- According to John Dalton all matter was composed of a small particle called an atom.
- Atom is a Greek word and its meaning Indivisible i.e. an ultimate particle which cannot be further subdivided.
- Atom is the smallest indivisible part of matter which takes part in the chemical reaction.
- Atom are neither created nor be destroyed.
- Atoms of the same element are similar in size, mass and characteristics however, atoms of different elements have different size, mass and characteristics.
- An atom is made up of three subatomic particle electron, proton & neutrons. These three particles are called fundamental particles of matter.
- There are two parts of an atom:
(ii) Outer part
- Nucleus: The size of the nucleus of an atom is very small in which neutrons and protons are present so the almost entire mass of the atom is situated in the nucleus. Protons & neutrons present in the nucleus are collectively termed as nucleus. Number of neutrons is called the mass number element. Size of the nucleus in the atom is 10-15m.
Mass number (A) = Number of protons in the nucleus (p) + Number of Neutrons (n)
- Outer Part : In the outer part, electrons move around the nucleus in fixed orbits. These orbits are called energy levels.
Thomson's Model of an Atom
Thomson's Model of an Atom
- Detailed model of the atom was first proposed by Sir J.J. Thomson.
- Thomson proposed that an atom consists of a uniform sphere of positive charge in which the electrons are distributed more or less uniformly.
- Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electron is sudden in the positively charged sphere, like the seeds in the watermelon.
➢ Drawback of the Thomson's Model
- An important drawback of this model is that the mass of the atoms is considered to be evenly spread over that atom.
- It is a static model. It does not explain the movement of electrons.
- It could not explain the stability of an atom.
Rutherford's Model of an Atom
- In 1911, scientist "Ernest Rutherford" gave a new picture for the structure of atoms by his α-particle scattering experiment & proposed the structure of atoms. α particle are charged particles having 2 unit of positive charge and 4 units of mass, that is α-particle (2He4) are doubly charged helium atom (He+2).
- When fast moving alpha particles strike very thin gold foil in vacuum, it was found that:
(i) Most of the fast moving α-particles passed straight through the gold foil, without any deflection from their original part.
(ii) Some of the α-particles were deflected by the foil by small angles and few are deflected through large angles.
(iii) A very few alpha particles completely rebound on hitting the gold foil and turn back on their path.
➢ Conclusion of Rutherford experiment
- Rutherford concluded from the α-particle scattering experiment.
- Most of the space inside the atom is empty because most of the α-particle passed through the gold foil without getting deflected.
- Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. A very small fraction of α-particles were deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
- On the basis of this experiment, Rutherford put forward the nuclear model of an atom, which has the following features:
(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of the an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbit.
(iii) The size of the nucleus is very small compared to the size of the atom.
➢ Drawback of the Rutherford Experiment
- According to Maxwell, if an electrically charged particle revolves around a circular path, then it always radiates out energy. Thus, If an electron moves around the nucleus, it must continuously radiate out energy and hence, gradually more towards nucleus in a spiral path, till it collides with the nucleus.
Bohr's Model of Atom
- Rutherford's Model of the atom was unable to explain certain observations with regard to the atom that is stability of the atom and the occurrence of the atomic spectra.
- Niels Bohr accepted Rutherford's idea that the positive charge and most of the mass of atom is concentrated in its nucleus with the electrons present at some distance away.
- It is a quantum mechanical model. This model was based on quantum theory of radiation or plank theory and classical law of physics.
➢ According to Bohr's Theory
- Electrons revolve around the nucleus in well defined orbits or shells each shells having a definite amount of energy associated with the electrons in it. Therefore these shells are also called energy levels.
- The energy associated with the electrons in an orbit increases as the radius of the orbit increases. These shell are known as K, L, M, N..... starting from the one closest to the nucleus.
- An electron in a shell can move to a higher or lower energy shell by absorbing or releasing a fixed amount of energy.
- The amount of energy absorbed or emitted is given by differences energies associated with the two levels. Thus
Energy absorbed ΔE = E2 – E1 = hν
Energy emitted ΔE = E2 – E1 = hv
where h is planck's constant and ν is the frequency of the radiation.
Discovery of Neutron
- In 1932, "Chadwick" bombarded beryllium with a stream of α-particle. He observed that penetrating radiation was produced which were not affected by electric & magnetic fields.
- These radiation consists of neutral particles, which are called neutrons. The nuclear reaction can be shown as as:
- The neutron is a fundamental constituent of atoms and is located in the nucleus.
➢ Characteristics of a Neutron
- Mass: The relative mass of a neutron is almost equal to that of proton. In fact the relative mass of neutron is 1.0087 (1.008) amu and that of proton is 1.0073 (1.008) amu.
- Charge: Neutrons is electrically neutral and has no electric charge. With the discovery of neutrons, we can explain why the atomic mass of helium is 4 amu.
- Atomic mass = Mass of proton + Mass of neutron.