SCIENCE (Chemistry) Topic: Structure of the Atom Question paper

Global International School

ANSWER KEY & MARKING SCHEME

Subject: SCIENCE (Structure of the Atom) | Class: IX

Section A: Multiple Choice Questions (1 Mark Each)

Q.No	Answer	Explanation
1.	(b) J.J. Thomson	J.J. Thomson is credited with the discovery of the electron.
2.	(c) No charge	The neutron is a sub-atomic particle that possesses no electrical charge (it is neutral).
3.	(b) 8	The maximum number of electrons for a shell is 2n2. For L-shell (n=2): 2(22) = 8.
4.	(c) Nucleus	Rutherford concluded that all positive charge and mass are concentrated in the central nucleus.
5.	(b) Isobars	Isobars are atoms of different elements with different atomic numbers but the same mass number.
6.	(c) 2000 times	The mass of a proton is approximately 2000 times the mass of an electron.
7.	(b) Proton	The proton carries a positive charge (+1), equal in magnitude but opposite to the electron.
8.	(c) 0	Atoms with a completely filled outermost shell (octet) show little chemical activity, so valency is zero.
9.	(c) Neutron	J. Chadwick discovered the neutron in 1932.
10.	(c) 6 protons	Atomic Number (Z) is defined as the total number of protons in the nucleus.

Section B: Short Answer Type I (2 Marks Each)

1. Define canal rays. Which sub-atomic particle was discovered as a result of the investigation of these rays?

Canal rays are new positively charged radiations. The investigation of these rays led to the discovery of the proton.

2. Explain why an atom, despite containing charged particles, is electrically neutral as a whole.

An atom is electrically neutral because the negative charges of the electrons are equal in magnitude to the positive charges of the protons, meaning the charges balance each other out perfectly.

3. Define Valency. What is the number of electrons in the outermost shell called?

Valency is defined as the combining capacity of an atom. The number of electrons present in the outermost shell is known as valence electrons.

4. Why did Rutherford choose a thin gold foil for his famous experiment?

Rutherford chose a thin gold foil because he needed a very thin layer (about 1000 atoms thick) to allow alpha particles to pass through for his scattering experiment.

5. If the K and L shells of an atom are completely filled, what would be the total number of electrons in the atom?

The maximum number of electrons in the K-shell (n=1) is 2.
The maximum number of electrons in the L-shell (n=2) is 8.
Total electrons = 2 + 8 = 10.

6. An atom contains 11 protons and 12 neutrons. Calculate the Mass Number (A).

Mass Number (A) = Protons + Neutrons
A = 11 + 12 = 23.

7. What is the main similarity between the chemical properties of all isotopes of an element?

The main similarity is that all isotopes of a given element have the same chemical properties because they have the same number of valence electrons.

8. Helium atom has an atomic mass of 4 u and two protons. Calculate the number of neutrons.

Neutrons = Mass Number - Protons
Neutrons = 4 - 2 = 2 neutrons.

Section C: Short Answer Type II (3 Marks Each)

1. Compare the three fundamental sub-atomic particles.

Particle	Charge	Mass (Approx.)	Location
Electron	Negative (-1)	Negligible (1/2000 of p+)	Revolves around nucleus
Proton	Positive (+1)	1 unit	Inside nucleus
Neutron	Neutral (0)	1 unit	Inside nucleus

2. Define Isotopes and give the symbols for the three atomic species of hydrogen.

Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A).
The three isotopes of hydrogen are:

1. Protium (H₁¹)
2. Deuterium (H₁²)
3. Tritium (H₁³)

3. Explain how an atom of Magnesium (Z=12) achieves stability and determine its valency.

Magnesium (Z=12) has an electron distribution of 2, 8, 2. To achieve a stable octet, it is easier for the atom to lose the 2 outer electrons than to gain 6. Therefore, its valency is two.

4. The atomic number of Sulphur (S) is 16. Determine the valency using the octet rule.

Sulphur (Z=16) has an electron distribution of 2, 8, 6. It needs to gain electrons to reach 8.
Valency = 8 (Octet) - 6 (Valence electrons) = 2.

5. State the defining property of an element. Why are protons and neutrons called nucleons?

The defining property is the Atomic Number (Z) (total number of protons). Protons and neutrons are called nucleons because they are both located inside the nucleus.

6. Define Isobars. Give an example.

Isobars are atoms of different elements with different atomic numbers but the same mass number.
Example: Calcium (Z=20, A=40) and Argon (Z=18, A=40).

7. What are the two key postulates that Neils Bohr put forward?

1. Only certain special orbits, known as discrete orbits, are allowed inside the atom.
2. While revolving in these discrete orbits, the electrons do not radiate energy.

8. Give two practical applications of isotopes.

1. Isotopes of Uranium are used as fuel in nuclear reactors.
2. An isotope of Iodine is used in the treatment of goitre.

Section D: Long Answer Type (5 Marks Each)

1. Describe Thomson’s Model. State its postulates and limitations.

Model: Thomson proposed the atom is a positively charged sphere with electrons embedded in it (like a watermelon or plum pudding).

Postulates: 1) An atom consists of a positively charged sphere and electrons are embedded in it. 2) The negative and positive charges are equal in magnitude, making the atom electrically neutral.

Limitation: It could not explain the results of scattering experiments (like Rutherford's) performed by other scientists.

2. Rutherford’s alpha-particle scattering experiment led to the Nuclear Model. State the observation and conclusions.

Observation: While most particles passed through, one out of every 12,000 particles appeared to rebound (deflect by 180°).

Conclusions:
1) Empty Space: Since most particles passed straight through, most space inside the atom is empty.
2) Nucleus: Since very few rebounded, all the positive charge and mass must be concentrated in a tiny volume called the nucleus.

3. Explain the drawback of Rutherford’s model regarding stability. How did Bohr fix this?

Drawback: Charged particles (electrons) undergoing acceleration in circular orbits should radiate energy. If they radiated energy, they would spiral into the nucleus, making the atom unstable.

Bohr's Solution: Bohr proposed that electrons revolve in discrete orbits and, while in these orbits, they do not radiate energy. This explains why atoms are stable.

4. Explain Atomic Number (Z) vs Mass Number (A). Use Nitrogen (N₇¹⁴) as an example.

Atomic Number (Z): Total number of protons in the nucleus. Defines the element's identity.

Mass Number (A): Sum of protons and neutrons (nucleons).

Example (N₇¹⁴):
Subscript 7 is Z (7 Protons).
Superscript 14 is A (Protons + Neutrons = 14).

5. Calculate the average atomic mass of Chlorine (Isotopes 35u and 37u in 3:1 ratio). Why is it not a whole number?

Average Mass = (35 × 3/4) + (37 × 1/4)
= 26.25 + 9.25
= 35.5 u

The mass is not a whole number because it is a weighted average calculated based on the natural abundance ratio of its isotopes. Individual atoms are whole numbers (35 or 37), but the bulk sample averages to 35.5.

6. Summarize the Bohr-Bury rules for electron distribution. Explain the maximum capacity formula.

Formula: The maximum capacity of a shell is 2n² (where n is the shell number).
e.g., K-shell (n=1) = 2 electrons; L-shell (n=2) = 8 electrons.

Rules:
1. The outermost shell can accommodate a maximum of 8 electrons.
2. Shells are filled step-wise; electrons do not occupy a new shell unless inner shells are filled.

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