The first question is that WHAT IS AN ATOM??

All substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element which can take part in chemical reactions and may or may not exist independently.

Now, What is the Structure of an Atom?

The history of the discovery of the atomic structure is outlined below:

John Dalton (1808) proposed the Atomic Theory. According to Dalton, matter is made up of tiny particles called atoms. The atom is the smallest particle of matter that takes part in a chemical reaction. Atoms are indivisible and cannot be created or destroyed. Further, atoms of the same element are identical in every respect.

J. J. Thomson (1897) discovered electrons in Cathode Ray experiments. According to Thomson, atoms are divisible. Atoms contain very tiny negatively charged particles called electrons.

E. Goldstein (1900) discovered protons in Anode Ray experiments. According to Goldstein, atoms contain positively charged particles called protons. Since atoms contain negatively charged particles, they must contain positively charged particles for them to be electrically neutral.

E. Rutherford (1911) discovered the nucleus and provided the basis for the modern atomic structure through his alpha particle scattering experiment. According to Rutherford, the atoms is made of two parts: the nucleus and the extra-nuclear part. His experiments proved that the atom is largely empty and has a heavy positively-charged body at the center called the nucleus. The central nucleus is positively-charged and the negatively-charged electrons revolve around the nucleus.

James Chadwick (1932) discovered neutrons. According to Chadwick, atoms contain neutral particles called neutrons in their nucleus along with the subatomic particles (i.e., electrons and protons).

N. Bohr (1940) provided the modern concept of the atomic model. According to Bohr, the atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged) revolve around the nucleus in different imaginary paths called orbits or shells.

A brief knowledge about Atomic Number and Atomic Weight Atomic number of an element is the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the number of protons equal the number of electrons in an atom.

Atomic weight (or relative atomic mass) of an element is the number of times an atom of that element is heavier than an atom of hydrogen. The atomic weight of hydrogen is taken to be unity or 1.

Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom.

The elements are arranged according to increasing atomic numbers (along with their atomic mass) in a table called the Periodic table.

Now, I would like to tell you about some experiments so that discovery of ELECTRON, PROTON & NEUTRON became possible.

1. Electron

Electron was discovered by J. J. Thomson in 1897 when he was studying the properties of cathode ray . Also he won Nobel Prize in 1906 for discovering the elementary particle electron. Interestingly, his son G. P. Thomson also won the Nobel Prize in 1937 for proving the wavelike properties of electron.

Now question arises what cathode ray is
Thomson constructed a glass tube which was partially evacuated i.e. much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of particle (ray) was coming out from the negatively charged electrode (cathode) to positively charged electrode (anode). This ray is called cathode ray and the whole construction is called cathode ray tube.

Properties of cathode ray particle
1. They travel in straight lines.
2. They are independent of the material composition of the cathode.
3. Applying electric field in the path of cathode ray deflects the ray towards positively charged plate. Hence cathode ray consists of negatively charged particles.

J. J. Thomson measured the charge-by-mass-ratio (e/m) of cathode ray particle using deflection in both electric and magnetic field.

e/m=−1.76×10^8 coulomb per gram

The cathode ray particle turned out to be 2000 times lighter than hydrogen.

Although we got e/m ratio for electron from J.J. Thomson’s Cathode Ray Tube experiment, we still don’t know the exact charge (e) for electron. American physicist Robert Millikan designed an experiment to measure the absolute value of the charge of electron. It is called Millikan Oil Drop Experiment. In this experiment he measured the charge of an electron using negatively charged oil droplets. The measured charge (e) of an electron is −1.60×10^(−19) Coulombs.

Using the measured charge of electron, we can calculate the mass of electron from e/m ratio given by J. J. Thomson’s cathode ray experiment.

e/m=−1.76×10^8 Coulomb-per-gram

Putting e=−1.60×10−19 Coulomb,

m=9.1×10−28 gram.

-Electron was discovered by J. J. Thomson in Cathode Ray Tube (CRT) experiment.
-Electrons are negatively charged particles with charge-to-mass ratio −1.76×108 C/gm
-The charge of an electron was measured by R. Millikan in Oil drop experiment.
-Charge of an electron is −1.60×10−19 C
-Mass of an electron is 9.1×10−28 gram.
-Electron is approximately 2000 times lighter than hydrogen.

2. Proton

In 1909, Rutherford did his famous gold foil experiment. In his gold foil experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide (ZnS) screen.

Most of the particles pass through the foil without any deflection.
Some of the alpha particles deflect at small angle.
Very few even bounce back (1 in 20,000).

Based on his observations, Rutherford proposed the following structural features of an atom:

-Most of the atom’s mass and its entire positive charge are confined in a small core, called nucleus.
-Most of the volume of an atom is empty space.
-The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.

Thus Rutherford only told about a positively charged Nucleus not about it’s contents.

3. Neutron

From the previous discussion, we can see that the gold foil experiment gave a clear picture of the structure of an atom which consists of some special particles in nucleus and same number of electrons outside of this nucleus.

But scientists soon realized that the atomic model offered by Rutherford is not complete. Various experiments showed that mass of the nucleus is approximately twice than the number of proton. What is the origin of this additional mass? Rutherford postulated the existence of some neutral particle having mass similar to proton but there was no direct experimental evidence.

Several theories and experimental observations eventually led the discovery of neutron. We can summarize some of the scientific observations behind the discovery of neutron.

In 1930, W. Bothe and H. Becker found an electrically neutral radiation when they bombarded beryllium with alpha particle. They thought it was photons with high energy (gamma rays).

In 1932, Irène and Frédéric Joliot-Curie showed that this ray can eject protons when it hits paraffin or H-containing compounds.

The question arose that how mass less photon could eject protons which are 1836 times heavier than electrons. So the ejected rays in bombardment of beryllium with alpha particles cannot be photon. In 1932, James Chadwick performed the same experiment as Irène and Frédéric Joliot-Curie but he used many different target of bombardment besides paraffin. By analyzing the energies of different targets after bombardment he discovered the existence of a new particle which is charge less and has similar mass to proton. This particle is called neutron. Beryllium undergoes the following reaction when it is bombarded with alpha particle. 4(Be)9 + 2(α)4 => ⟶[6(C)13] => ⟶ 6(C)12 + 0(n)1. Here the symbol z(A)x is used where Z = atomic number and X = atomic mass of the element A.

Atomic mass = mass of protons + mass of neutron. For a neutral atom, number of proton=number of electron.

Now, last but not least
Measured masses and charges of the three elementary particles are given as following-

Particle:: Symbol::Charge::Mass
Electron ::e- :: -1.60×10-19 C :: 9.1×10^(-31) kg
Proton :: p+ :: (H+) ::1.60×10-19 C ::1.672×10^[-27] kg
Neutron :: n:: 0.00 C :: 1.674×10^(-27) kg


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Maha Gupta

Maha Gupta

Founder of www.examscomp.com and guiding aspirants on SSC exam affairs since 2010 when objective pattern of exams was introduced first in SSC. Also the author of the following books:

1. Maha English Grammar (for Competitive Exams)
2. Maha English Practice Sets (for Competitive Exams)