Question

a) Describe the Democritian model for the atom.

b) Describe the Thomson model for the atom. What experiment led to its formulation?

c) Describe the Rutherford model for the atom. What experiment led to its formulation?

d) Describe the Bohr atom. What experimental evidence led to its formulation?

Answer 1

A) Democritus’s model stated that matter consists of invisible particles called atoms and a void (empty space). He stated that atoms are indestructible and unchangeable. Also that they are homogenous, meaning they have no internal structure. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void exists between them.

B) Thomson’s Atomic Model
According to the postulates of Thomson’s atomic model, an atom resembles a sphere of positive charge with electrons (negatively charged particles) present inside the sphere.
The positive and negative charge is equal in magnitude and therefore an atom has no charge as a whole and is electrically neutral.
Thomson’s atomic model resembles a spherical plum pudding as well as a watermelon. It resembles a plum pudding because the electrons in the model look like the dry fruits embedded in a sphere of positive charge just like a spherical plum pudding. The model has also been compared to a watermelon because the red edible part of a watermelon was compared to the sphere having a positive charge and the black seeds filling the watermelon looked similar to the electrons inside the sphere.

C) Rutherford’s Alpha Scattering Experiment;

Rutherford’s conducted an experiment by bombarding a thin sheet of gold with α-particles and then studied the trajectory of these particles after their interaction with the gold foil.

The observations made by Rutherford led him to conclude that:

A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume.
Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180° angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.

D) Bohr model;

Thomson’s atomic model and Rutherford’s atomic model failed to answer any questions related to the energy of an atom and its stability.

Salient features of Niels Bohr atomic model are:

Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level.
An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state.
An electron emits or absorbs energy when it jumps from one orbit or energy level to another. When it jumps from a higher energy level to lower energy level it emits energy while it absorbs energy when it jumps from a lower energy level to a higher energy level.
The energy absorbed or emitted is equal to the difference between the energies of the two energy levels (E1, E2) and is determined by Plank’s equation.
ΔE = E2-E1 = hv

Where,

ΔE = energy absorbed or emitted
h= Plank’s constant
v= frequency of electromagnetic radiation emitted or absorbed

The angular momentum of an electron revolving in energy shells is given by:
mvr = nh/2π

Where,
n= number of corresponding energy shell; 1, 2, 3 …..
m= mass of the electron
v= velocity
r=radius
h= Plank’s constant