Study Notes On Chemistry: Structure of a Atom

Here we are providing the short notes on Chemistry which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.

Structure of a Atom

The atomic theory of matter was first proposed by John Dalton.Fundamental particles of an atom are Electron, Proton and Neutron.

  1. Proton(p): Discovered by E. Goldstein.
  • Protons are positively charged.
  • The absolute charge on the electron to be + 1.6 × 10–19 C.
  1. Electron (e): Discovered by J.J. Thomson when he was studying the properties of cathode ray.
  • Irish physicist George Johnstone Stoneynamed this charge ‘electron‘ in 1891.
  • Electrons are negatively charged.
  • The absolute charge on the electron to be – 1.6 × 10–19 C.
  • e/me as: = 1.758820 × 1011 C kg–1
  • The charge of an electron was measured by R. Millikan in Oil drop experiment.


  1. Neutrons(n) – J. Chadwick
  • It has no charge and a mass nearly equal to that of a proton.
  • The mass of a neutron is taken as one unit each.


  1. Atomic nucleus – Rutherford
  • The fast moving alpha (α)-particles (doubly-charged helium ions) were made to fall on a thin gold foil.
  • The mass of an atom is the sum of the masses of protons and neutrons present in the nucleus.


  1. Valency
  • The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, is called valency.
  • The atoms of elements, having a completely filled outermost shell show little chemical activity,their valency is zero.
  • An outermost-shell, which had eight electrons is said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell.
  • The chemical behavior of an atom depend upon the number of electrons orbiting around the nucleus.
  1. Atomic Number

The atomic number is defined as the total number of protons present in the nucleus of an atom. It is denoted by “Z”. 

  1. Mass number

The mass number is defined as the sum of the total number of nucleons (protons and neutrons) present in the nucleus of an atom.

  1. Isotopes
  • Atoms which have the same atomic number but different mass numbers.The chemical properties of isotopes are similar but their physical properties are different.But some isotopes have special properties which find them useful in various fields. Some of them are :
  • (i) An isotope of uranium is used as a fuel in nuclear reactors.
  • (ii) An isotope of cobalt is used in the treatment of cancer.
  • (iii) An isotope of iodine is used in the treatment of goiter

Radioactive isotopes

Arsenic–74 → detect tumors

Sodiun–24 → Blood clot

Iodine–131 → Activity of thyroid gland

Cobalt–60 → Treat of cancer

  1. ISOBARS- Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.


  1. Isotones – atoms having same number of neutrons.
  2. Isoelectronics – atoms/molecules/ions containing sae number of electrons.
  3. Mass defect – The mass defect is the difference between the rest mass of a nucleus and the sum of the rest masses of its constituent nucleons.
  4. Binding Energy
  • The binding energy of a nucleus is the energy required to separate a nucleus into its constituent parts.
  • For heavier nuclei, energy is released when they break up (fission).
  • For lighter nuclei, energy is released when they fuse together (fusion).
  • Nuclear particles are held together by a nuclear strong force.A stable nucleus remains forever, but as the ratio of N/Z gets larger, the atoms decay. Elements with Z > 82 are all unstable.
  • As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with A > 82 are radioactive.

Examples are:

Alpha particles – (2 proton and 2 neutron) least penetrating

beta-minus particles – (electron) penetrating

beta- plus particles – (positron) penetrating

Gamma rays – most penetrating, high electromagnetic radiation.

Half-Life period – The half half-life of an isotope is the time in which one one- half of its unstable nuclei will decay.

N = N₀(1/2)n     , Where n is number of  half-lives

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