NEET - Chemistry
Structure of the Atom
Introduction
- By 1900, it was discovered that the atom was not a simple, indivisible particle, but rather it contains sub-atomic particles.
- J.J. Thomson discovered the sub-atomic particle namely ‘electron.’
- J.J. Thomson was the first person who proposed a model for the structure of an atom.
- In 1886, E. Goldstein discovered the presence of new radiations in a gas discharge and named them canal rays.
- Another positively charged sub-atomic particle was discovered with experiments of canal rays and named it proton.
Thomson’s Model of Atom
- Thomson proposed that an atom consists of a positively charged sphere and the electrons (negative charge) are embedded in it (as shown in the image given below).
- Further, Thomson said that the negative and positive charges are equal in magnitude. Thus, the atom as a whole is electrically neutral.
Rutherford’s Model of Atom
- E. Rutherford is popular as the ‘Father’ of nuclear physics.
- Rutherford is largely known for his work on radioactivity and the discovery of thenucleus of an atom with the gold foil experiment (as shown in the image given below.
- Rutherford said that in an atom, there is a positively charged center known as thenucleus.
- Rutherford said that nearly all the mass of an atom exists in in the nucleus.
- According to Rutherford, the electrons revolve around the nucleus in well-defined orbits.
Bohr’s Model of Atom
- Neils Bohr further extended Rutherford’s model and improved his drawbacks.
- According to Bohr, only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
- Bohr said that electrons do not radiate energy while revolving in discrete orbits.
- Bohr named orbits or shells as energy levels (as shown in the image given below).
- Bohr represented these orbits or shells are by the letters K, L, M, N,… or the numbers, n = 1,2,3,4,….
Neutron
- In 1932, J. Chadwick discovered a new sub-atomic particle i.e. neutron.
- Neutron has no charge and a mass nearly equal to that of a proton.
- Neutrons are present in the nucleus of all atoms, except hydrogen.
Electrons Distributed in Different Orbits (Shells)
- The maximum number of electrons that can be present in a shell is given by the formula 2n2.
- ‘n’ is the orbit number or energy level index, i.e. 1, 2, 3,….
- According to the given formula −
- First orbit i.e. K-shell will be = 2 × 12= 2
- Second orbit i.e. L-shell will be = 2 × 22 = 8
- Third orbit i.e. M-shell will be = 2 × 32= 18
- Fourth orbit i.e. N-shell will be = 2 × 42 = 32
- Likewise, the maximum number of electrons that can be accommodated in the outermost orbit is 8.
- Electrons are not filled in a given shell, unless the inner shells are filled. It means, the shells are filled in a step-wise manner; starting from inner shell to outer shell.
Valence
- The electrons, those are present in the outermost shell of an atom, are known as the valence electrons.
- According to Bohr-Bury model, the outermost shell of an atom can have a maximum of 8 electrons.
Atomic Number
- The total number of protons, present in the nucleus of an atom, is known as atomic number.
- The number of protons of an atom determines the atomic number.
- Atomic number is denoted by ‘Z’.
- Protons and neutrons collectively are known as nucleons.
Mass Number
- The sum of the total number of protons and neutrons, present in the nucleus of an atom, is known as mass number.
Isotopes
- The atoms of the same element, having the same atomic number but different mass numbers, is known as isotopes. E.g. Hydrogen atom has three isotopes namely protium, deuterium, and tritium.
- The chemical properties of isotopes of an atom are similar but their physical properties are different.
Isobars
- Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. E.g. calcium’s atomic number is 20and argon’s atomic number is 18; further, the number of electrons in these atoms is different, but the mass number of both these elements is 40.
