Inside the Atom – Maharashtra Board Class 9 Solutions for Science and Technology (English Medium)
- 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.
- 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.
- 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.
- 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.
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.
The path in which electrons revolve around the nucleus of an atom is called an orbit.
The number of electrons or protons in an atom is called the atomic number and is denoted by Z.
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.
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.
- 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.
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.