Atomic Structure, Isotopes and Radioactivity
(i) What is the maximum number of electrons that can be accommodated in the third shell of elements?
Answer: (b) 18
Explanation: The maximum number of electrons in any shell (n) is determined by the formula `2n²`. For the third shell (n=3), the maximum number of electrons is 2(3)² = 2×9 = 18.
(ii) What information was obtained from discharge tube experiments?
Answer: (c) Electrons and protons were discovered.
Explanation: Discharge tube experiments (by J.J. Thomson and others) led to the discovery of cathode rays (electrons). Later, the gold foil experiment (by Rutherford) led to the discovery of the nucleus and protons. Neutrons were discovered later by Chadwick.
(iii) Why are isotopes not shown separately in the periodic table?
Answer: (c) All isotopes have the same atomic number, so they don’t need separate placement.
Explanation: All isotopes of an element have the same number of protons (atomic number Z), so they have identical chemical properties and are placed at the same position in the periodic table. The periodic table arranges elements according to their atomic numbers, not atomic masses.
(iv) Which particle has a different number in isotopes?
Answer: (b) Neutron
Explanation: Isotopes are atoms of the same element that have the same number of protons (Z) but different numbers of neutrons (N), which results in different atomic masses (A). In neutral atoms, the number of electrons equals the number of protons, so it remains the same.
(v) In which isotope of oxygen are the numbers of protons, electrons, and neutrons equal?
Answer: (d) None of these
Explanation: Oxygen has an atomic number of 8, meaning every oxygen isotope has 8 protons and (in un-ionized state) 8 electrons. The number of neutrons depends on the isotope.
(vi) Nitrogen has two isotopes, N-14 and N-15, with abundances of 99.64% and 0.35% respectively. What is its relative atomic mass?
Answer: (b) 14.0021
Explanation: Assume N-14 has an abundance of 99.64% and mass of 14 amu. N-15 has an abundance of 0.35% and mass of 15 amu.
(vii) How is radiocarbon dating useful for archaeologists?
Answer: (a) It helps determine the age of organic material.
Explanation: Radiocarbon dating works on the principle of half-life of carbon-14 isotope. It measures the ratio of C-14 to C-12 in ancient organic remains (bones, wood, charcoal, etc.) to estimate their approximate age.
(viii) What holds the particles together in the nucleus?
Answer: (a) Particles are bound by a strong nuclear force.
Explanation: Positively charged protons in the nucleus repel each other through electrostatic force. The force that holds them together is the strong nuclear force, which is much more powerful than the repulsive electric force but acts only at very short distances.
(ix) How do electrons stay away from the oppositely charged nucleus?
Answer: (b) By revolving around the nucleus
Explanation: According to Bohr’s model, electrons revolve around the nucleus in specific orbits (shells). This revolution creates a centripetal force that balances the attraction (electric force) toward the nucleus, preventing electrons from falling into the nucleus. In modern quantum mechanics, this concept is more complex but the basic idea is that they remain in motion.
(x) Rubidium has two isotopes, ^85Rb and ^87Rb. The percentage abundance of the lighter isotope is 72.2%. What is the percentage abundance of the heavier isotope? Its atomic mass is 85.47.
Answer: (b) 27.8%
Explanation: The total percentage abundance of both isotopes must be 100%. If the abundance of the lighter isotope (assume Rb-85) is 72.2%, then the abundance of the heavier isotope (Rb-87) is 100% – 72.2% = 27.8%. The given atomic mass (85.47) is to verify this answer.
i. Why is it said that almost the entire mass of an atom is concentrated in its nucleus?
Answer: Because almost the entire mass of an atom consists of protons and neutrons, which are present in the nucleus. The mass of electrons is negligible (the mass of one proton is approximately equal to 1836 electrons).
ii. Why are elements different from each other?
Answer: Elements are different from each other because each element’s atoms have a different number of protons (atomic number Z). This number determines its chemical properties.
iii. How many neutrons are present in \(_{83}^{210}\text{Bi}\)?
Answer: Bi has an atomic number of 83. If we take A = 210 and Z = 83, then the number of neutrons N = A – Z = 210 – 83 = 127.
iv. Why is tritium (\(_{1}^{3}\text{H}\) or T) a radioactive element?
Answer: Tritium’s nucleus contains one proton and two neutrons. This ratio of protons to neutrons is unstable. To achieve stability, it transforms into a neutron, an electron (beta particle), and an antineutrino.
v. How can an atom absorb and emit energy?
Answer: When an atom absorbs energy (such as a photon of light), its electron moves from a lower energy orbit (lower shell) to a higher energy orbit (higher shell) (excited state). This excited state is unstable, so the electron returns to its original state (ground state) and emits a photon of energy.
i. Why does the energy of an electron increase when it moves from the first shell to the second shell?
Answer: As the distance from the nucleus increases, the electromagnetic potential energy of the electron increases. In the second shell (n=2), the electron is farther from the nucleus compared to the first shell (n=1), so its total energy (which is negative) becomes less negative, meaning it increases (becomes greater).
ii. Why is it necessary to reduce the gas pressure inside a discharge tube?
Answer: At low pressure, gas atoms/molecules are far apart from each other. This provides enough space for electrons and ions to travel long distances in the tube to gain sufficient energy from the electric field and become capable of producing cathode rays. At high pressure, particles frequently collide and cannot gain enough energy.
iii. What was the classical concept of the electron? How has this concept changed over time?
Answer:
Initially, the electron was viewed as a very small, negatively charged particle that revolved around the nucleus in orbits.
De Broglie proposed the wave-particle duality concept. Heisenberg proved with the Uncertainty Principle that the exact position and velocity of an electron cannot be determined simultaneously. Later, Schrödinger presented the wave equation in which the electron was described as a wave function, which led to the concept of orbitals. This new quantum mechanical model is completely different from the classical model.
iv. Why are the nuclei of radioactive elements unstable?
Answer: Radioactive nuclei are unstable because they have an unbalanced ratio of protons to neutrons. Either the total number of protons/neutrons is so large that the strong nuclear force cannot bind them stably. To achieve stability, these nuclei emit their excess mass or energy in the form of alpha particles (helium nuclei), beta particles (electrons/positrons), or gamma rays (energy).
v. During discharge tube experiments, how did scientists conclude that all elements contain the same type of electrons and protons?
Answer: Scientists observed that when different gases (different elements) were used in discharge tubes, the e/m ratio of particles emitted from the cathode (electrons) was always the same. Similarly, the e/m ratio of anode rays (protons) was also the same when obtained from different elements. This proved that these fundamental particles (electrons and protons) are identical in all elements.
i. Describe the structure of a hydrogen atom.
Answer: The hydrogen atom is the simplest of all elements.
1. Its nucleus contains only one proton.
2. One electron revolves around the nucleus or is found in an orbital.
3. It has no neutrons (except for its isotopes deuterium and tritium).
4. Its atomic number is 1 and atomic mass is (approximately) 1 amu.
5. It is the first and lightest element.
ii. How does the atomic structure theory explain the ionization of atoms by a radioactive isotope?
Answer: Radiation emitted from a radioactive isotope (such as alpha particles, beta particles, gamma rays) contains very high energy. When these particles or rays collide with another atom, they can transfer energy to its outer electron. If the transferred energy is greater than the electron’s binding energy, the electron separates from the atom. This results in the formation of a positive ion (cation). This process is ionization.
iii. What is radioactivity? Describe any three applications of radioactive isotopes.
Answer:
It is the process by which an unstable atomic nucleus continuously emits particles (alpha, beta) or rays (gamma) to transform into a more stable state.
1. Medicine: Radiotherapy in cancer treatment (such as gamma rays from Cobalt-60), radioactive tracers in disease diagnosis (such as Iodine-131 for thyroid).
2. Industry: Checking leaks in pipelines, measuring thickness of metals.
3. Agriculture: For pest control and developing new varieties, studying fertilizer absorption.
4. Archaeology/Geology: Determining the age of ancient samples through carbon dating (Carbon-14).
iv. Determine the relative atomic mass of mercury from the following data.
Isotopes: Hg-196, Hg-198, Hg-199, Hg-200, Hg-201, Hg-202, Hg-204
Table showing isotopes and their percentage abundances would be here
Answer: Calculate relative atomic mass using weighted average.
Relative Atomic Mass = 200.59 amu (approximately)
(This value matches the value found on the periodic table).
i. How can scientists prepare elements in the laboratory?
Answer: Scientists can prepare new elements (usually heavy and unstable) in the laboratory through processes like nuclear fusion or neutron capture. If they create a new heavy nucleus through nuclear fusion, it becomes a new element. These new nuclei are usually very unstable and break down immediately.
ii. “A system exactly like our solar system exists within the atom.” Comment on this statement.
Answer: This statement was valid up to Bohr’s atomic model, which showed electrons revolving around the nucleus (sun) in specific orbits like planets. However, according to the modern quantum mechanical model, this statement is not correct. Electrons do not move in specific paths like planets but are found in orbitals as clouds. Their exact location cannot be determined, only the probability region of their presence can be indicated. Therefore, comparing the structure of an atom to the solar system is an old and incorrect concept.
Calculate the number of neutrons, protons, and electrons in the nuclei of platinum (195)Pt, manganese (55)Mn, and silicon (127)Si.
Solution:
1. (195)Pt (Platinum):
Atomic number (Z) = 78 (from periodic table)
Protons (P) = 78
Electrons (E) = 78 (in un-ionized atom)
Neutrons (N) = A – Z = 195 – 78 = 117
2. (55)Mn (Manganese):
Atomic number (Z) = 25
Protons (P) = 25
Electrons (E) = 25
Neutrons (N) = 55 – 25 = 30
3. (127)Si (Silicon):
Atomic number (Z) = 14
Protons (P) = 14
Electrons (E) = 14
Neutrons (N) = 127 – 14 = 113
(Note: The common stable isotope of silicon is Si-28. This is an unstable (radioactive) isotope).
Why do the chemical properties of isotopes of an element remain the same while their physical properties differ?
Answer: Chemical properties depend on the atomic number (Z), i.e., the number of protons and the arrangement of electrons in the outer shell (valence electrons). Since all isotopes of an element have the same number of protons and electrons, their chemical properties remain the same. Physical properties (such as density, boiling/melting point) also depend on mass. Different isotopes have different masses due to different numbers of neutrons, which can cause some of their physical properties to differ.
1. Why does a radioactive isotope emit radiation?
Answer: The nucleus of a radioactive isotope is unstable because it has an unbalanced ratio of protons to neutrons or their total number is too large. To achieve stability, the nucleus emits its excess mass or energy in the form of alpha particles, beta particles, or gamma rays. This process is the emission of radiation.
2. Give an example of a radioactive isotope that gives a stable atom after disintegration.
Answer: Carbon-14 is a radioactive isotope. It disintegrates through beta decay and forms a stable Nitrogen-14 atom.
Diagram of beta decay would be here
How would you compare the mass of magnesium (Mg) and chlorine (Cl) atoms?
Answer: The mass of atoms is compared using their relative atomic mass (average atomic mass), which is listed on the periodic table.
Relative atomic mass of magnesium (Mg) = 24.31 amu
Relative atomic mass of chlorine (Cl) = 35.45 amu
Therefore, on average, a chlorine atom has more mass than a magnesium atom.
Calculate the relative atomic mass of lead (Pb). The isotopic abundances of (204)Pb, (206)Pb, (207)Pb, (208)Pb are 2.0, 24.0, 22.0, 52.0 respectively.
Solution:
Relative atomic mass = [(mass1 × abundance1) + (mass2 × abundance2) + …] / 100
(This value is close to the value 207.2 amu listed on the periodic table).