By the end of the 19th century, it was discovered that the atom is not an indivisible particle.
It consists of two fundamental particles, namely, electrons and protons. How these subatomic particles are arranged in an atom? In this blog, we will learn about this arrangement of subatomic particles in an atom i.e. the structure of an atom.

Thomson�s Model of an Atom.

In the year 1904, J.J Thomson was the first one to propose a model for the structure of an atom. Thomson proposed that an atom was a sphere of positive charge in which a number of electrons were embedded, which were sufficient to neutralize the positive charge.

It is like a Christmas Pudding with dry fruit. This model can be compared with a watermelon in which the seeds are embedded. Thomson proposed that:-

�    An atom consists of a positively charged sphere and the electrons are embedded in it
�    The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Although Thomson�s model explained that the atom is electrically neutral, the result of experiments carried out by other scientists could not be explained by this model.

Rutherford�s model of an Atom

Rutherford�s scattering experiment: In 1911, Rutherford performed an experiment in which he bombarded a thin foil of a heavy metal like gold with a beam of fast-moving alpha particles which are doubly ionized helium ions or He2+ ions,  each having 2 units of positive charge and 4 units of mass i.e. having a mass of 4 AMU. These were obtained from radium placed in the cavity of block of lead and made into a fine beam with a slit.

He observed the scattering of the rays after hitting the foil by placing a circular zinc sulfide screen around the metal foil. Wherever alpha particles struck the screen a flash of light is produced at that point on the screen.

From these experiments, Rutherford made the following observations:-

�    Most of the particles that are 99.9% of the particles passed through the foil without undergoing any deflection.
�    Some particles were deflected through small angles and a few were deflected through large angles.
�    Very few i.e. only one in 12000 were deflected back i.e. through an angle greater than 90 degrees.

From these observations, Rutherford drew the following conclusions:-

�    Since most of the alpha particles passed through the foil without undergoing any deflection, there must be sufficient empty space within the atom.
�    As some of the alpha particles were deflected by small angles, and since alpha particles are positively charged they could be deflected only by some other positively charged body present within the atom.

The alpha particles which deflected through small angles were those which passed at a distance pretty far from this positive body. The alpha particles which deflected through large angles were those which passed close to the positive body.

�    Since some alpha particles were deflected back and we know that alpha particles are heavy particles, they could only be deflected back upon striking some heavy object inside the atom.
�    Since the number of alpha particles that deflected back is extremely small, which shows that the heavy body present in an atom must be occupying a very small volume.

This small heavy positively charged body present within the atom was called a nucleus.

Rutherford�s Nuclear Model of an Atom

On the basis of scattering experiments, Rutherford put forward a model of atom known as Rutherford�s Nuclear Model

�    There is a positively charged center in an atom called the nucleus in which all the mass of an atom resides.
�    The size of the nucleus is very small as compared to the total size of an atom.�    The electrons revolve around the nucleus in well-defined orbits.

Drawbacks of Rutherford�s model of an atom

According to Rutherford�s model, an atom consists of a small heavy positively charged nucleus in the center and the electrons were revolving around it. Whenever a charged particle-like electron is revolving around a central force like that of the nucleus, it losses energy continuously in the form of radiations.

Thus, the orbit of the revolving electron will keep on becoming smaller and smaller, following a spiral path and ultimately the electron should fall into the nucleus, which means the atom should collapse. But this actually does not happen and the atom is quite stable.

Bohr�s Model of an Atom

To explain the stability of an atom, Neils Bohr, a Danish Physicist in 1913, proposed a new model of an atom. Postulates of Bohr�s model of an atom are:-

�    An atom consists of a small, heavy, positively charged nucleus in the center and the electrons revolve around it in a circular path known as orbits
�    These discrete orbits have fixed radii and energy and are called energy levels or shells.

These are numbered as 1, 2, 3, 4, etc., or K, L, M, N, and so on as we move outwards from the nucleus.

�    While revolving in discrete orbits, electrons do not radiate energy and so the atom doesn�t collapse.

The state of an atom with the lowest energy is known as the ground state of an atom�    Energy is gained or lost by an electron when it jumps from one orbit to the other, the atom is then said to be in the excited state.

Charged particles in matter

When we run a comb with dry hairs it gets charged and when it is brought near the small pieces of paper, it attracts the small pieces of paper, as it induces an opposite charge on the paper. Now, where does this charge come from? Experiments were performed during the years 1895-1905 which showed that the atom is not the smallest particle but is further made up of smaller particles called subatomic particles.

Protons

In 1886 Goldstein discovered the anode rays, originating from the anode. On studying various properties of anode rays it was observed that anode rays carry a positive charge. These positively charged particles are called protons.

Electrons

In 1897, J.J Thomson discovered the Cathode Rays. On studying various properties of Cathode rays it was observed that cathode rays carry a negative charge. These negatively charged particles are called electrons. So, it seemed likely that an atom was composed of protons and electrons, both in equal numbers for mutually balancing their charges.

Neutrons

In the year 1932 J.Chandwick bombarded the nuclei of some light elements like beryllium (Be) and boron (B) with fast-moving particles. He found that some neutral particles were ejected from the nucleus. Each of these particles carried no charge but had a mass nearly equal to that of protons. This particle was named a neutron.

Neutron may be defined as the sub-atomic or fundamental particle which carries no charge, i.e., it is a neutral particle but has a mass equal to that of the proton. The presence of such neutral particles was already assumed on the basis of the atomic mass of the atom, which was assumed to be equal to the numbers of protons present in the nucleus of the atom, but it was observed that the actual mass of the atom is more than the assumed mass.

So it was suggested by Rutherford in the year 1920 that there must be some neutral particles present in the nucleus because the entire mass of the atom is present in the nucleus. So it is believed that the nucleus of an atom contains not only protons but it also contains neutrons. This solved the problem of explaining the relative mass of an atom.

Characteristics Sub of Atomic Particles

Comparison of the characteristics of electrons, protons, and neutrons:

Particles, charge on the particle, relative charge, the mass of the particle, symbol, location in the atom.

• The particle is electron, the charge of the electron is minus 1.602 * 10-19 Coulomb, the relative charge on electron is -1, mass of the electron is 9.11 * 10-31 Kilograms i.e. 1/1840 AMU, the symbol of electron is. Electron is located outside the nucleus.
• The particle is Proton, the charge of the proton is plus 1.602 * 10-19 Coulomb, the relative charge on proton is +1; mass of the proton is 1.673 * 10-27 Kilograms i.e. 1 AMU, the symbol of proton is. Proton is located inside the nucleus.
• The particle is Neutron, there is no charge on neutron, the relative charge of neutron is zero, mass of the neutron is 1.675 * 10-27 Kilograms which is often taken as 1 AMU; the symbol of neutron is. Neutron is located inside the nucleus.

Read More: Atoms and Molecules: Ions - Explanation | Science Class 9