Chapter 5: Inside the Atom
Solved textbook questions with step-by-step explanations.
Grade 8 Q&A: Chapter 5: Inside the Atom
Welcome to the Questions and Answers section for Grade 8 Science, Chapter 5: "Inside the Atom." This chapter delves into the fundamental building blocks of matter, exploring the historical development of atomic models, the structure of an atom, its subatomic particles, and related concepts like atomic number, mass number, isotopes, and valency.
Important Questions and Answers
Q1: What was the main postulate of Dalton's Atomic Theory regarding the divisibility of atoms?
Answer: Dalton's Atomic Theory stated that matter is made up of extremely small, indivisible particles called atoms.
Q2: Describe Thomson's Atomic Model (Plum Pudding Model).
Answer: Thomson's model proposed that an atom is a positively charged sphere with negatively charged electrons embedded in it, similar to plums in a pudding or seeds in a watermelon. The positive and negative charges were considered equal in magnitude, making the atom electrically neutral.
Q3: What was the key experiment conducted by Rutherford that led to his atomic model?
Answer: Rutherford conducted the famous alpha-particle scattering experiment, where he bombarded a thin gold foil with alpha particles.
Q4: What were the main observations from Rutherford's alpha-particle scattering experiment?
Answer:
- Most alpha particles passed straight through the gold foil without deflection.
- Some alpha particles were deflected by small angles.
- A very few alpha particles (about 1 in 20,000) were deflected back by large angles or bounced back.
Q5: What conclusions did Rutherford draw from his experiment about the structure of the atom?
Answer:
- Most of the space inside the atom is empty.
- There is a small, dense, positively charged center called the nucleus.
- The size of the nucleus is very small compared to the atom.
Q6: What was the major limitation of Rutherford's Atomic Model?
Answer: Rutherford's model could not explain the stability of the atom. According to classical physics, an electron revolving in a circular orbit would continuously radiate energy and eventually spiral into the nucleus, causing the atom to collapse.
Q7: What are the main postulates of Bohr's Atomic Model?
Answer:
- Electrons revolve around the nucleus in specific, fixed orbits called shells or energy levels.
- While revolving in these discrete orbits, electrons do not radiate energy.
- Each shell has a fixed amount of energy.
Q8: Name the three subatomic particles and their respective charges.
Answer:
- Proton (p⁺): Positive charge (+1)
- Neutron (n⁰): No charge (neutral)
- Electron (e⁻): Negative charge (-1)
Q9: Where are protons, neutrons, and electrons located within an atom?
Answer:
- Protons and Neutrons: Located in the central part of the atom called the nucleus.
- Electrons: Revolve around the nucleus in specific orbits or shells.
Q10: Define 'Atomic Number (Z)'. How is it related to the number of protons and electrons in a neutral atom?
Answer: Atomic Number (Z) is the number of protons present in the nucleus of an atom. In a neutral atom, the Atomic Number is equal to the number of protons, which is also equal to the number of electrons.
Q11: Define 'Mass Number (A)'. How is it calculated?
Answer: Mass Number (A) is the total number of protons and neutrons in the nucleus of an atom. It is calculated as:
Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n)
Q12: What are 'Isotopes'? Give an example.
Answer: Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different number of neutrons).
Example: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H).
Q13: What are 'Isobars'? Give an example.
Answer: Isobars are atoms of different elements that have the same mass number but different atomic numbers. This means they have the same total number of nucleons but different numbers of protons.
Example: Argon (⁴⁰₁₈Ar) and Calcium (⁴⁰₂₀Ca) are isobars, both having a mass number of 40.
Q14: Define 'Valency'. How is it determined?
Answer: Valency is the combining capacity of an element. It is determined by the number of electrons an atom gains, loses, or shares to achieve a stable outermost electron shell (usually 8 electrons, or 2 for hydrogen/helium).
Q15: What is 'Electronic Configuration'? What formula is used to determine the maximum number of electrons in a shell?
Answer: Electronic configuration is the distribution of electrons in different shells or energy levels around the nucleus of an atom. The maximum number of electrons that can be accommodated in a shell is given by the formula 2n², where 'n' is the shell number.
Q16: What are the maximum electron capacities of the K, L, and M shells?
Answer:
- K-shell (n=1): Maximum 2 electrons (2 x 1² = 2)
- L-shell (n=2): Maximum 8 electrons (2 x 2² = 8)
- M-shell (n=3): Maximum 18 electrons (2 x 3² = 18)
Q17: Write the electronic configuration and valency for an element with atomic number 11 (Sodium).
Answer: For Sodium (Na) with Z=11:
- Electronic Configuration: K=2, L=8, M=1 (2, 8, 1)
- Valency: 1 (Sodium tends to lose 1 electron to achieve a stable octet in its L shell).
Q18: Why is an atom electrically neutral?
Answer: An atom is electrically neutral because in a neutral atom, the number of positively charged protons in the nucleus is exactly equal to the number of negatively charged electrons revolving around the nucleus. The positive and negative charges cancel each other out.
Q19: What is the significance of the atomic number (Z)?
Answer: The atomic number (Z) is significant because it uniquely identifies an element. Every atom of a particular element has the same atomic number. It also tells us the number of protons in the nucleus and, for a neutral atom, the number of electrons.
Q20: How are chemical symbols of elements determined? Give two examples.
Answer: Chemical symbols are usually one or two letters derived from the element's English or Latin name. The first letter is always capitalized, and the second letter (if present) is lowercase.
Examples: Hydrogen (H), Oxygen (O), Sodium (Na - from Natrium), Iron (Fe - from Ferrum).
Q21: What is the relative mass of an electron compared to a proton or neutron?
Answer: The relative mass of an electron is approximately 1/1837 of the mass of a proton or neutron, making its mass negligible when considering the total mass of an atom.
Q22: What happens when an electron jumps from a higher energy level to a lower energy level?
Answer: When an electron jumps from a higher energy level to a lower energy level, it emits energy, usually in the form of light or other electromagnetic radiation.
Q23: What is the difference between isotopes and isobars?
Answer:
- Isotopes: Atoms of the *same* element (same Z, different A) with different numbers of neutrons.
- Isobars: Atoms of *different* elements (different Z, same A) with the same mass number.
Q24: What is the valency of an inert (noble) gas like Neon (Atomic Number 10)?
Answer: The electronic configuration of Neon (Z=10) is 2, 8. Its outermost shell is complete (octet), so it is stable and has a valency of 0.
Q25: If an atom has 17 protons and 18 neutrons, what are its atomic number and mass number?
Answer:
- Atomic Number (Z) = Number of Protons = 17
- Mass Number (A) = Number of Protons + Number of Neutrons = 17 + 18 = 35
Exercise Solutions (From Screenshot Page 38)
Q1: Fill in the blanks.
- The central part of an atom is called the nucleus.
- The positively charged particles in the nucleus are called protons.
- The neutral particles in the nucleus are called neutrons.
- The negatively charged particles revolving around the nucleus are called electrons.
- The number of protons in an atom is called its atomic number (Z).
- The sum of protons and neutrons in an atom is called its mass number (A).
- Atoms of the same element with different mass numbers are called isotopes.
- Atoms of different elements with the same mass number are called isobars.
- The combining capacity of an element is called its valency.
- The maximum number of electrons in the K-shell is 2.
- The maximum number of electrons in the L-shell is 8.
Q2: Match the pairs.
(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)
- Dalton - Indivisible atom
- Thomson - Plum pudding model
- Rutherford - Nuclear model
- Bohr - Fixed orbits/shells
- Proton - Positive charge
- Electron - Negative charge
- Neutron - No charge
- Atomic number - Number of protons
- Mass number - Protons + Neutrons
- Valency - Combining capacity
Q3: Give scientific reasons.
- An atom is electrically neutral.
Reason: An atom is electrically neutral because in a neutral atom, the number of positively charged protons in its nucleus is exactly equal to the number of negatively charged electrons revolving around the nucleus. The total positive charge balances the total negative charge, resulting in a net charge of zero for the atom. - Rutherford's atomic model was modified by Bohr.
Reason: Rutherford's model had a major limitation: according to classical electromagnetic theory, an electron revolving in a circular orbit should continuously radiate energy and eventually spiral into the nucleus, making the atom unstable. This contradicts the observed stability of atoms. Bohr modified this model by proposing that electrons revolve in specific, discrete orbits (shells) without radiating energy, thus explaining atomic stability and the discrete line spectra of elements. - Isotopes of an element have the same chemical properties.
Reason: The chemical properties of an element are primarily determined by the number of electrons in its outermost shell (electronic configuration), which in turn depends on the number of protons (atomic number). Isotopes of an element have the same atomic number (same number of protons and electrons) but differ only in the number of neutrons. Since their electronic configuration is identical, their chemical properties are also the same.
Q4: Answer the following questions.
- Explain Dalton's atomic theory.
Answer: John Dalton's atomic theory, proposed in 1803, was the first scientific theory about the atom. Its main postulates were:- Matter consists of indivisible particles called atoms.
- Atoms of the same element are identical in mass, size, and chemical properties.
- Atoms of different elements are different in mass, size, and chemical properties.
- Atoms cannot be created or destroyed in a chemical reaction.
- Atoms combine in simple whole-number ratios to form compounds.
- Explain Rutherford's atomic model with a diagram.
Answer: (Diagram explanation - as I cannot draw, I will describe the model.) Rutherford's Nuclear Model: Based on his alpha-particle scattering experiment, Rutherford proposed that an atom consists of:- A tiny, dense, positively charged center called the nucleus, which contains almost all the mass of the atom.
- Negatively charged electrons revolve around the nucleus in circular paths, similar to planets orbiting the sun.
- Most of the atom's volume is empty space.
- Explain Bohr's atomic model with a diagram.
Answer: (Diagram explanation - as I cannot draw, I will describe the model.) Bohr's Atomic Model: To overcome the limitations of Rutherford's model, Niels Bohr proposed:- Electrons revolve around the nucleus in specific, fixed circular paths called shells or energy levels (designated as K, L, M, N... or 1, 2, 3, 4...).
- While in these discrete orbits, electrons do not radiate energy, thus explaining atomic stability.
- Each shell has a fixed amount of energy. Electrons can jump between these energy levels by absorbing or emitting energy.
- Explain the structure of an atom.
Answer: An atom consists of a central nucleus and electrons orbiting around it.- Nucleus: It is the small, dense, positively charged core of the atom. It contains two types of subatomic particles:
- Protons (p⁺): Positively charged particles, each with a relative mass of 1 amu.
- Neutrons (n⁰): Electrically neutral particles, each with a relative mass of 1 amu.
- Electrons (e⁻): Negatively charged particles that revolve rapidly around the nucleus in specific, fixed energy shells or orbits. Their mass is negligible (about 1/1837th of a proton). In a neutral atom, the number of electrons is equal to the number of protons.
- Nucleus: It is the small, dense, positively charged core of the atom. It contains two types of subatomic particles:
- Explain the concepts of atomic number, mass number, and valency.
Answer:- Atomic Number (Z): It is the number of protons present in the nucleus of an atom. It uniquely identifies an element. For a neutral atom, Z = Number of Protons = Number of Electrons.
- Mass Number (A): It is the total number of protons and neutrons in the nucleus of an atom. It represents the approximate atomic mass. Formula: A = Number of Protons + Number of Neutrons.
- Valency: It is the combining capacity of an element. It represents the number of electrons an atom gains, loses, or shares to achieve a stable electronic configuration (usually 8 electrons in the outermost shell, or 2 for the first shell).
References
- Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 5: Inside the Atom.
- Maharashtra State Board 8th Standard Science Syllabus.
- Balbharati Science and Technology Textbook.