Chemistry Formula for class 11 chapter- Atomic Structure

Jul 07, 2023, 16:45 IST

Thomson Model of Atom

J.J. Thomson proposed a model in which atom was assumed to be uniform sphere with radius 10 cm of positive electricity (positive charge) with electrons embedded into it in such a way as to give the most stable electrostatic arrangement.

positive charge

In this model the atom is visualized as a pudding or cake of positive charge with raisins (electrons) embedded into it. This model is also called the ‘raising pudding’ model.

Drawback of Thomson Model of Atom

This model of atom could account for the electrical neutrality of atom, but it could not explain the results of gold foil experiment carried out by Rutherford.

a-particles

They are positively charged (+2) helium ions having high energy emitted during radio-active decay of unstable elements like Uranium. It has +2 charge and mass 4u.

Rutherford's a-Scattering Experiment (Discovery of Nucleus)

Rutherford bombarded high energy a-particles from radioactive source on thin foil (thickness 10^–7m) of gold metal. The thin gold foil had a circular flurescent ZnS screen around it. Whenever an a-particle struck the screen, a tiny flash of light was produced at that point. He observed that :

deflected ray

Most of a -particles passed through the gold foil undeflected.
A small fraction of a-particles was deflected by small angles.
A very few a-particles (1 in 2000) bounded back, i.e., were deflected by nearly 180°.

Rutherford's Conclusions regarding Model of Atom:

Most of space in atom is empty because most of a-particles passed through the foil.
The positive charge has to be concentrated in a very small volume that repelled and deflected few positively charged a-particles. This very small portion of the atom was called nucleus.
The volume of nucleus is very small as compared to total volume of atom. The radius of nucleus (10^–13 cm) is about 1/100,000 of that atom (10^–8 cm). If cricket ball is nucleus then radius of atom will be about 5 km.
The volume of nucleus is very small as compared to total volume of atom. The radius of nucleus (10–13 cm) is about 1/100,000 of that atom (10–8 cm). If cricket ball is nucleus then radius of atom will be about 5 km.

Rutherford’s Nuclear Model of Atom:

An atom consists of tiny positively charged centre called nucleus.
The nucleus consists of protons and neutrons called nucleons.
Nucleons (proton and neutron) are much heavier than an electron indicating that mass of atomlies in its nucleus. The total number of nucleons is termed as mass number.
The electrons are outside the nucleus revolving around nucleus at high speed in certain fixed circular path called orbits in the same manner as planets move around sun.

Quantization

The restriction of a property to discrete values and not continuous values is called quantization.

Hydrogen Spectrum

If a discharge is passed through hydrogen gas at a low pressure, some hydrogen, atoms are formed, which emit light in the visible region. This can be studied by using Spectroscope or Spectrometer, and is found to comprise of series of lines of different wavelength. The four lines can be seen by eyes, but many more observed photographically in ultraviolet region.

Quantum Numbers

Atomic orbitals can be specified by giving their corresponding energies and angular momenta which are quantised (i.e., they can bave some specific values). The quantised values can be expressed in terms of quantum number. They are used to get complete information about electron. i.e., location, energy, spin, etc.

Principal Quantum Number

It specifies the location and energy of an electron. It is the measure of the effectiv volume of the electron cloud. It is denoted by `n’. Its possible values are 1, 2, 3, .... for K, L, M, N....

Pauli Exclusion Principle

No two electrons in an atom can have the same four quantum numbers. It can also be stated as—An orbital can bave maximumtwo electrons and they must be of opposite spin quantum numbers.

Chemistry formula for chapter-Atomic Structure is prepared by expert of Physics Wallah and consist of all-important formula use in Atomic Structure chapter. This chemistry formula sheet for Atomic Structure is highly recommended for the quick revision of the entire chapter-Atomic Structure.