Inside the Atom – Maharashtra Board Class 9 Solutions for Science and Technology (English Medium)
AlgebraGeometryScience and TechnologyHindi
Solution 1:
- According to the atomic model of J. J. Thomson electrons are embedded in a gel of positive charge.
- All atoms except normal hydrogen contain neutrons in their nuclei.
- The maximum capacity of M shell is 18 electrons.
- The atom of sodium has 1 electrons in the outer most shell.
Solution 2.1:
Solution 3:
- True
- False. The mass of an atom is concentrated in the nucleus.
- False. The mass of an electron is much less than that of a proton.
- False. The electrons in the K shell have minimum energy.
- True
Solution 4.a:
- An atom has three sub-atomic particles. Protons are positively charged and present in the nucleus due to which the nucleus is positively charged.
- Neutrons are neutral, while electrons are negatively charged which are in the extra nuclear part of an atom.
- The number of protons is equal to the number of electrons. Thus, the positive charge and negative charge are equal in magnitude nullifying each other’s effect.
- Hence, the atom is electrically neutral though it contains charged particles.
Solution 4.b:
- The electronic configuration of Helium is (2). These two electrons are accommodated in the K shell as its maximum capacity of accommodating electrons is two.
- The electronic configuration of Lithium is (2, 1). Of these three electrons, two get accommodated in the K shell as its maximum capacity of accommodating electrons is two. The remaining one electron goes into the next shell, i.e. the L shell.
- So, three electrons of Lithium occupy the K and L shells.
Solution 5:
Solution 6.a:
The smallest particle of an element which can exist independently and take part in a chemical reaction is known as the atom of that element.
Solution 6.b:
The path in which electrons revolve around the nucleus of an atom is called an orbit.
Solution 6.c:
The number of electrons or protons in an atom is called the atomic number and is denoted by Z.
Solution 6.d:
The sum of the number of protons and neutrons in the nucleus of an atom is called the atomic mass number. It is denoted by A.
Solution 7.a:
Rutherford’s Scattering Experiment
- Rutherford selected a gold foil as he wanted a very thin layer.
- The gold foil used by Rutherford was 0.00004 centimetres in thickness. That is, the foil was about 1000 atoms thick.
- In his experiment, fast-moving alpha particles (α-particles) were bombarded on a thin gold foil.
- Alpha particles are helium ions with +2 charge. Their atomic mass is 4 u; hence, a high-velocity beam of α-particles has a lot of energy.
- These particles were studied by the flashes of light they produced on striking a zinc sulphide screen.
- The α-particles are much heavier than the sub-atomic particles present in the gold atoms.
- Hence, he expected the α-particles to pass through the gold foil with little deflection and to strike the fluorescent screen.
- However, the observations he made were quite unexpected.
Observations made by Rutherford:
- Rutherford observed that most of the α-particles passed straight through the gold foil.
- Some α-particles were deflected by the foil through small angles, while some were deflected through very large angles.
- Very few α-particles bounced back towards the source.
Rutherford’s Atomic Model
- Based on the results of the α-particles scattering experiments, Rutherford put forth his ‘Theory of atom’.
- An atom contains a positively charged centre called the nucleus of the atom. Almost all the mass of the atom is concentrated in the nucleus.
- Electrons of the atom revolve around the nucleus in fixed, circular orbits.
- This is much like a Solar System where planets revolve around the Sun.
Solution 7.b:
- Atoms of the same element having the same atomic number but different atomic mass numbers are called isotopes.
- Such isotopes have the same number of protons in their nuclei but different number of neutrons. Example: Carbon element has two isotopes and. Both have six protons, but the number of neutrons is 6 and 8, respectively.
- Isotopes have the same chemical properties, but some physical properties may differ.
- Isotopes are used for different purposes.
Examples:
Isotopes of iodine are used in the treatment of goitre.
Isotopes are used in the treatment of cancer.
Uranium-235 is used for nuclear fission and in the production of energy.