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Nuclei or atomic nucleus is the central core of every atom which contains the entire positive charge and more than 99% of the mass of the atom. The size of the atom is of order 10-10m, the size of the atomic nucleus is of order 10-15 m which means most of the space around the nucleus of an atom is empty space. Atomic nucleus was discovered by Ernest Rutherford in 1911 from the study of large scattering of alpha particles from thin metal foils. The deflection of alpha particles from the centre of the atom of the gold foil gave Rutherford the conclusion that nuclei are positively charged and contain most of the mass of an atom.
Atomic number is the number of protons present in the nucleus of an atom of the element, it is also equal to the number of electrons revolving in various orbits around the nucleus of the atom, It is represented by Z.
Nuclei Class 12 Important Notes PDF
Nuclei Class 12 Important Notes
MCQ Based on Nuclei
Ques: For a nuclear fusion process, suitable nuclei are____
- Any nuclei
- Heavy nuclei
- Lighter nuclei
- Nuclei lying in the middle of periodic table
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Ans: (c) lighter nuclei
Explanation: In a nuclear fusion reaction two or more lighter nuclei combine to make a more heavy and stable nucleus and for this reaction to occur extreme conditions of temperature and pressure are required. Lighter nuclei make collisions occur more frequently than the heavier ones.
Ques: Particles that can be added to the nucleus of an atom without changing its chemical properties are called ____
- neutrons
- electrons
- protons
- alpha particles
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Ans: (a) neutrons
Explanation: Chemical properties of an atom depends on the number of electrons revolving around its nucleus and proton and the number of electron should be the same to maintain the balance in the atom if we add neutrons that are neither positively charged nor negatively charged then atoms chemical properties will not change, atom will just become heavier than the earlier.
Ques: Nucleus of an atom whose atomic mass is 24 consists of ____
- 11 electrons, 11 protons and 13 neutrons
- 11 electrons, 13 protons and 11 neutrons
- 11 protons and 13 neutrons
- 11 protons and 13 electrons
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Ans: (c) 11 protons and 13 neutrons
Explanation: Nucleus of an atom consists of protons and neutrons. Electrons are absent inside the nucleus of an atom. Electrons revolve around the nucleus in different orbits, they are not present inside the nucleus. The nucleus is positively charged due to the presence of protons.
Ques: A nuclei having the same number of neutrons but different number of protons/ atomic numbers are called ____
- Isobars
- Isomers
- Isotones
- Isotopes
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Ans: (c) isotones
Explanation: Isotones are nuclides which contain the same number of neutrons while their atomic number and mass number are different. For example, 17cl 37 and 19k39 are isotones. This is because of the equal number of neutrons in chlorine = 37-17=20 and in potassium=39-19=20.
Ques: Which one of the following has the identical property for isotopes?
- Physical property
- Chemical property
- Nuclear property
- Thermal property
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Ans: (b) Chemical properties
Explanation: Chemical properties are the property of the atom which remains the same in an isotope because the electronic configuration of an isotope remains same, that is the number of electrons and the number of protons in an isotope remains the same and chemical properties of the atom mainly depends on the proton and electron of the atom. Neutrons add to the overall mass of the atom.
Ques: When the number of nucleons in nuclei increases, the binding energy per nucleon
- Increases continuously with mass number.
- Decreases continuously with mass number.
- Remains constant with mass number.
- First increases and then decreases with the increase of the mass number.
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Ans: (d) First increases then decreases with an increase of the mass number.
Explanation: Binding energy of a nucleus is the energy with which nucleons are bound in the nucleus. The binding energy per nucleon of a nucleus increases then decreases with the increases in the mass number and it is visible from the average binding energy graph, this phenomenon happens because of the Coulomb repulsion between the proton. The heavy nuclei are therefore less stable.
Ques: Which of the following statements is not true regarding Einstein's mass energy relation?
- Mass disappears to reappear as energy.
- Energy disappears to reappear as mass.
- Mass and energy are two different forms of the same entity.
- Mass and energy can never be related to each other.
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Ans: (d) mass and energy can never be related to each other.
Explanation: Einstein was the first to establish the relation between mass and energy and the famous equation is E=MC2 where C= speed of light in vacuum. The relation means that when a certain mass M disappears then a certain amount of energy E appears and vice -versa.
Ques: The curve of binding energy per nucleon as a function of atomic mass number has a sharp peak for helium nucleus. This implies that helium ___
- Can easily be broken up
- Is very stable
- Can be used as fissionable material
- Is radioactive
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Ans: (b) is very stable
Explanation: The average binding energy per nucleon of a nucleus has a sharp peak for helium nucleus as helium is a very stable atom and therefore the energy required to separate the nucleons from nucleus to an infinite distance is higher for a stable nucleus.
Ques: Nuclear forces are ____
- Spin dependent and have no non-central part
- Spin dependent and have a non-central part
- Spin independent and have no non-central part
- Spin independent and have a non-central part
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Ans: (b) spin dependent and have a non-central part
Explanation: Nuclear forces are strong forces of attraction which exist between the nucleons. These forces are spin dependent as forces between nucleons having parallel spin are greater than forces between nucleons with antiparallel spin. These forces are non-central as the distribution of nucleons in the nucleus is not spherically symmetric.
Ques: Nuclear forces exist between ____
- Neutron – neutron
- Proton-proton
- Neutron-proton
- All of these
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Ans: (d) All of these
Explanation: Nuclear forces act upon neutron-neutron, proton-proton, neutron-proton showing that these forces are independent of charges and exert it irrespective of it. They hold together nucleons.
Ques: γ-rays are deflected by ____
- An electric field but not by a magnetic field
- A magnetic field but not by an electric field
- Both electric and magnetic field
- Neither by electric field nor by magnetic field
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Ans:(d) Neither electric field nor the magnetic field
Explanation: Gamma rays are neither deflected by electric field nor by magnetic field showing they do not carry any charges on them. Gamma rays travel with the speed of light.
Ques: Beta rays emitted by a radioactive material are ____
- Electromagnetic radiations
- The electrons orbiting around the nucleus
- Charged particles emitted by nucleus
- Neutral particles
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Ans: (c) charged particles emitted by the nucleus.
Explanation: Beta rays are negatively charged particles emitted by the nucleus of radioactive material. A beta particle carries a charge 1.6 x 10-19 C of negative charge (which is the charge on one electron).
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