Haryana State Board HBSE 9th Class Science Notes Chapter 4 Structure of the Atom Notes.
Haryana Board 9th Class Science Notes Chapter 4 Structure of the Atom
→ Matter is composed of atoms and molecules together.
→ Electrons and protons were discovered by J.J. Thomson and E. Goldstein respectively.
→ J.J.Thomson proposed that electrons are struck into the +ve sphere.
→ The mass of proton is single unit and the charge is written as +1, whereas the mass of electron is negligible and the charge is taken as -1.
→ The defraction experiment of Rutherford’s alpha particles discovered the nucleus.
→ Rutherford’s atomic model proposed that atom has a very small nucleus in its centre and electrons revolve around the nucleus. The stability of atom cannot be described by this model.
→ According to Rutherford, the radius of nucleus is 105 times smaller than the radius of an atom.
→ The model presented by Neils Bohr was more successful. He proposed that electrons have been distributed around the nucleus in different shells with a definite energy. If the outermost shells of the atom gets accommodated, the atom will be most stable and will be less active. ’
→ J. Chadwick in 1932 discovered the existence of neutron in an atom.
→ The nucleus of hydrogen does not possess neutron.
→ Atom has three sub-atomic particles namely electron, proton and neutron.
→ According to Bohr and Bury’s law, the maximum number of electrons in an orbit cannot exceed 2n2. Here, n is the number of orbit or shell.
→ The orbits of an atom have been named as K, L, M, N, ……………..
→ The outermost shell of an atom can possess maximum electrons.
→ The elements whose outermost shells remain completely filled up, are chemically inactive.
→ Hydrogen has three isotopes protium (1H1), Deuterium (1H2), and Tritium (1H3).
→ Chlorine has two 17Cl35 and 17Cl37 isotopes.
→ Isotopes have same chemical properties and different physical properties.
→ The mass of an atom of a natural element is equal to the average mass of all existed natural forms of that element.
→ In nuclear plant, the isotopes of uranium are used up as a fuel.
→ The isotopes of cobalt are used in treatment of cancer.
→ The isotopes of iodine are used in treatment of goitre.
→ Calcium and argon are isobaric atoms.
→ Elements are defined according to the number of protons present in them.
→ Valence Electrons: Electrons present in the outermost shell of an atom are called valence electrons.
→ Valency: Valency is the combining capacity of an atom.
→ Non-reacting Elements: Elements which have complete valence shells are known as non-reacting elements.
→ Atomic Number: The total number of protons present in the nucleus of an atom is called its atomic number (Z).
→ Mass Number: The sum total of protons and neutrons present in the nucleus of an atom is known as mass number (A).
→ Nucleons: Protons and neutrons present in the nucleus of an atom are called nucleons.
→ Isotopes: Atoms of the same element whose atomic number is the same, but mass number is different are called isotopes of an element.
→ Isobars: Isobars are those atoms whose mass number is same, but atomic number is different.