CBSE Class 9 - Science - CH4 - Gist on Structure of The Atom

Structure of The Atom

Study Points

1. Democritus, Greek Philosopher in 400 BC named the smallest piece of matter “ATOMOS ,” meaning “not to be cut.”
2. Dalton's theory (1803) was based on the premise that the atoms of different elements could be distinguished by differences in their weights.
3. WIth the invention of Scanning Tunneling Microscope we can see atoms.
4. Electrons, protons and neutrons are the fundamental particles of an atom.
5. Electrons were discovered by JJ Thomson in discharge tube experiments as cathode rays.
6. Cathode rays are streams of negatively charged particles.
7. They are deflected by electric and magnetic fields.
8. The specific charge of electron was determined by J. J. Thomson
9. The charge of the electron is 1.602 x 10-19 coulombs.
10. The absolute mass of electron is 9.108 x 10-28 g or 9.108 x 10-31kg
11. Protons are positive rays that were discovered by Goldstein in the discharge tube experiments when perforated metal cathode was used.
12. Positive rays are also called as anode rays.
13. Positive rays are deflected by electric and magnetic fields.
14. The lightest positive ray having the highest specific charge was taken in the discharge tube.
15. Positive ray carrying unit charge and unit mass is called proton.
16. The charge of a proton is equal to that of electron in magnitude but they differ in sign.
17. The magnitude of charge of a proton is 1.602 x 10-19 coulombs or 4.8 x 10-10 e.s.u
18. The absolute mass of a proton is 1.672 x 10-24 g or 1.672 x 10-27 kg.
19. The mass of a proton on atomic mass unit scale is 1.00727 a.m.u./u
20. The mass of a proton is approximately 1837 times greater than that of an electron.
21. Neutrons were discovered by James Chadwick.
22. Neutrons were produced when atoms of low atomic weight elements were bombarded by α-particle.
23. Neutrons are unaffected by electric or magnetic field because these are neutral charged.
24. On atomic mass unit scale, mass of a neutron is 1.00866 amu.
25. According to Rutherford’s atomic model the electrons revolve around the nucleus in circular paths.
26. Neil Bohr’s atomic theory model says the electrons revolve around the nucleus in stationary circular paths, called orbits or shells or energy levels.
27. The orbits numbered as K, L, M, N… shells. As long as electrons revolve in the same orbit, it does not lose or gain energy.
28. Maximum number of electrons that can be accommodated in an energy level (n) is given by 2n2
29. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
30. The outermost shell or orbit of an atom is known as valence shell or valence orbit.
31. The electrons present in the outer shell are called valence electrons.
32. The number representing the valence electrons is used to calculate the valency of the element. This valency is regarded as the combining capacity of elements.
33. Bohr’s theory became the basis for modern physics known as Quantum theory.
34. Atomic number of an element is defined as the number of unit positive charges on the nucleus (nuclear charge) of the atom of that element or as the number of protons present in the nucleus.
35. Atomic number, Z = Number of unit positive charge on the nucleus = Number of protons in the nucleus (p) = Number of electrons revolving in the orbits (e)
36. Mass number (A) is defined as the sum of the number of protons & neutrons present in the nucleus of an atom.
37. Mass Number = Mass of protons + Mass of neutrons
38. Isotopes are elements with same atomic number but different atomic mass i.e. same Z but mass number (A) are different.
39. Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.
40. In late 1800s Henri Becquerel and Madam Curie discovered radioactivity.
41. Energy in any form from any source is radiated continuously in the form of packets called quanta.
42. Each packet or bundle of energy is called a photon.
43. The energy associated with a photon is E = hv (equation given by Max Planck)
44. Einstein’s theory is applicable to all types of radiations.