⚗️ Grade 9 Science · Chemistry

Samacheer Class 9 Science - Atomic Structure

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Textbook Exercises
14 Sections
23 Questions

Complete Grade 9 Science questions and answers for Atomic Structure. Click any question to expand the answer.

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📋 Sections in this chapter
I. Choose the Correct AnswerII. State Whether True or FalseIII. Fill in the BlanksIV. Match the FollowingV. Complete the Following TableVI. Answer Very BrieflyVII. Answer BrieflyVIII. Answer in DetailIntext ActivitiesActivity – 1Activity – 2Activity – 4Activity – 5Activity – 6
Your Progress — Atomic Structure0% complete
1I. Choose the Correct Answer5 questions
Q.1Among the following, the odd pair is▾
✓ Answer
  • (a) (,^{18}*{8}O,\ ^{19}*{9}F)
  • (b) (,^{40}*{18}Ar,\ ^{14}*{7}N)
  • (c) (,^{30}*{14}Si,\ ^{31}*{15}P)
  • (d) (,^{40}*{20}Ca,\ ^{39}*{19}K)

Answer:
(b) (,^{40}*{18}Ar,\ ^{14}*{7}N)

Other pairs are isotones (same neutrons).
Q.2Change in number of neutrons changes an atom into▾
✓ Answer
  • (a) ion
  • (b) isotope
  • (c) isobar
  • (d) another element

Answer:
(b) isotope

Q.3The term nucleons refers to▾
✓ Answer
  • (a) protons and electrons
  • (b) only neutrons
  • (c) electrons and neutrons
  • (d) protons and neutrons

Answer:
(d) protons and neutrons

Q.4Number of protons, neutrons and electrons in (,^{80}_{35}Br) are respectively▾
✓ Answer
  • (a) 80, 80, 35
  • (b) 35, 55, 80
  • (c) 35, 35, 80
  • (d) 35, 45, 35

Answer:
(d) 35, 45, 35

Q.5Correct electronic configuration of potassium is▾
✓ Answer
  • (a) 2, 8, 9
  • (b) 2, 8, 1
  • (c) 2, 8, 8, 1
  • (d) 2, 8, 8, 3

Answer:
(c) 2, 8, 8, 1

2II. State Whether True or False5 questions

If false, correct the statement.

Q.1Electrons revolve around nucleus in fixed orbits.▾
✓ Answer

Answer: True

Q.2Isotopes have different atomic numbers.▾
✓ Answer

Answer: False

Correct Statement:
Isotopes have same atomic number but different mass numbers.

Q.3Electrons have negligible mass and charge.▾
✓ Answer

Answer: False

Correct Statement:
Electrons have negligible mass but possess negative charge.

Q.4Smaller orbit size means lower energy.▾
✓ Answer

Answer: True

Q.5Maximum number of electrons in L-shell is 10.▾
✓ Answer

Answer: False

Correct Statement:

Using formula:

$$2n^2$$

For L-shell:

$$2(2)^2 = 8$$

Maximum electrons = 8

3III. Fill in the Blanks0 questions
  • Calcium and Argon are examples of isobars.
  • Maximum electrons in an orbit is given by:
$$2n^2$$
  • Uranium-235 is used in nuclear reactors.
  • Number of neutrons in (,^{7}_{3}Li) is 4.
  • Valency of Argon is zero.
4IV. Match the Following0 questions
ScientistContribution
DaltonFirst atomic theory
ChadwickDiscovery of neutron
RutherfordDiscovery of nucleus
Niels BohrHydrogen atom model
5V. Complete the Following Table0 questions
ParticleSymbolChargeRelative Mass
Electron(e^-)(-1)Negligible
Proton(p^+)(+1)1 amu
Neutron(n^0)01 amu
6VI. Answer Very Briefly5 questions
Q.1Element Having Equal Electrons in K and L Shells▾
✓ Answer

Answer:

Beryllium

Atomic number = 4

Electronic configuration:

$$2,2$$
Q.2Electronic Configuration of K and Cl▾
✓ Answer

Potassium (K)

Atomic number = 19

$$2,8,8,1$$

Chlorine (Cl)

Atomic number = 17

$$2,8,7$$
Q.3Meaning of Symbols▾
✓ Answer

(,^{1}_{1}H)

Hydrogen atom

  • Atomic number = 1
  • Mass number = 1

(,^{1}_{0}n)

Neutron

  • Charge = 0
  • Mass = 1 amu

(,^{0}_{-1}e)

Electron

  • Charge = (-1)
  • Negligible mass
Q.4Electrons in Completely Filled K, L and M Shells▾
✓ Answer

Using:

$$2n^2$$
  • K = 2
  • L = 8
  • M = 18

Total:

$$2 + 8 + 18 = 28$$

Answer:

28 electrons

Q.5Common Feature in Electron Structures▾
✓ Answer

(a) Lithium, Sodium, Potassium

All have:

$$1 \text{ valence electron}$$

(b) Beryllium, Magnesium, Calcium

All have:

$$2 \text{ valence electrons}$$
7VII. Answer Briefly5 questions
Q.1How Did Rutherford Show Atom Has Empty Space?▾
✓ Answer

Answer:

Rutherford bombarded thin gold foil with alpha particles.

Observations:

  • Most particles passed through foil.
  • Few deflected.
  • Very few bounced back.

Conclusion:

Most of atom is empty space.

Q.2Why Do (,^{35}*{17}Cl) and (,^{37}*{17}Cl) Have Same Chemical Properties?▾
✓ Answer

Answer:

Both isotopes have:

  • same atomic number
  • same electronic configuration

Hence chemical properties are same.

They differ in number of neutrons.

Q.3Structure of Oxygen and Sulphur Atoms▾
✓ Answer

Oxygen

Atomic number = 8

Electronic configuration:

$$2,6$$

Sulphur

Atomic number = 16

Electronic configuration:

$$2,8,6$$
Q.4Calculate Protons, Neutrons and Electrons▾
✓ Answer

(i) Atomic Number = 3, Mass Number = 7

Protons:

$$3$$

Electrons:

$$3$$

Neutrons:

$$7 - 3 = 4$$

(ii) Atomic Number = 92, Mass Number = 238

Protons:

$$92$$

Electrons:

$$92$$

Neutrons:

$$238 - 92 = 146$$
Q.5What are Nucleons?▾
✓ Answer

Answer:

Particles present inside nucleus are called nucleons.

They include:

  • protons
  • neutrons

Nucleons in Phosphorus

Mass number of phosphorus:

$$31$$

Hence phosphorus has:

$$31 \text{ nucleons}$$
8VIII. Answer in Detail3 questions
Q.1Conclusions from Rutherford’s Gold Foil Experiment▾
✓ Answer

Conclusions

  • Atom contains very small nucleus.
  • Most of atom is empty space.
  • Entire positive charge concentrated in nucleus.
  • Most mass concentrated in nucleus.
  • Electrons revolve around nucleus.
Q.2Postulates of Bohr’s Atomic Model▾
✓ Answer

Main Postulates

(i)

Electrons revolve around nucleus in fixed circular orbits.

(ii)

Electrons in fixed orbits neither lose nor gain energy.

(iii)

Electrons absorb or emit fixed energy while moving between shells.

(iv)

Shells are represented as:

$$K,L,M,N$$

or

$$n = 1,2,3,4...$$

Energy Levels

  • K-shell has least energy
  • Energy increases outward
Q.3Gay-Lussac’s Law of Combining Volumes▾
✓ Answer

Statement

Volumes of reacting gases bear simple whole-number ratio under same temperature and pressure.

Example: Formation of Water

Step 1:

Word equation:

Hydrogen + Oxygen → Water

Step 2:

Chemical equation:

$$2H_2 + O_2 \rightarrow 2H_2O$$

Volume Ratio

$$2 : 1 : 2$$

Thus gases combine in simple whole-number ratio.

9Intext Activities0 questions
10Activity – 10 questions

Symbolic Representation

ElementSymbolic Form
Carbon(,^{12}_{6}C)
Oxygen(,^{16}_{8}O)
Silicon(,^{28}_{14}Si)
Beryllium(,^{9}_{4}Be)
11Activity – 20 questions

Valency

ElementValency
Phosphorus3 or 5
Chlorine1
Silicon4
Argon0
12Activity – 40 questions

Isotopes of Oxygen

Oxygen-16

  • Protons = 8
  • Neutrons = 8

Oxygen-18

  • Protons = 8
  • Neutrons = 10
13Activity – 50 questions

Isotones

(i) Fluorine and Neon

(ii) Sodium and Magnesium

(iii) Aluminium and Silicon

All pairs contain equal neutrons.

14Activity – 60 questions

Gay-Lussac Law Using Ammonia

$$N_2 + 3H_2 \rightarrow 2NH_3$$

Volume ratio:

$$1 : 3 : 2$$

# Test Yourself

# Number of Neutrons

(a) (,^{27}_{13}Al)

$$27 - 13 = 14$$

(b) (,^{31}_{15}P)

$$31 - 15 = 16$$
Correction made:
Original notation incorrectly showed 34 instead of 31.

(c) (,^{190}_{76}Os)

$$190 - 76 = 114$$

(d) (,^{54}_{24}Cr)

$$54 - 24 = 30$$
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