This PSEB 9th Class Science Notes Chapter 4 Structure of the Atom will help you in revision during exams.
PSEB 9th Class Science Notes Chapter 4 Structure of the Atom
→ It was known by 1900 that the atom was not a simple, indivisible particle but contains at least one sub-atomic particle i.e., electron identified by J.J. Thomson.
→ Even before the electron was identified by J.J. Thomson, E. Goldstein in 1886 discovered the presence of new radiations in a discharge tube called canal rays.
→ The canal rays were positively charged radiations which led to the discovery of the proton.
→ Proton has a charge, equal in magnitude but opposite in sign to that of electron and a mass approximately 2000 times as that of the electron.
→ Generally, the electron is represented by ‘e’ and a proton as ‘p’.
→ The mass of a proton is taken as one unit and charge +1.
→ The mass of an electron is considered to be negligible and its charge -1.
→ α-particles are doubly-charged helium ions having mass 4u \(\left({ }^{4}{ }_{2} \mathrm{He}^{2+}\right)\)
→ E. Rutherford discovered the nucleus of an atom on the basis of an α-ray scattering experiment.
→ E. Rutherford was awarded the Nobel prize in chemistry for his famous work discovery of radioactivity and the discovery of the nucleus of the atom.
→ On the basis of his experiment, Rutherford put forward the nuclear model of an atom.
→ According to Rutherford’s model, there is a positively charged centre of an atom called a nucleus and contains nearly all the mass and all the positive charge.
→ The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom.
→ Neils Bohr’s model of the atom was more successful.
→ He suggested that only certain special orbits known as discrete orbits of electrons are allowed inside the atom. While revolving electrons do not radiate energy.
→ J. Chadwick discovered another sub-atomic particle that had no charge but mass nearly equal to a proton. This particle is called the neutron.
→ The orbits or shells in an atom are designated as K, L, M, N ………… shells starting from the nucleus side.
→ J.J. Thomson suggested that an atom is a uniform sphere of positive electricity in which electrons are embedded it.
→ The total positive charge is equal to the total negative charge and the atom, on the whole, is electrically neutral.
→ Electron is a negatively charged particle with 1.602 × 10-19 coulomb negative charge (-1 unit) and mass 9.1089 × 10-19 kg (negligible mass). It is represented by the symbol ‘e’. It is a fundamental particle of an atom.
→ Proton is a positively charged particle with a 1.602 × 10-19 coulomb positive charge (+1 unit) and mass 1.672 × 10-27 kg, it is represented by the symbol ‘p’. It is a fundamental particle of an atom.
→ Neutron is a neutral particle with no charge and mass equal to 1.678 × 10-27 kg. It is represented by the symbol ‘n’. It is a fundamental particle of an atom.
→ The nucleus is the small, positively charged, and heavy central portion in an atom that contains in it protons and neutrons.
→ Nucleons. The neutrons and protons present in the nucleus of an atom are collectively known as nucleons.
→ An atomic number of an element represents the number of protons in an atom. It is denoted by the symbol Z.
→ Shells of an atom are designated as K, L, M, N, etc. These are also called energy levels.
→ The valence shell of an atom represents the outermost shell where electrons are present and the electrons are called valence electrons.
→ A mass number of an element is the sum of the number of protons and neutrons present in the nucleus of the atom. It is denoted by A.
→ The valence shell of an atom represents the outermost shell where electrons are present and the electrons are called valence electrons.
→ Valency. It is the combining capacity of an atom of the element.
→ Isotopes are the atoms of the same element having the same atomic number but different mass numbers.
→ Isobars are the atoms of the different elements having the same mass number but different atomic numbers.