Dalton’s atomic theory suggested that an atom was indivisible. However, the discovery of two fundamental particles named electrons and protons, inside the atom, led to the failure of Dalton’s atomic theory.
Fundamental particles of an atom
Three particles; electron, proton and neutron from which an atom is consisted of, are called fundamental particles of an atom or subatomic particles.
Discovery of Electron
- He carried a cathode ray experiment in which observed a stream of negatively charged particles coming out of the cathode towards the anode.
- These particles were named electrons.
Apparatus of Cathode Ray Experiment
Discovery of Proton
- By Ernest Goldstein in 1886.
- He observed in the same gas discharge tube, with different situations that the anode emitted positive particles which he named Canal Rays.
- His experiment led to the discovery of the proton.
Discovery of Neutron
- By J. Chadwick in1932.
- The neutron is present in the nucleus of all atoms.
Thomson’s Model of Atom
- This model is also known as the watermelon model, plum pudding model and apple pie model.
- According to Thomson's Model of Atom: An atom is a uniform sphere of positive charges (due to the presence of protons) as well as negative charges (due to the presence of electrons).
- Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.
Limitations of Thomson’s Model
- It failed to explain how protons and electrons were arranged in an atom so close to each other.
Rutherford’s Model of Atom
This was based on an experiment in which α – particles were bombarded on a thin gold foil.
- Most of the α – particles passed without any hindrance.
- Some of the α – particles deflected from their original path at a noticeable angle.
- Very few of the α – particles bounced back at their original path.
- Most of the part in an atom is empty.
- There is a positively charged centre in an atom, which contains nearly the whole mass of atom. The centre is called the nucleus.
- The size of nucleus is very small compared to an atom.
- Electrons revolve around the nucleus
- According to Rutherford’s Model, the electron revolves around the positively charged nucleus which is not expected to be stable.
- But a charged particle in an accelerated motion along a circular path would continuously undergo loss of energy and finally would fall into the nucleus.
- This makes an atom unstable while atoms are quite stable.
- Rutherford model could not solve the problem of the atomic mass of an atom as it proposed only the existence of protons in the nucleus.
Bohr’s Model of Atom
- An atom consists of a heavy positively charged nucleus.
- The whole mass of the atom is concentrated in the nucleus.
- The electrons in an atom revolve around the nucleus in definite circular paths called orbits or energy level.
- Each energy level is associated with a definite amount of energy.
- The change in energy takes place when an electron jumps from one energy level to another energy level.
Arrangement of electrons in an atom
- The arrangement of electrons in various shells (energy levels) of an atom of the element is known as Electronic configuration.\
- The maximum number of electrons that could be put in a particular shell, i.e., energy levels, was given by Bohr and Bury.
- According to Bohr-Bury Scheme:→ The maximum number of electrons that can be accommodated in any energy level is given by 2n2where n = 1, 2, 3, 4, …. (for K, L, M, N…..)
- The maximum number of electrons in the outermost orbit will be 8 electrons even if it has the capacity to accommodate more electrons.
- Electrons are not accommodated in a given shell. Unless earlier shells are filled, that is stepwise filling of shells is followed.
- The circular path around the nucleus is called orbit, energy level or shell. The energy level is represented by English alphabets: K, L, M, N, …. and so on.
- Therefore, 1st orbit is denoted by K→ 2nd orbit is denoted by L
- 3rd orbit is denoted by M, and so on.
- The distribution of electrons in an orbit can be obtained by using formulae 2n 2 where ‘n’ is a number of that orbit.
- The combining capacity of an atom is called its valency.
- The number of bonds that an atom can form as part of a compound is expressed by the valency of the element.
- Valence electrons are those electrons that are present in the outermost orbit of the atom.
- The total number of protons in the nucleus of an atom gives us the atomic number of that atom.
- It is represented with the letter ‘Z.’
- The number of protons and neutrons combined to give us the mass number of an atom.
- Mass Number is also called Atomic Mass
- It is represented using the letter ‘A.’
- Atoms of the same element with the same atomic number but a different mass number, are called isotopes.
- Chemical properties → same
- Physical properties → different
Applications of Isotopes:
(a) An isotope of Uranium is used as fuel in nuclear reactors.
(b) An isotope of Cobalt is used in the treatment of cancer.
(c) An isotope of Iodine is used in the treatment of goitre
- Atoms of different elements with the same mass number but different atomic numbers are called isobars.
Examples of Isobars
Ques. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.
Ans. Yes, it is true for the hydrogen atom which is represented as 1H1 . It is having one electron, one proton and no neutron.
Ques. What do you understand by the ground state of an atom?
Ans. The state of an atom where all the electrons in the atom are in their lowest energy levels is called the ground state.
Ques. Who identified the sub-atomic particle electron?
Ans. J.J. Thomson discovered the sub-atomic particle electron and proved that it existed without ever being able to see or isolate one.
Ques. Who discovered the nucleus of the atom?
Ans. Rutherford and his co-workers performed alpha-particle scattering experiments which led to the discovery of the atomic nucleus of atom.