Structure Of The Atom - Class 9

Powerpoint Presentation on the Topic - Structure Of The Atom.
Made By - NehaRohtagi1

1. STRUCTURE OF THE ATOM CLASS – 9

2. what IS AN ATOM ? Atoms are defined as “ Basic Building Blocks of Matter ”. It is the Smallest Constituent Unit of Matter that posses the properties of the chemical element. Atoms are very small. Atomic Radius is measured in Nanometers (1 m =10 9 nm). Atom is made up of three Sub – Atomic Particles :- Protons – It is found in the Nucleus with a Positive Charge and mass of 1 A.M.U . Neutrons – It is found in the Nucleus with a Neutral Charge and mass of 1 A.M.U . Electrons – It is a Sub – Atomic Particle with Electrically Negative Charge and of Negligible Mass ( Mass of the Proton ).  

3. The Sub – Atomic Particles

4. PROTONS In 1886 E. Goldstein discovered evidence for the existence of this Positively Charged Particle ( Protons ) . He used a Cathode Ray Tube with Holes . Then he noticed that these rays were travelling in the Opposite Direction from the Cathode Rays . He called them as the Canal Rays and showed that they were composed of Positively Charged Particles . The mass of the Proton is approximately 2000 times as that of the Electron . In general, Proton is represented as ‘ p + ’. Its mass is taken as 1 Atomic Mass Unit and the Charge of +1 .

5. NEUTRONS In 1932 , J. Chadwick discovered the Sub – Atomic Particle which had Neutral Charge and a Mass of 1 A.M.U. which is equal to the mass of the Protons. It was named as Neutron . Neutrons are present in the nucleus of all atoms, except Hydrogen . In general, a Neutron is represented as ‘ n ’. ( Atomic Mass = No. of Protons + No. of Neutrons )

6. ELECTRONS In 1897 , J.J. Thompson discovered Electrons which had a Negative Charge and mass which is than that of the Protons . In general, Electron is represented as ‘ e - ’. ( Atomic Number = No. of Protons = No. of Electrons )  

7. DALTON’S ATOMIC THEORY The Atomic Theory of John Dalton suggested that the atom was Indivisible and Indestructible . But the discovery of Sub – Atomic Particles led to the failure of Dalton’s aspect. Then, it was considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom. Then, Ernest Rutherford proposed a model for the structure of an atom by doing Certain Experiments . After that, Neils Bohr postulated a model of an atom which overcome the Objections raised by Rutherford .

8. J.J. THOMSON’S MODEL J.J. 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. It was just like a Watermelon where the Red Part was the Positive Sphere and the Seeds are the Electrons which are embedded in it. Thomson proposed that :- An atom consists of a positively charged sphere and the electrons are embedded in it. The Negative and Positive Charges are equal in Magnitude . So, the atom as a whole is Electrically Neutral .

9. RUTHERFORD’S MODEL Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. So, he carried out an experiment in which fast moving Alpha (α)-Particles were made to fall on a thin Gold Foil . He selected a Gold Foil because he wanted as thin a layer as possible. This gold foil was about 1000 Atoms Thick . α-Particles are Doubly-Charged Helium Ions . As, they have a mass of 4 u , the fast-moving α-particles have a considerable amount of energy. It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections.

10. RUTHERFORD’S OBSERVATIONs From the – particles Experiment Rutherford concluded that :- Most of the fast moving – particles passed straight through the Gold Foil because most of the space inside the atom is Empty . Some of the α-particles were deflected by the foil by Small Angles , indicating that the Positive Charge of the atom occupies Very Little Space . Surprisingly one out of every 12000 Particles appeared to Rebound by 180 º , indicating that all the Positive Charge and Mass of the Gold Atom were concentrated in a Very Small Volume within the atom.  

11. DRAWBACKs OF RUTHERFORD’S MODEL On the basis of his Experiment , Rutherford put forward the Nuclear Model of An Atom , which had the following features: There is a Positively Charged Center in an atom called the Nucleus . Nearly all the Mass of An Atom resides in the Nucleus . The Electrons revolve around the Nucleus in Circular Paths . The Size of the Nucleus is very small as compared to the Size of the Atom . Drawbacks of Rutherford’s Model of the Atom The Revolution of the Electron in a Circular Orbit 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 Loses Energy and finally fall into the Nucleus . If this were so, the atom should be Highly Unstable . But atoms are Quite Stable .

12. BOHR’S MODEL OF ATOM In order to overcome the objections raised against Rutherford’s Model of the Atom , Neils Bohr put forward the following postulates about the model of an atom: Only Certain Special Orbits known as Discrete Orbits of Electrons , are allowed inside the atom. While revolving in Discrete Orbits the Electrons Don’t Radiate Energy . These orbits or shells are also known as Energy Levels . These orbits or shells are represented by the Letters K,L,M,N or by the Numbers, n=1,2,3,4 .

13. ELECTRONIC CONFIGURATION The Electronic Configuration was suggested by Neils Bohr and Charles Rugeley Bury . The following rules are followed for writing the number of electrons in different Discrete Orbits : The maximum number of electrons present in a shell is given by the formula , where ‘n’ is the orbit number with 1, 2, 3, 4 increases according to the: 1 st Orbit (K – Shell) : 2 * 1 2 = 2 Electrons 2 nd Orbit (L – Shell) : 2 * 2 2 = 8 Electrons 3 rd Orbit (M – Shell) : 2 * 3 2 = 18 Electrons 4 th Orbit (N – Shell) : 2 * 4 2 = 32 Electrons The maximum number of electrons that can be accommodated in the Outermost Orbit is 8 . Electrons are not filled in the next shell unless previous shells are filled.  

14. VALENCY The combining capacity of the atoms of other elements, that is, their tendency to react and form molecules with atoms of the same or different elements is called Valency . Valency is determined by the outermost shell, which has to posses an Octet ( 8 Electrons ). This is done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the Valency. If the element is a Non – Metal having 5,6,7 - Electrons in the Outermost shell then the valency is determined by subtracting No. of Electrons in Outermost Shell from Octet . If the element is a Metal having 1,2,3 – Electrons in the Outermost shell then the valency is said to be the No. of electrons in the Outermost Shell .

15. Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells

16. ISOTOPES Isotopes are atoms of the same element that have the same number of protons (i.e., atomic number, "Z") but a different number of neutrons, meaning that their mass number, "A", varies. For example, take the case of hydrogen atom, it has three atomic species, Protium ( 1 1 H ), Deuterium ( 2 1 H ), Tritium ( 3 1 H ). Applications Since the chemical properties of all the isotopes of an element are the same, normally we are not concerned about taking a mixture. But some isotopes have special properties which find them useful in various fields. Some of them are : An isotope of uranium is used as a fuel in nuclear reactors. An isotope of cobalt is used in the treatment of cancer. An isotope of iodine is used in the treatment of goitre .

17. ISOBARS Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of nucleons is the same in the atoms of this pair of elements. Atoms of different elements with different atomic numbers, which have the same mass number, are known as Isobars .