NCERT Solution Class 9th Science Chapter – 4 Structure of the Atom
| Textbook | NCERT |
| Class | 9th |
| Subject | Science |
| Chapter | 4th |
| Chapter Name | Structure of the Atom |
| Category | Class 9th Science |
| Medium | English |
| Source | Last Doubt |
| NCERT Solution Class 9th Science Chapter – 4 Structure of the Atom Notes What is structure of atom Class 9, What is the structure of atoms, What are the 5 parts of an atom, What is atom full answer, What is atom Class 9 Topper, What are atoms made of, What are the 4 types of structures in chemistry, Who discovered the structure of atom, How big is atom, Who discovered atom First Class 9, What is the smallest unit of matter. |
NCERT Solution Class 9th Science Chapter – 4 Structure of the Atom
Chapter – 4
Structure of the Atom
Notes
Atom
| • Electron • Proton • Neutron |
Electron
| • Thomson (discovery) • From cathode rays • -1.6×10-19 C (charge) • 9.1×10-31 Kg (mass) |
Proton
| • Goldstein (discovery) • Anode rays or canal rays • +1.6×10-19 C (charge) • 1.673×x10-27 kg (mass) |
Neutron
| • Chadwick (discovery) • particles on lighter • elements • neutral • 1.673×x10-27 kg (mass) |
Aromic models
| Thomson’s Atomic model Rutherford’s Atomic model Bohr’s Atomic model |
Thomson’s Atomic model
| Water-melon model Electrons present in positive sphere |
Rutherford’s Atomic model
| Atom has maximum empty space Positive charge occupies less space Nucleus is very small |
Bohr’s Atomic model
| Distribution of electrons in shells Rule of 2n2 Valence shells have valence electrons K, L, M, N shells |
Atom
| Atomic Number Atomic Mass ⇓ ⇓ No .of protons No .of protons or + No. of electron No. of electron Isotopes Same Ay No. Diff Atomic No. Isobars Same Mass NO. Diff Mass No. John Dalton considered atom to be an indivisible entity, but his concept had to be discarded at the end of nineteenth century, when scientists through experiments were able to find existence of charged (electrons and protons) and neutral particles (neutrons) in the atom. These particles were called the ‘Sub-atomic particles’. Atoms • Smallest particle of an element that has all the properties of that element. • Atoms are the basic building block of matter that make up everyday objects. Atoms are made of subatomic particles:- protons, neutrons, electrons |
| Discovery of Electrons – Cathode Rays (By J.J. Thomson) Thomson explained presence of electrons by cathode rays experiment. Facts about Electrons Charge on electron = -1.6 × 10⁻¹⁹ (C=Coloumb) (As calculated by Robert E. Millikan) Mass of electron = 9.1 × 10⁻³¹ kg |
| Discovery of Protons – Anode Rays/Canal Rays (By E. Goldstein) E. Goldstein by his famous anode rays/canal rays experiment was able to detect the presence of positively charged particles called protons in the atom. Facts about Protons • Charge on proton = +1.6 × 10⁻¹⁹C • Mass of proton = 1.673 × 10⁻²⁷ kg i.e., Mass of proton = 1840 × Mass of electron |
Discovery of Neutrons (By J. Chadwick)
| • J. Chadwick bombarded lighter elements (like lithium, boron, etc.) with a-particles and observed emission of new particles having zero charge but having mass equal to that of proton. • These particles were called ‘Neutron’ i.e., neutral particle of the atom. • Neutron are absent in Protium isotope of hydrogen atom. (₁H¹) • Since, mass of electrons are negligible as compared to that of proton and neutrons hence, sum of masses of protons and neutrons in an atom will compose its atomic mass. |
Atomic Models
| From the knowledge of existence of subatomic particles viz., electron, proton and neutron in an atom, various atomic models were proposed by different scientists. Following are some of the atomic models a. Thomson’s Model of Atom b. Rutherford’s Model of Atom c. Bohr’s Model of Atom • The most trusted and scientifically established model of atom which is adopted these days is ‘Quantum Mechanical Model of Atom’. It will be dealt in higher classes. 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 (made up of protons), just same as seeds of watermelon are embedded in red edible part of watermelon.  Rutherford’s Atomic Model • In his famous ‘a-ray Scattering Experiment’, Rutherford bombarded a-ray (Helium nucleus₂H⁴) upon thin gold foil. Rutherford made following observations from this experiment : (i) Most of a-particles passed through gold foil undeflected. (ii) Some of the a-particles deflected by foil by small angles.   • Atom consists of predominantly empty space as most of a-particles passed through gold foil undeflected. • Atom contains centrally placed positively charged nucleus (carrying positively charged particles), because few alpha particles were deflected and very few i.e., one in 12000 bounced back. • Since a minute fraction of a-particles suffered deflections and very few bounced back, this lead to conclusion that most of the space an atom is empty and the space occupied by nucleus is neigligible compared to this empty space. • Size of nucleus was about 10° times that of size of atom. • Whole of the atomic mass is concentrated in the nucleus. On the basis of his experiment, Rutherford proposed model of atom having following features :- • There is positively placed nucleus in an atom. Nearly all the mass resides in nucleus(Proton+Neutron). • Electrons revolves round the nucleus in well defined orbits. • Size of nucleus is very small compared to the size of atom. Drawbacks of Rutherford’s Model (Unstability of Atom) • According to Rutherford, electrons revolve round the nucleus in well- defined orbits, but electrons being charged particles will lose their energy and finally will fall into the nucleus. This will make atom highly unstable. • This was the major drawback of Rutherford which was unexplained by him  (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. (iii) Energy is emitted or absorbed by an atom only when an electron moves from one orbit to another.  |
Atomic Number
| The total number of proton lying in the nucleus of any atom is called the atomic number. • An atomic number is the identity of an atom, changing atomic number means changing the atom. • Atomic number is denoted by‘Z’.(Z = no. of Proton) • For a neutral atom, no. of protons and electrons are equal. Mass Number • It is the sum of total number of protons and neutrons lying in the nucleus of an atom. • Mass Number = Number of Proton + Number of Neutron ↓ ↓ It is denoted A.(A=np+nN) |
Representation of Atom
| A/ZE (E=Symbol of element) E.g., 27/13 Al (z) Atomic No. of Aluminium (A) = 13 (z=np) (A) Mass No. of Aluminium (A/)=27 (A=np+np) ↓ ↓ (A=13+14) Example. Calculate number of protons, electrons and neutrons for : (a) 35 /17 Cl (b) 23/11 Na Solution- (a) 35/17Cl zCl = 17 (np) Here, since Cl is neutral, so ne = np-17. Now, ACl=35Or 35=np+nN Or 35=17+nN Or nN=35-17-18 (b) 23/11Na zNa=11 ANa=23 23=np+nN 23=11+nN nN=23-11-12 |
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