Atomic Structure — Exercise Solutions
Comprehensive solutions for TN State Board Class 7 Science Term 1 Chapter 4: Atomic Structure. Covers MCQs, fill in the blanks, match the following, true/false, analogy, assertion-reason, very short answers, short answers, detailed answers, and HOTS.
I. Choose the Appropriate Answer (MCQ)
Q1. The basic unit of matter is ____
(a) Element (b) Atom ✓ (c) Molecule (d) Electron
Q2. The subatomic particle that revolves around the nucleus is ____
(a) Atom (b) Neutron (c) Electron ✓ (d) Proton
Q3. ____ is positively charged.
(a) Proton ✓ (b) Electron (c) Molecule (d) Neutron
Q4. The atomic number of an atom is ____
(a) Number of neutrons (b) Number of protons ✓ (c) Total number of protons and neutrons (d) Number of atoms
Q5. Nucleons comprise of ____
(a) Protons and electrons (b) Neutrons and electrons (c) Protons and neutrons ✓ (d) Neutrons and Positron
Additional MCQs
Q6 (Additional). ____ is made up of two or more atoms.
(a) Atoms (b) Molecule ✓ (c) Compound (d) None
Q7 (Additional). The average diameter of an atom is ____
(a) 10⁻⁸ m (b) 1 m (c) 1 Å ✓ (d) 10
Q8 (Additional). Whose theory does not propose anything about the positive and negative charges of atoms?
(a) J.J. Thomson (b) Dalton ✓ (c) Rutherford (d) Bohr
Q9 (Additional). ____ is the smallest unit used to measure small lengths.
(a) Metre (b) Nanometre ✓ (c) Millimetre (d) Centimetre
Q10 (Additional). One nanometre is equal to ____
(a) 10⁸ m (b) 10⁻⁹ m ✓ (c) 10⁻⁵ m (d) 10⁹ m
Q11 (Additional). One metre is equal to ____ nm.
(a) 10⁹ ✓ (b) 10⁸ (c) 10⁵ (d) 10¹⁰
Q12 (Additional). Who discovered the existence of negatively charged particles in an atom?
(a) John Dalton (b) Rutherford (c) Neils Bohr (d) J.J. Thomson ✓
Q13 (Additional). J.J. Thomson's atom model is also called ____
(a) Plum pudding model ✓ (b) gold foil (c) alpha particle (d) none
Q14 (Additional). Protons and neutrons are called ____
(a) atom (b) nucleons ✓ (c) positive charge (d) negative charge
Q15 (Additional). Proton was discovered by ____
(a) Rutherford ✓ (b) Goldstein (c) Chadwick (d) J.J. Thomson
II. Fill in the Blanks
Textual Evaluation:
1. The smaller particles found in the atom are called sub-atomic particles. 2. The nucleus has protons and neutrons. 3. The electrons revolve around the nucleus. 4. If the valency of carbon is 4 and that of hydrogen is 1, then the molecular formula of methane is CH₄. 5. There are two electrons in the outermost orbit of the magnesium atom. Hence, the valency of magnesium is 2.
Additional Fill in the Blanks:
6. Atom is the smallest particle of a chemical element. 7. The molecules are combinations of atoms of various elements or the same element. 8. According to John Dalton, atom is a hard, solid, ball and it is indivisible. 9. J.J. Thomson compared an atom to a watermelon. 10. The particles that make up the atom are called sub-atomic particles.
III. Match the Following
Set 1 (Textual):
| Column A | Column B | Answer |
|---|---|---|
| 1. Valency | (a) Fe | c — Electrons in the outermost orbit |
| 2. Neutral Particle | (b) Proton | d — Neutron |
| 3. Iron | (c) Electrons in the outermost orbit | a — Fe |
| 4. Hydrogen | (d) Neutron | e — Monovalent |
| 5. Positively charged Particle | (e) Monovalent | b — Proton |
Set 2 (Additional — Element/Symbol/Atomic No.):
| Element | Symbol | Atomic No. | Answer |
|---|---|---|---|
| i) Hydrogen | a) 0 | 1. 11 | 1 → e (H) |
| ii) Helium | b) Al | 2. 8 | 2 → c (He) |
| iii) Aluminium | c) He | 3. 1 | 3 → b (Al) |
| iv) Oxygen | d) Na | 4. 2 | 4 → a (O) |
| v) Sodium | e) H | 5. 13 | 5 → d (Na) |
Set 3 (Additional — Particle/Charge/Discoverer):
| Particle | Charge | Discoverer | Answer |
|---|---|---|---|
| i) Proton | a) n | 1. James Chadwick | 1 → b (P) |
| ii) Electron | b) P | 2. Rutherford | 2 → c (e) |
| iii) Neutron | c) e | 3. J.J. Thomson | 3 → a (n) |
IV. True or False (with corrections)
Q1. The basic unit of an element is molecule.
False. The basic unit of an element is atom.
Q2. The electrons are positively charged.
False. Electrons are negatively charged.
Q3. An atom is electrically neutral.
True. An atom has equal number of protons and electrons, making it electrically neutral.
Q4. The nucleus is surrounded by protons.
False. The nucleus is surrounded by electrons. The nucleus contains protons and neutrons.
Additional True/False
Q5. A neutron is a neutral particle.
True. Neutrons carry no charge.
Q6. J.J. Thomson proposed that an atom consists of a positively charged sphere in which electrons are embedded.
True. This is the plum pudding model of the atom.
Q7. The outermost orbit of an atom can have more than 8 electrons.
False. The outermost orbit can contain a maximum of 8 electrons (octet rule).
Q8. Most of the space in an atom is occupied by electrons.
True. According to Rutherford's model, most of the atom is empty space through which electrons move.
Q9. Rutherford used beta particles in his scattering experiments.
False. Rutherford used alpha particles in his gold foil scattering experiment.
V. Complete the Analogy
1. Sun : Nucleus, planets : electrons 2. Atomic number : Number of protons, Mass number : number of protons and neutrons 3. K : Potassium, C : Carbon
VI. Assertion and Reason
Q1. Assertion: An atom is electrically neutral. Reason: Atoms have equal number of protons and electrons.
Q2. Assertion: The mass of an atom is the mass of nucleus. Reason: The nucleus is at the centre.
Q3. Assertion: The number of protons and neutrons is atomic number. Reason: The mass number is sum of protons and neutrons.
VII. Very Short Answer Questions
Q1. Define an atom.
An atom is the smallest particle of a chemical element that retains its chemical properties.
Q2. Name the sub-atomic particles.
Proton, Electron, and Neutron.
Q3. What is atomic number?
The number of electrons or protons in an atom is its atomic number, represented by the letter Z.
Q4. What is the characteristic of a proton?
A proton is a positively charged particle located in the nucleus. Its positive charge has the same magnitude as an electron's negative charge (+1 unit).
Q5. Why are neutrons called neutral particles?
Neutrons carry no charge — they are neither positive like protons nor negative like electrons — hence they are called neutral particles.
Additional Very Short Answers
Q6. What is atomic number?
It is the number of protons an atom contains. It is denoted by Z.
Q7. What is mass number of an atom?
It equals the sum of protons and neutrons in the nucleus. A = Number of protons + Number of neutrons.
Q8. What is a neutron?
A neutron is an elementary particle that carries no charge, located in the nucleus of an atom.
Q9. Name the 3 sub-atomic particles of an atom.
Electron, Proton, and Neutron.
Q10. What are valence electrons?
Electrons present in the outermost shell (orbit) of an atom are called valence electrons.
Q11. What are nucleons?
Protons and neutrons residing in the nucleus are collectively called nucleons.
Q12. What is an electron?
An electron is a negatively charged particle that revolves around the nucleus in specific orbits (shells).
Q13. What is the mass of a proton?
1.6726 × 10⁻²⁷ kg
VIII. Short Answer Questions
Q1. Distinguish Isotopes from Isobars.
| Isotopes | Isobars |
|---|---|
| Atoms of the same element with same atomic number but different mass number | Atoms with same mass number but different atomic numbers (different elements) |
| Same number of protons, different number of neutrons | Different number of protons and neutrons |
| Example: Hydrogen isotopes — Protium, Deuterium, Tritium (Z = 1, A = 1, 2, 3) | Example: Calcium (Z = 20) and Argon (Z = 18) both have A = 40 |
Q2. What are isotones? Give one example.
Isotones are atoms of different elements that have the same number of neutrons. Example: ¹³₆C (neutrons = 13 − 6 = 7) and ¹⁴₇N (neutrons = 14 − 7 = 7).
Q3. Differentiate mass number from atomic number.
| Atomic Number (Z) | Mass Number (A) |
|---|---|
| Number of protons in an atom | Number of protons and neutrons in the nucleus |
| Represented by Z | Represented by A |
| Example: Helium has 2 protons, so Z = 2 | Example: Sodium has 11 protons and 12 neutrons, so A = 23 |
Q4. The atomic number of an element is 9, it has 10 neutrons. Find the element and its mass number.
- Atomic number Z = 9 → Element is Fluorine
- Mass number A = protons + neutrons = 9 + 10 = 19
Q5 (Additional). Is the structure of the atom similar to the structure of the solar system?
Yes, it is similar. The nucleus is at the centre like the Sun, and electrons orbit around it like planets. This is why Rutherford's model is called the planetary model.
Q6 (Additional). What is atomic mass number? Mention its formula.
Atomic mass number (A) equals the sum of protons and neutrons in the nucleus. Formula: A = Number of Protons + Number of Neutrons
Q7 (Additional). Define isotopes with an example.
Isotopes are atoms of the same element having different numbers of neutrons, resulting in the same atomic number but different mass numbers. Example: Hydrogen has three isotopes — Protium (¹H), Deuterium (²H), and Tritium (³H).
Q8 (Additional). Define isobars with an example.
Isobars are atoms with the same mass number but different atomic numbers (they belong to different elements). Example: Calcium-40 (Z = 20) and Argon-40 (Z = 18).
Q9 (Additional). What are the limitations of Thomson's model?
Thomson's theory did not address the positive and negative charges within an atom adequately. It could not explain many properties of substances, such as the scattering of alpha particles observed in Rutherford's experiment.
IX. Detailed Answer Questions
Q1. Draw the atom structure and explain the position of sub-atomic particles.
An atom consists of three sub-atomic particles: Protons, Electrons, and Neutrons.
1. Nucleus — at the centre of the atom, contains protons (positively charged) and neutrons (no charge). The nucleus is very small compared to the overall size of the atom. 2. Electron shells — the outermost regions contain electrons (negatively charged) that revolve around the nucleus in specific circular paths called orbits or shells. 3. Protons carry positive charge (+1); neutrons are neutral (0); electrons are negatively charged (−1) and orbit the nucleus. 4. An atom is electrically neutral because the number of protons equals the number of electrons.
Q2. Atomic number = 26, Mass number = 56. Calculate electrons, protons, neutrons. Draw the structure.
- Element: Iron (Fe)
- Number of protons = Atomic number = 26
- Number of neutrons = Mass number − Protons = 56 − 26 = 30
- Number of electrons = Number of protons = 26
- Electron configuration: 2, 8, 14, 2 (K, L, M, N shells)
Q3. What are nucleons? Why are they so called? Write their properties.
Nucleons are sub-atomic particles (protons and neutrons) located in the nucleus. They are called nucleons because they reside within the nucleus.
Properties: 1. Protons and neutrons have nearly the same mass, differing only in electric charge. 2. Proton carries charge +1; neutron is neutral (charge 0). 3. Nucleon size is approximately 1.6 fm (femtometres). 4. The nuclear force holding nucleons together is very short-range and is the strongest known force.
Q4. Define valency. What is the valency of element with atomic number 8? What compound does it form with hydrogen?
Valency is the combining capacity of an element. It is determined by the number of electrons in the outermost shell.
- Element with atomic number 8 is Oxygen.
- Electronic configuration of Oxygen: K = 2, L = 6
- Valency = 8 − 6 = 2 (since it has 6 electrons in the outermost shell, it needs 2 more to complete the octet)
- The compound formed with hydrogen is water (H₂O) — two hydrogen atoms (valency 1 each) combine with one oxygen atom (valency 2).
X. Higher Order Thinking Skills (HOTS)
Q1. An atom has no electron. Will it have mass? Can an atom exist without electrons? Give an example.
Yes, an atom without electrons will still have mass because mass depends on protons and neutrons in the nucleus, which are much heavier than electrons. Atoms can exist without electrons. Example: He²⁺ (helium ion) has 2 protons and 2 neutrons but no electrons.
Q2. Find details about common salt — its elements, formula, atomic numbers, mass numbers, and ions.
- Common salt is sodium chloride (NaCl), composed of sodium and chlorine.
- Atomic number of Na = 11; Atomic number of Cl = 17
- Mass number of Na = 23; Mass number of Cl = 35
- Ions present: Na⁺ (sodium ion) and Cl⁻ (chloride ion)
Try Yourself Q1: Why are atomic numbers and mass numbers always whole numbers?
Atomic number must be a whole number because you cannot have a fraction of a proton — they always come in whole units. The same reasoning applies to mass number, since protons and neutrons cannot exist as fractions.
Try Yourself Q2: A sulphur atom has 16 protons and 16 neutrons. Give its atomic number and mass number.
- Atomic number = Number of protons = 16
- Mass number = 16 + 16 = 32
Additional HOTS Q1: In Rutherford's experiment, can we use foil of another metal instead of gold?
Yes, any highly malleable metal could work. Gold was chosen because it is extremely malleable; the foil used was only about 1000 atoms thick, which allowed alpha particles to pass through and some to be deflected.
Additional HOTS Q2: How is Rutherford's model different from Thomson's?
Rutherford proposed that most of the atom's mass is concentrated in a small central nucleus, with electrons orbiting around it. Thomson believed the positive charge was uniformly distributed throughout the atom with electrons embedded in it (plum pudding model). Rutherford's gold foil experiment showed that most of the atom is empty space.
Intext Activities
Activity 1: Identify objects and their materials.
- Hammer → made of Iron
- Bangles → made of Glass
- Tap → made of Steel
- Vessels → made of Aluminium or Steel
Activity 2: Label sub-atomic particles:
1. Positively charged particle: Proton 2. Negatively charged particle: Electron 3. Neutral particle: Neutron
Activity 3: Identify elements from clues:
1. Used for breathing: Oxygen (O), atomic number 8 2. Used in filling balloons: Helium (He), mass number 4 3. Element in bananas: Potassium (K), atomic number 19 4. Found in crackers: Phosphorus (P), protons 15 5. Most valuable element: Carbon (C), mass number 12
Common Mistakes to Avoid
1. Confusing atomic number and mass number — Atomic number (Z) = protons only; Mass number (A) = protons + neutrons. 2. Saying electrons are in the nucleus — Electrons orbit around the nucleus in shells; protons and neutrons are IN the nucleus. 3. Confusing isotopes and isobars — Isotopes have same Z but different A (same element); isobars have same A but different Z (different elements). 4. Writing valency incorrectly — Valency equals electrons in the outermost shell (if ≤ 4) or 8 minus that number (if > 4). Not just the shell number. 5. Forgetting that atoms are electrically neutral — Number of protons = number of electrons in a neutral atom. This is why atomic number also gives the number of electrons. 6. Confusing Thomson's and Rutherford's models — Thomson: positive charge spread throughout with electrons embedded (plum pudding). Rutherford: positive charge concentrated in nucleus with electrons orbiting. 7. Saying neutrons have charge — Neutrons are neutral (charge = 0). Only protons are positive and electrons are negative.
Exam Checklist
- [ ] Can define atom and name the three sub-atomic particles
- [ ] Can explain the position and charge of protons, neutrons, and electrons
- [ ] Can distinguish atomic number (Z) from mass number (A)
- [ ] Can calculate number of protons, neutrons, and electrons given Z and A
- [ ] Can define and distinguish isotopes, isobars, and isotones
- [ ] Can define valency and determine valency from electron configuration
- [ ] Can write electron configuration for elements (2, 8, 8... pattern)
- [ ] Can explain Thomson's plum pudding model and Rutherford's nuclear model
- [ ] Can explain why atoms are electrically neutral
- [ ] Can identify elements from atomic number and calculate mass number
- [ ] Know the discoverers: electron (Thomson), proton (Rutherford), neutron (Chadwick)