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 the nucleus 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 the nucleus.

• 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.