9th Class Science Structure of the Atom

Structure of the Atom

Synopsis

• Atoms are the building blocks of all matter. The word atom, meaning indivisible was coined by Greek philosopher Democritus. Scientists, then assumed that these are the smallest units of matter. The first theory about atom was put forward by Dalton.

• Atom contains fundamental particles electrons, protons and neutrons.

• Thomson's Model of an Atom

        (a) It is also known as water melon model or plum pudding model.

         (b) Electrons are embedded in the positively charged mass distributed uniformly thoughout the atomic sphere.

• Demerits

• It could not explain how both positive and negatively charged particles protect their identity being together.

• Rutherford bombarded a - particles at very high speed to strike a thin 'Au' foil of 10-4 mm thickness.

          Note: An a-particle is a doubly Ionised 'He' nuclei  \[(H{{e}^{++}})\]

• Rutherford's model or planetary model of an atom

          (a) The entire positively charged particles are present in the centre of an atom called nucleus.

           (b) Electrons revolve around the nucleus just as planets revolve around the sun. Hence, it is called as planetary model or solar model.

           (c) Electrons revolve around the nucleus with a high speed to overcome the electrostatic force of attractions between protons and electrons.

• Bohr's Model of Atom

• Neils Bohr overcame the difficulties which Rutherford model faced and proposed his atomic model based on Planck's quantum theory. It explained the origin of stationary orbits which also led to an entirely new concept of atomic structure.

          This theory is based on Planck's quantum theory and the postulates are as given below.

          (a) Electrons revolve around the nucleus in definite, circular paths known as orbits or shells.

          (b) Each orbit is associated with a definite amount of energy. Hence, they are known as stationary orbits.

          (c) These shells are designated as K, L, M, N....... or 1, 2, 3, 4 .........

          (d) As the size of the orbit number increases, energy of the orbit also increases.

• Atomic Number is the number of protons or electrons in an atom.

• Mass Number is the sum of protons and neutrons present in the nucleus of an atom

• Isotopes are atoms having the same atomic number but have different mass number

• Isobars are atoms having the same mass number but different atomic number.