Atomic structure

Atom

The fundamental building blocks of matters are atoms- not be further divisible. The word ‘atom’ was derived from the Greek word ‘a-tomio’ which literally means “non-divisible”. The atomic theory of matter was first proposed by a British schoolteacher John Dalton in 1808 which is known to us as Dalton’s atomic theory.

This theory explains the law of conversion of mass, constant composition, and the law of multiple proportions of an atom. However, it failed to explain why substances like glass or ebonite get electrically charged when rubbed with silk or fur.

Sub-Atomic Particles:

The structure of an atom was obtained from various experiments on electrical discharge through gases. The experiments which were followed to discover the subatomic particles are as follows-

• Michael Faraday and many scientists in 1850, known as Cathode ray discharge tube which conclude that- the movement of cathode rays from cathode to anode in the presence of electrical or magnetic field led us to a point that the cathode rays consists of negatively charged particles, called electron, which is a basic constituent of an atom.
• The Discovery of an electron from cathode rays led to the discovery of canal rays, the behavior of these particles in the presence of an electrical or magnetic field is just opposite to the electron or cathode rays. This positive ion was obtained form hydrogen, which is characterized in 1919 and introduced as a proton.
• In 1932, Chadwick discovered electrically neutral particles having higher mass than protons which was emitted after the bombardment of α-particles on a thin sheet of beryllium. He introduced these particles as neutrons.

Atomic models:

After understanding the subatomic particles of Dalton’s atom, scientists started performing different models to observe stability, physical and chemical properties of the atoms, and also to explain the distribution of these charged particles in an atom. The proposed models are as follows:

• Thomson model of atom:

In 1889, J. J. Thomson proposed that an atom has a circular shape where positively charged neutrons were uniformly distributed and negatively charged electrons were embedded in them in such a way to maintain the most stable electrostatic arrangement. This model is also known as plum pudding, raisin pudding, or watermelon. This model gave an important feature that the mass of an atom is may be uniformly distributed over the atom.

• Rutherford’s nuclear model of atom:

Rutherford and his students, Hans Geiger and Ernest Marsden performed an α-particle scattering experiment and drew an observation over the model that-

1. most of the α-particles were passed through the gold foil without any deflection
2. and a few positively charged α-particles were deflected by a small angle and few were bounced back to the source.

After observations and calculations of the above model, Rutherford proposed the nuclear model of atom, which depicts:

• The positive charge and most of the mass of the atom was concentrated in a small region called the nucleus by Rutherford.
• And the negatively charged electrons were moved around the nucleus with high speed in a circular path called
• An electrostatic force of attraction helps the electron and the nucleus to stable together.

Atomic number and mass number:

• Atomic number(Z) = number of protons in the nucleus of an atom= number of electrons in an atom
• The mass of an atom is due to the protons and neutron present in the nucleus, known as Therefore,

                          mass number(A)= number of protons(Z)+number of neutrons(n)

The composition of an atom is denoted by ^AX_Z, where, X= denote any normal element

                                                                                           A= atomic mass number

                                                                                           Z= atomic number

Isobars and Isotopes:

• Atoms having same mass number but a different atomic number are called isobars Examples: ¹⁴C₆, ¹⁴N₇
• And atoms having the same atomic number but different mass numbers are known as isotopes Examples: isotopes of a hydrogen atom- protium (¹H₁), deuterium (²D₁), tritium (³T₁).

The proportion of electron and neutron in these isotopes are the same, but the change in the mass number in these isotopes is due to the different number of neutrons present in their nucleus. No. of neutrons in, protium=1, deuterium=2, tritium=3.

The chemical property of an atom is characterized by its electron numbers which are determined by the proton number or atomic number(Z).

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