Ch#2 : Structure Of Atoms

Question 1

Explain Rutherford’s experiment, its conclusions and defects.

Answer 1

Rutherford’s experiment : In 1911, Rutherford performed a series of experiments to know the arrangement of electrons and protons in atoms.

He bombarded a thin gold foil of thickness (! 0.0004cm) with a-particles obtained from the disintegration of polonium. a-particles are doubly positively charged helium nuclei, (He++) . Most of these particles passed straight through the foil. Only a few partciles were slighrly deflected. Bit 1 in 1 million particles were deflected at a angle greater than 90° from thier straight paths. He performed similar experiments with foils of other experiments.

Conclusions:
1. Since majority of the a-particles passed through the foil undeflected, most of the space occupied by an atom must be empty.
2.The deflection of a few a-particles at abgles greater than 90° shows that they were deflected by the electrostatic repulsion between the positively charged a-particles and positively charged part of an atom.
3. Massive a-particles are not deflected by electrons.
On the basis of these results, he proposed a new model for a atom. He proposed a planetary model similar to our solar system for an atom. An atom is an electrically neutral particle. The mass of an atom is concentrated in a very small densely positively charged regions which he named nucleas. The electrons revolve around the nucleas in circles.These circles are called Orbits. The centripetal force due ti the revolution of electrons balances out the electrostatic force of attract between the nucleas and electrons.

Defects:

1. Classical Physics suggests that electrons being charged particles emit energy continously while revolving around the nucleas. The orbit of the electron would become smaller and smaller until it would fall into the nucleas. This would collapse the atomic structure.
2. If a revolving electron emits energy continously it should form a continuous spectrum but a line spectrum is obtained.

Question 2

State the main postulates of Bohr’s Atomic theory.

Answer 2

In 1913, Neil Bohr proposed a model for an atom which was consistent witb Rutherford’s but explained the observed kine spectrum of the Hydrogen atom.

1. The electron in an atom revolves around the nucleas in one of the circular paths called orbits. Each orbit has a fixed energy so orbits are also called energy levels.
2. The energy of an electron in an orbit is directly proportional to its distance frkn the nucleas. The farther the electron, the more energy it has..
3. The electron revolves only in orbits for which the angular momentum is an integral multiple of h/2Pi (h is plank’s constant and its value is 6.626 * 10^-34Js)
4. Light is absorbed when an electron jumps to a higher energy orbit and emitted when an electron falls into a lower energy orbit. An electron in a particular orbit doesnt radiate energy
5. The energy of the light emitted is exactly equal to the difference between the energies of the orbits.
   ΔE = E2 - E1

Question 3

What are Shells and Subshells?, define the Auf Bau Principle and electronic configuration.

Answer 3

Shells:According to Bohr’s theory, an electron in an atom revolves around the nucleas in circular paths called shells or orbits. Each shell has a fixed energy, so it is called an energy level as well. Each shell is described by an n value, n can be 1, 2, 3,…

N = 1, K shell
N = 2, L shell
N = 3, M shell and so on
As the value of n increases, the distance of the electrln from the nucleas increases and the energy of the shell increases.

Sub-shell : a shell or energy level is further divided into sub shells or sub energy levels.n value of the shell is placed before the symbol for the subshell.

n = 1, K shell. 1s
n = 2,  L shell. 2s, 2p
n = 3, M shell. 3s, 3p, 3d
n = 4, N shell. 4s, 4p, 4d, 4f

Maximum electron capacity,
s : 2
p : 6
d : 10
f : 14

Electronic Configuration : The arrangement of electrons in sub-shells is called electron configuration..
We can fill the electrons in the various elements with the Auf Bau Principle.

Auf Bau Principle : Electrons fill up the lowest energy subshell that is avaliable first.