CBSE Class 9 Chemistry, Structure of Atom

•   Sub-atomic particles of atom includes

(a) Electrons – negatively charged with charge of –1 and negligible mass.

(b) Protons – Positively charged with charge of +1 and mass is 1 u.

(c) Neutrons – Particles are with no charge and mass of 1 u.

•   Atom is electrically neutral consisting of positively charged sphere with electrons embedded in it.

•   If failed to explain the results of experiments carried out by other scientists.

•   An atom has a positively charged nucleus and electrons revolve in permitted circular orbits with fixed radii and energy.

•   Bohr explained the drawback of Rutherford’s model of an atom.

•   He performed the α-particle scattering experiment and proposed that an atom is electrically neutral, with a positively charged nucleus having protons and neutrons and negatively charged electrons revolving around the nucleus.

•   The number of electrons that can be present in a given shell is 2n², when ‘n’ is the number of shell.

•   Maximum number of electrons in the outermost orbit should not exceed 8.

•   The combining capacity of the atoms of an element is valency.

•   Elements having 1, 2, 3, and 4 electrons in the valence shell, has valency equal to number of electrons.

•   Elements having more than 4 electrons in the valence shell has valency equal to 8 minus the number of valence electrons.

•   Atomic number (z) is number of protons in one atom of an element.

•   Mass number (A) is the sum of protons and neutrons present in the atom of the element.

•   Notation for an atom

               

•   Isotopes are the atoms of the same element, having the some atomic number but different mass number.

•   Isotopes are useful as nuclear fuel, in medical field, in carbon dating, etc.