Class 9th Science: Chapter 4 Structure of the Atom
When two objects are rubbed together they get electrically charged. The origin of this charge can be explained, if it is assumed that the constituent particles of matter, the "atoms" are divisible and consist of charged particles.
Sub atomic particles: Atom is defined as the smallest particle of the element that can exist independently and retain all its chemical properties.
According modern atomic theory atoms are composed of particles. The three main sub-atomic particles are proton, neutron and electron.
Cathode rays- Discovery of electrons: In 1878 William Crooks carried out discharge tube experiments and discovered new radiations and called them cathode rays. Since these rays travel from the cathode towards anode. Later J.J Thomson studied on characteristics of cathode rays and concluded that cathode rays are negatively charged particles, now called electrons. The name electron was given by Johnson Stoney.
Discharge tube experiment
Canal rays Or Anode rays: In 1886, E. Goldstein carried out discharge tube experiments and discovered new radiations and called them canal rays. These rays were made up of positively charged particles and led to the discovery of proton.
Properties of electron, proton and neutron:
Present outside the nucleus and revolves in the orbits
Present inside the nucleus
Present inside the nucleus
-1.602 X10-19 coulombs
1.602 X10-19 coulombs
Atomic models: Atomic models proposed by scientists show the arrangement of the various sub-atomic particles in an atom.
Thomson’s atomic model: J.J. Thomson was the first to put forward a model to explain the structure of an atom.
Thomson's atomic model is also called water melon model or Christmas pudding model.
He compared the electrons with the raisins in the Spherical Christmas pudding and to seeds in a watermelon.
Postulates of Thomson’s Model:
• An atom consists of a positively charged sphere, with electrons set within the sphere.
• An atom is electrically neutral as the positive and negative charges within it are equal.
Draw backs of Thomson’s Model
• It could not explain the stability of an atom, i.e how a positive charge in the atom holds the negatively charged electrons.
• It could not explain the position of nucleus in an atom.
• It could not explain the scattering of alpha particles
Rutherford’s Experiment: Thomson’s student, Ernest Rutherford conducted an experiment using gold foil which disproved Thomson’s model.
To study the structure of atom, Rutherford performed a thin gold foil scattering experiment. For his experiments Rutherford used a gold foil. He made a narrow beam of alpha particles to fall on the gold foil.
Observations made from the alpha ray scattering experiment
• Most of the alpha particles passed straight through the gold foil without getting deflected.
• A small fraction of the alpha particles were deflected through small angles.
• A few alpha particles bounced back.
Based on his observations, Rutherford proposed the nuclear model of an atom.
Postulates of Rutherford nuclear model:
• Positive charge is concentrated in the center of the atom, called nucleus.
• Electrons revolve around the nucleus in circular paths called orbits.
• The nucleus is much smaller in size than the atom.
Drawbacks of Rutherford’s Model
• The orbital revolution of the electron is not expected to be stable.
• According to Rutherford’s model, the electrons, while moving in their orbits, would give up energy.
This would make them slow down, gradually and move towards the nucleus. The electrons will follow a spiral path and then fall into the nucleus. Ultimately, the atom would collapse. But in reality the atom is stable.
Bohr’s Model: Keeping the shortcomings of the Rutherford’s model in mind Niels Bohr formed his postulates about the structure of an atom as the following.
Postulates of Bohr’s Model:
• Electrons revolve in discrete orbits called shells.
• Electrons revolve in their orbits without radiating energy. Within a particular orbit, the energy of an electron is constant. This is why orbits are called stationary orbits or stationary shells.
• Orbits or shells are also known as energy levels.
• These orbits or shells are represented by the letters K, L, M, N,… or the numbers n=1, 2, 3, 4,….
Drawbacks of Bohr’s Model:
• Bohr’s model did not apply to elements like helium and lithium and the higher elements containing more than one electron.
• The model was also unable to explain the structure of chemical bonds.
The discovery of neutrons: Consider the element helium (He24). It was found that helium has two protons and two electrons however its mass was found to be four times that of Hydrogen. Similarly, the masses of some other elements were also found to be double or more than double the number of protons.
This problem was solved on the discovery of another particle ‘neutron’ by James Chadwick In 1932 by bombarding beryllium with alpha particles.
Be49 + He24 → C612 + n01
‘Neutron’ is a neutral particle with a mass equal to that of a proton and is present in the nucleus along with a proton.