Electrons: Negative, outside nucleus, determines chemical behavior
Calculating Subatomic Particles
For neutral atoms:
Protons = Atomic Number (Z)
Electrons = Atomic Number (Z)
Neutrons = Mass Number (A) – Atomic Number (Z)
For ions:
Cations (positive ions): Electrons = Z – charge
Anions (negative ions): Electrons = Z + charge
Electron Shell Capacity
Use the formula 2n² to remember maximum electrons in each shell:
K shell (n=1): 2 × 1² = 2 electrons
L shell (n=2): 2 × 2² = 8 electrons
M shell (n=3): 2 × 3² = 18 electrons
N shell (n=4): 2 × 4² = 32 electrons
Distinguishing Isotopes and Ions
Isotopes: Same element, different number of neutrons
Ions: Same element, different number of electrons
Atomic Notation
Remember the format: AZX
A = Mass Number (protons + neutrons)
Z = Atomic Number (protons)
X = Element symbol
Multiple Choice Questions
(i) How many electrons can be accommodated at the most in the third shell of the elements?
(a) 8
(b) 18
(c) 10
(d) 32
Answer: (b) 18
Explanation: The maximum number of electrons in a shell is given by the formula 2n², where n is the shell number. For the third shell (n=3), 2 × 3² = 18 electrons.
(ii) What information was obtained from discharge tube experiments?
(a) Structure of atom was discovered
(b) Neutrons and protons were discovered
(c) Electrons and protons were discovered
(d) Presence of nucleus in an atom was discovered
Answer: (c) Electrons and protons were discovered
Explanation: Discharge tube experiments led to the discovery of cathode rays (electrons) by J.J. Thomson and canal rays (protons) by E. Goldstein.
(iii) Why have isotopes not been shown in the periodic table?
(a) Periodic table cannot accommodate a large number of isotopes of different elements
(b) Some of the isotopes are unstable and they give rise to different elements
(c) All the isotopes have same atomic number, so there is no need to give them separate places
(d) Isotopes do not show periodic behavior
Answer: (c) All the isotopes have same atomic number, so there is no need to give them separate places
Explanation: The periodic table is arranged based on atomic number. Since isotopes of an element have the same atomic number, they occupy the same position in the periodic table.
(iv) Which particle is present in different number in the isotopes?
(a) Electron
(b) Neutron
(c) Proton
(d) Both neutron and electron
Answer: (b) Neutron
Explanation: Isotopes are atoms of the same element with same number of protons but different number of neutrons.
(v) In which isotope of oxygen there are the equal number of protons, electrons and neutrons?
(a) ¹⁶O
(b) ¹⁸O
(c) ¹⁷O
(d) None of these
Answer: (b) ¹⁸O
Explanation: Oxygen has atomic number 8, so it has 8 protons and 8 electrons. For ¹⁸O, mass number is 18, so neutrons = 18 – 8 = 10. This is not equal to protons and electrons (8). Actually, none of oxygen isotopes have equal protons, electrons and neutrons. The correct answer should be (d) None of these.
(vi) What will be the relative atomic mass of nitrogen given the abundances of its two isotopes, ¹⁴N and ¹⁵N are 99.64 and 0.35 respectively?
(viii) What does keep the particles present in the nucleus intact?
(a) Particles are held together by strong nuclear force
(b) Particles are held together by weak nuclear force
(c) Particles are held together by electrostatic force
(d) Particles are held together by dipolar force
Answer: (a) Particles are held together by strong nuclear force
Explanation: The strong nuclear force is an attractive force that acts between nucleons (protons and neutrons) and overcomes the electrostatic repulsion between protons to hold the nucleus together.
(ix) How do electrons keep themselves away from the oppositely charged nucleus?
(a) By keeping themselves stationary
(b) By revolving around the nucleus
(c) Due to their wave-like nature
(d) A magnetic field around the nucleus keeps them away
Answer: (b) By revolving around the nucleus
Explanation: According to Bohr’s model, electrons revolve around the nucleus in specific orbits. The centrifugal force due to their motion balances the electrostatic attraction toward the nucleus.
Short Answer Questions
(i) Why is it said that almost all the mass of an atom is concentrated in its nucleus?
Answer: Almost all the mass of an atom is concentrated in its nucleus because:
The nucleus contains protons and neutrons, which are much heavier than electrons
A proton is about 1836 times heavier than an electron
Electrons have negligible mass compared to nucleons (protons and neutrons)
For example, in a carbon atom, the nucleus contains 99.95% of the total mass
(ii) Why are elements different from one another?
Answer: Elements are different from one another because they have different atomic numbers, which means:
Different number of protons in their nuclei
Different number of electrons in neutral atoms
Different electron configurations
These differences lead to variations in chemical and physical properties
(iii) How many neutrons are present in 21084Po?
Answer: Number of neutrons = Mass number – Atomic number = 210 – 84 = 126 neutrons
Descriptive Questions
(i) Explain the structure of a hydrogen atom.
Answer: The structure of a hydrogen atom:
Simplest atom: Consists of one proton and one electron
Nucleus: Contains a single proton with positive charge
Electron: Revolves around the nucleus in the first shell (K shell)
Size: The diameter of the atom is about 10⁻¹⁰ m, while the nucleus is about 10⁻¹⁵ m
Isotopes: Hydrogen has three isotopes:
Protium (¹H): 1 proton, 0 neutrons, 1 electron
Deuterium (²H): 1 proton, 1 neutron, 1 electron
Tritium (³H): 1 proton, 2 neutrons, 1 electron (radioactive)
Electron configuration: The single electron occupies the 1s orbital in the ground state
Bohr’s model: Successfully explains the hydrogen spectrum by quantizing electron orbits
(ii) What is radioactivity? Explain any three applications of radioactive isotopes.
Answer:
Radioactivity: The spontaneous disintegration of unstable atomic nuclei through emission of radiation (alpha particles, beta particles, or gamma rays) to achieve stability.
Three applications of radioactive isotopes:
Medical Applications:
Radiotherapy: Cobalt-60 and other radioisotopes are used to destroy cancerous tumors
Medical Imaging: Technetium-99m is used as a tracer in diagnostic imaging
Sterilization: Gamma rays from cobalt-60 are used to sterilize medical equipment
Archaeological Dating:
Radiocarbon Dating: Carbon-14 is used to determine the age of organic materials up to 50,000 years old
Potassium-Argon Dating: Used for dating rocks and geological formations
Industrial Applications:
Tracers: Radioactive isotopes are used to trace the flow of materials in pipelines or biological systems
Thickness Gauges: Beta emitters are used to measure and control thickness in manufacturing processes
Food Preservation: Gamma radiation is used to kill bacteria and extend shelf life of food
Constructed Response Questions
(i) Why does the energy of electron increase as we move from first shell to second shell?
Answer: The energy of electrons increases as we move from the first shell to higher shells because:
Distance from nucleus: Electrons in higher shells are farther from the positively charged nucleus
Electrostatic attraction: The attractive force between the nucleus and electrons decreases with distance
Energy requirement: More energy is needed to keep electrons in orbits farther from the nucleus
Potential energy: Electrons in higher shells have higher potential energy, similar to objects at greater heights in a gravitational field
Quantized energy levels: According to Bohr’s model, each shell has a specific energy level, with energy increasing as n increases (E ∝ 1/n²)
(ii) Why is it needed to lower the pressure of the gas inside the discharge tube?
Answer: Lowering the pressure of gas inside the discharge tube is necessary because:
Reduce collisions: At normal pressure, gas molecules are too close together, causing frequent collisions that prevent the formation of cathode rays
Longer mean free path: At low pressure (around 0.01 mmHg), gas molecules are far apart, allowing electrons to travel longer distances without collisions
Ionization: Low pressure allows the applied high voltage to ionize the gas molecules more effectively
Visible effects: The glow inside the tube and the formation of cathode rays become visible only at sufficiently low pressures
Experimental observation: The properties of cathode rays (deflection in electric and magnetic fields) can only be studied when the pressure is reduced
(iii) What is the classical concept of an electron? How has this concept changed with time?
Answer:
Classical Concept of Electron:
Initially, electrons were considered as tiny, negatively charged particles
They were thought to revolve around the nucleus in fixed orbits, similar to planets around the sun
According to classical physics, accelerating charged particles should continuously emit radiation and lose energy
This would cause electrons to spiral into the nucleus, making atoms unstable – which contradicted experimental observations
Evolution of the Concept:
Bohr’s Model (1913): Introduced quantized orbits where electrons don’t radiate energy while in stationary states
de Broglie’s Hypothesis (1924): Proposed that electrons have wave-like properties (wave-particle duality)
Schrödinger’s Quantum Mechanical Model (1926): Described electrons as probability clouds (orbitals) rather than precise orbits
Heisenberg’s Uncertainty Principle (1927): Stated that we cannot simultaneously know both the exact position and momentum of an electron
Modern Concept: Electrons are described by quantum numbers and exist in orbitals where we can only predict the probability of finding them
Investigative Questions
(ii) A system just like our solar system exists in an atom. Comment on this statement.
Answer: This statement refers to the analogy between the solar system and the Rutherford/Bohr model of the atom, but it has both similarities and limitations:
Similarities:
Central body: Both have a massive central body (sun/nucleus) with smaller bodies (planets/electrons) revolving around it
Inverse square law: Both systems follow an inverse square law for the central force (gravity/electrostatic force)
Empty space: Both consist mostly of empty space between the central body and orbiting bodies
Stability: Both systems are stable over long periods
Limitations of the Analogy:
Scale: The atom is quantum mechanical while the solar system follows classical mechanics
Energy radiation: According to classical physics, accelerating electrons should radiate energy and spiral into the nucleus, unlike planets
Quantization: Electron orbits are quantized (specific energy levels), while planetary orbits can have any energy
Wave-particle duality: Electrons exhibit both particle and wave properties, unlike planets
Uncertainty principle: We cannot precisely determine both position and momentum of electrons simultaneously
Probability distribution: Modern quantum mechanics describes electrons as probability clouds rather than precise orbits
Conclusion: While the solar system analogy was useful in early atomic models (especially Rutherford’s), it fails to explain many quantum phenomena observed in atoms. The modern quantum mechanical model provides a more accurate description of atomic structure.