CLASS 09 (SCIENCE)

                        CHAPTER 04

                  STRUCTURE OF ATOM

 

      Jhon Dalton considered atom to be an indivisible entity, but his concept had to be discarded at the end of nineteenth century, when scientist  were able to find existence of charged proton and electron and neutral particle neutron in the atom through experiment. These particles are called subatomic particles.

 

Discovery of electron (By Thomson) :-  J.J.Thomson explained presence of electrons by cathode  rays experiment.

** He using a cathode ray tube, which is a vacuum sealed tube with a cathode and anode on one end that created a beam of electrons travelling towards the other end of the tube.

** In this experiment he observed a stream of negatively charged particle coming out cathode towards the anode. These particles were named as electrons.

ELECTRON:-

##  It is negatively charged sub atomic particle of  an atom.

## The mass of electron is considered to be  negligible, and its charge is -1.

## The symbol for electron is e^-

## Electrons are extremely small.

## They found outside the nucleus.

 

Discovery of Proton (By Goldstein):-

 E. Goldstein by his famous anode rays/ canal rays experiment was able to detect presence of positively charged particles called proton in the atom.

 

PROTON :-

## It is positively charged subatomic particle .

## The charge of proton is +1.

## The symbol of proton is P⁺.

## They found inside the nucleus.

 

Discovery of neutrons (BY Chadwick):- J. Chadwick  bombarded lighter elements with alpha particles and observed emission of new particles having zero charge but having mass equal to that of proton.

 

NEUTRON:-

## It is neutral sub atomic particle.

## There is no charge.

## The symbol of neutron is n.

## They found inside the nucleus.

 

ATOMIC MODEL->

(1)* Thomson’s atomic model

(2)* Rutherford’s atomic model

(3)* Bohr’s atomic model

 

 (1)* Thomson’s atomic model :- This model is often called the “water melon model”

      In this model, Thomson predicted the presence of electrons inside positive sphere, which is made up by proton, just same seeds of watermelon are embedded in red edible part of water melon.

 

 Thomson’s model propose that:

*a* An atom is a uniform sphere of positive charges (due to presence of proton) as well as negative charges (due to presence of electron).

*b* Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.

 

Limitation of thomson’s model:-

  It failed to explain how protons and electrons were arranged in atom so close to each other.

 

(2)* Rutherford’s atomic model:- This was based on an experiment in which alpha particles were bombarded on a thin gold foil.

 

Observations made by Rutherford in his experiment:

1- Most of alpha particles passed without any hindrance.

2- Some of the alpha particles deflected from their original path at a noticeable angle.

3- Very few of the alpha particles bounced back at their original path.

 

Postulates of Rutherford’s Model of atom:-

1- Most of the part of atom  is empty.

2- There is a positively charged center in atom, which contains nearly the whole mass of atom. The centre is called nucleus.

3- The size of nucleus is very small compared to an atom.

4- Electrons revolve round the nucleus.

 

Drawbacks of Rutherford Model:-

 1-  According to Rutherford, electron revolve round the nucleus in well defined orbits, but electrons being charged particles will lose their energy and finally will fall into the nucleus.

2- This will make atom highly unstable.

3- This was the major drawback of Rutherford which was unexplained by him.

4- To overcome drawbacks  of Rutherford’s Model, Neil Bohr in 1912 proposed modified model of structure of atom.

 

(3)* Bohr’s atomic model:- This model of atom states that-

1- An atom consists of heavy positively charged nucleus. The whole mass of the atom is concentrated in the nucleus.

2- The electrons in an atom revolve around the nucleus in definite circular paths called orbits or energy level.

3- Each energy level is associated with a definite amount of energy.

4- The change in energy takes place when electron jumps  from one energy level to another energy level.

 

Arrangement of electrons in atom:-

                The arrangement of electrons in various shells (energy level) of an atom of the element is known as Electronic configuration.

                The maximum number of electrons that could be put in a particular shell, i.e, energy level, was given by Bohr and Bury.

  

According to Bhor-Bury Scheme:

** The maximum number of electrons that can be accommodated in any energy level is given by 2n² where  n = 1,2,3,4…(for K, L, M, N…..)

 

** The maximum number of electrons in the outermost orbit will be 8 electrons even if it has capacity to accommodate more electrons.

 

** Electrons, are not accommodated in a given shell. Unless earlier shells are filled, that is stepwise filling of shells is followed.

 

ORBIT:- The circular path around the nucleus is called orbit, energy level or shell. Energy level are represented by English alphabets: K, L, M, N …. and so on.

 

> 1^st orbit is denoted by K

> 2^nd orbit is denoted by L

> 3^rd  orbit is denoted by M

> 4^th orbit is denoted by N and so on.

 

Distribution of electrons in a orbit or shell:- The distribution of electrons in an orbit can be obtained by using formulae 2n² where ‘n’ is number of that orbit.

Ex.

> Number of electrons in K-shell (1^st orbit)

      n = 1

Therefore, 2n² = 2x1² = 2x1  = 2

Maximum number of electrons in K shell is 2.

>Number of electrons in L-shell (2^nd orbit)        n = 2

Therefore, 2n² = 2x2² = 2x4  = 8

Maximum number of electrons in L shell is 8.

 > Numbero of electrons in M-shell (3^rd orbit)        n = 3

Therefore, 2n² = 2x3² = 2x9  = 18

Maximum number of electrons in M shell is 18.

> Number of electrons in N-shell (4^th orbit)    n = 4 .

Therefore, 2n² = 2x4² = 2x16  = 32

Maximum number of electrons in N shell is 32.

                The shells are always filled in a step- wise manner from the lower to higher energy levels. Electrons are not filled in the next shell unless previous shells are filled.

 

VALENCY :- 

=> The electron present in the outermost shell of an atom are known as the valence electrons.

 

=> The combining capacity of the atoms or their tendency to react and form molecules with atoms of the same or different elements is known as valency of the atom.

 

=> Atoms of elements, having a completely filled outermost shell show little chemical activity. Their combining capacity or valency is zero.

 

 => For the atoms having valence electrons less than or equal to 4, valency is same as that of the number of valence electrons in that atom.

Ex.  Valency of Magnesium is 2 (2,8,2)

 

=> If number of valence electrons exceed 4, then valency is = 8 – number of valence electrons.

Ex. Valency of Oxygen is 2 (2,6) = 8-6 = 2

Name of elements	Symbol	Atomic number	No. of protons	No. of electrons	No. of neutrons	Distribution of electrons				Valency
						K	L	M	N
Hydrogen	H	1	1	1	-	1	-	-	-	1
Helium	He	2	2	2	2	2	-	-	-	0
Lithium	Li	3	3	3	4	2	1	-	-	1
Beryllium	Be	4	4	4	5	2	2	-	-	2
Boron	B	5	5	5	6	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
Fluorine	F	9	9	9	10	2	7	-	-	1
Neon	Ne	10	10	10	10	2	8	-	-	0
Sodium	Na	11	11	11	12	2	8	1	-	1
Magnesium	Mg	12	12	12	12	2	8	2	-	2
Aluminium	Al	13	13	13	14	2	8	3	-	3
Silicon	Si	14	14	14	14	2	8	4	-	4
Phosphorus	P	15	15	15	16	2	8	5	-	3,5
Sulphur	S	16	16	16	16	2	8	6	-	2
Chlorine	Cl	17	17	17	18	2	8	7	-	1
Argon	Ar	18	18	18	22	2	8	8	-	0

 

ATOMIC NUMBER (Z) :- Atomic number of an elements is equal to the number of protons present in the nucleus of an atom.

Atomic number (Z) = number of protons = number of electrons.

 

MASS NUMBER (A) :- The total number of the protons and neutrons present in the nucleus of an atom is called mass number.

                  _Z X^A

Z = Atomic number

A = Mass number

X = Atom

Ex. Nitrogen is written as  ₇N¹⁴

 

ISOTOPES :- Atoms of the same elements with same atomic number but a different mass number, are called isotopes.

    In these chemical properties are same but physical properties are different.

EX.  ₆C¹², ₆C¹⁴

       ₁H¹, ₁H², ₁H³,

 

Application of isotopes:-

>>  Isotope of Uranium used as fuel.

>> Isotope of Cobalt is used in the treatment of cancer.

>> Isotope of iodine is used in treatment of goiter.

 

ISOBARS :- Atoms of different elements with same mass number but different atomic numbers are called isobars.

Ex.  ₂₀Ca^40,  ₁₈Ar⁴⁰