• explain the atomic number, isotopes and isobars; • recognize the fundamental particles of atom; • describe Rutherford’s experiment and explain its results; • define electromagnetic radiation; • list and define the characteristic parameters of electromagnetic radiation; • discuss line spectrum of hydrogen; • explain Bohr’s postulates and discuss his model; • draw energy level diagram of hydrogen atom showing different series of lines in its spectrum;

Atomic Number, Isotopes, and Isobars:

• The atomic number of an element is the number of protons in the nucleus of an atom. It defines the chemical properties of an element and its place in the periodic table.
• Isotopes are atoms of the same element that have different numbers of neutrons. They have the same atomic number but different mass numbers.
• Isobars are atoms (nuclei) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number.

Fundamental Particles of Atom:

• Protons: Positively charged particles present in the nucleus.
• Neutrons: Neutral particles present in the nucleus.
• Electrons: Negatively charged particles orbiting the nucleus.

Rutherford’s Experiment:

• Rutherford conducted an experiment by bombarding a thin gold foil with alpha particles. He found that most of the alpha particles passed straight through the foil, some were deflected slightly and a few were deflected back.
• This led to the conclusion that most of the space in an atom is empty, with a tiny, dense, positively charged nucleus at the center.

Electromagnetic Radiation:

• Electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy.

Characteristic Parameters of Electromagnetic Radiation:

• Wavelength: The distance between successive crests of a wave.
• Frequency: The number of complete wave cycles per second.
• Speed: The speed at which the wave propagates.
• Amplitude: The maximum extent of a vibration or oscillation, measured from the position of equilibrium.

Line Spectrum of Hydrogen:

• When an electric discharge is passed through hydrogen gas at low pressure, a bluish light is emitted. If this light is passed through a prism, it splits into several lines of different colors. This is known as the line spectrum of hydrogen.

Bohr’s Postulates and His Model:

• Bohr proposed that an electron in an atom revolves in certain stable orbits without the emission of radiant energy, contrary to the predictions of electromagnetic theory.
• He also postulated that an electron could jump from one orbit to another by absorbing or emitting energy, giving rise to spectral lines.

Energy Level Diagram of Hydrogen Atom:

• The energy level diagram of a hydrogen atom shows different series of lines in its spectrum. These series (Lyman, Balmer, Paschen, Brackett, and Pfund) correspond to the transition of an electron from a higher energy level to a lower one, with the emission of a photon of light.