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• Discovery of subatomic particles:
  

  Scientist	Subatomic particle	Symbol	Charge
  E. Goldstein	Proton	p+	+1
  J.J.Thomson	Electron	e–	-1
  James Chadwick	Neutron	n	No charge

• Thomson’s Model of Atom:
  • Atom is positively charged sphere and electrons are fixed in it.
  • Positive and negative charges are in same quantity.
  • Charges are equally distributed in entire atomic space.
  • Atom is electrically neutral.
• Rutherford’s Model of Atom:

  Rutherford carried out experiment in which ⍺ particles i.e. He²⁺ions were bombarded on thin gold foil.

  

  Observation	Conclusion
  Most of ⍺ particles passes through the coil without deflection	Atom contains large empty space
  Some ⍺ particles got deflected by small angles	Atoms contain positive charge in small space
  Few particles (1 / 12000 ) were reflected back	Positive charge and most of mass of atoms is present in very small volume in an atom

Click here to watch video on Thomson’s Model and Rutherford’s Model

• Nuclear model of atom:
  • Dense positive charge situated at the center of atom is called nucleus.
  • Size of nucleus is very small with respect to size of atom.
  • Electrons revolve around nucleus in circular path.
• Drawback of Rutherford’s Model:In an atom electron revolve nucleus. While revolving electron emits energy and energy must be constantly decreasing. Due to loss of energy, radius of revolution will decreases and electron will fall in the nucleus. This will makes atom highly unstable. But in reality atoms are stable.
• Bohr’s Atomic Model:
  • Electrons revolve in circular path, in discrete orbit with stable energy.
  • Orbits are also called energy shells or energy levels.
  • Energy of electron may change if electron jumps from one orbit to another.
  • Orbits or shells are represented by alphabets K,L,M,N,… or by numbers n= 1,2,3,…
• Distribution of electrons in shell:
  • Electrons in one orbit = 2n²; n = shell number or energy level
  • Electrons fill from inner shell to outer shell
  • Duplet: Orbit allows 2 electrons in it
  • Octet: Orbit allows 8 electrons in it
• Valency:
  • Combining capacity of an atom
  • It is number of electron in outermost shell or eight minus electrons in the outermost shell.
  • For elements with completely filled outermost shell, valency is zero. E.g. Neon, Argon, etc.

  Element	Electrons in outermost shell	Valency
  Sodium	1	1
  Carbon	4	4
  Chlorine	7	(8-7) = 1

• Atomic number(Z): Number of protons present in the nucleus of atom.
• Mass number(A):Sum of protons and neutrons in the nucleus.
• Atomic number and mass number are together represented by _Z^AX

Element	Atomic Number (Z)	Mass Number (A)	Symbol
H	1	1	11H
He	2	4	24He
Li	3	7	37Li
Be	4	9	49Be

• Isotope: Atoms of same element having same atomic number but different mass number
• Isobar: Element with same mass number and different atomic number

Examples of Isotopes	Examples of Isobars
126C , 146C	4017Cl, 4018Ar , 4019K, 4020Ca
1735Cl , 1737Cl	5826Fe, 5827Ni
11H, 12H, 13H	168O, 167N
2655Fe, 5626Fe,5826Fe	2411Na, 2412Mg
63Li, 73Li, 83Li	7632Ce, 7634Se

• Comparison between isobars and isotopes:
  

  Isotope	Isobar
  Same atomic number different mass number	Same mass number different atomic number
  Same number of protons different number of neutrons	Different number of electrons
  Same chemical properties	Different chemical properties
  E.g. 11H, 12H, 13H	E.g. 4017Cl, 4018Ar, 4019K, 4020Ca