1A group of students is trying to understand the gold foil experiment. They are discussing the observations and Rutherford's conclusions. One student says, "The fact that most alpha particles passed straight through meant that atoms are mostly solid." Another student counters, "No, it means there's a lot of empty space because they went through so easily." A third student adds, "And the few that bounced back showed the positive charge is concentrated."
Based on your understanding of the gold foil experiment:
(a) Which student's statement about empty space is correct? [1 mark]
(b) What observation led to the conclusion that the positive charge is concentrated? [1 mark]
(c) Why did Rutherford's model fail to explain the stability of an atom, even though it correctly identified the nucleus? [2 marks]
2Assertion (A): Thomson’s model of the atom, the 'plum pudding model', failed to explain the results of the gold foil experiment.Reason (R): Thomson's model proposed that the positive charge was spread uniformly throughout the atom, which contradicts the sharp deflection of some alpha particles observed by Rutherford.(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
3A scientist is designing an experiment to study the subatomic particles. They have a cathode ray tube and are observing the behavior of cathode rays. They notice that the rays are deflected towards a positively charged plate.(a) What can be concluded about the charge of the particles in cathode rays? [1 mark](b) What is the name given to these particles, which are a fundamental component of all atoms? [1 mark](c) If the cathode ray tube were filled with a different inert gas, would the nature of cathode rays change? Justify your answer. 2marks
4Consider an atom with atomic number Z and mass number A.(a) If this atom has 15 electrons, what is its atomic number? [1 mark](b) If this atom also has 16 neutrons, what is its mass number? [1 mark](c) What would be the number of protons in this atom? 1mark
5Case Study: A sample of element 'X' is found to have two isotopes. Isotope X-35 constitutes 75% of the sample, and Isotope X-37 constitutes 25% of the sample.
(a) If the atomic number of element X is 17, how many protons and neutrons does X-35 have? [2 marks]
(b) How many protons and neutrons does X-37 have? [2 marks]
(c) Calculate the average atomic mass of element X, considering the given abundances. [2 marks]
6Bohr's model of the atom introduced the concept of energy levels or shells.(a) What is the maximum number of electrons that can be accommodated in the M-shell (n=3) of an atom? [1 mark](b) Explain why electrons in the K-shell are considered to have lower energy than electrons in the L-shell. 2marks
7A student is trying to draw the electronic configuration of an atom. They know the atomic number is 11.(a) What is the electronic configuration of this atom? [1 mark](b) How many valence electrons does this atom have? [1 mark](c) Based on its electronic configuration, predict its valency. 2marks
8Assertion (A): Atoms with a complete octet in their outermost shell are generally reactive.Reason (R): Atoms tend to lose or gain electrons to achieve a stable electron configuration.(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
9You are given two elements, A and B. Element A has 6 protons and 6 neutrons. Element B has 6 protons and 8 neutrons.(a) What is the atomic number of element A? [1 mark](b) What is the mass number of element B? [1 mark](c) Are elements A and B isotopes or isobars? Justify your answer. 2marks
10Consider the following information about an atom: Atomic number = 17, Mass number = 35.(a) How many protons does this atom have? [1 mark](b) How many neutrons does this atom have? [1 mark](c) How many electrons does this atom have, and what is its electronic configuration? 2marks
11A scientist is investigating a radioactive element. They observe that it emits alpha particles.(a) What is an alpha particle? [1 mark](b) What subatomic particles are present in an alpha particle, and what is its charge? 2marks
12Scenario: In a chemistry lab, students are studying different elements. They observe that Calcium (Ca) has an atomic number of 20 and a mass number of 40. Argon (Ar) has an atomic number of 18 and a mass number of 40.(a) How many neutrons does Calcium have? [1 mark](b) How many neutrons does Argon have? [1 mark](c) What is the relationship between Calcium and Argon based on this information? 1mark
13Application Question: The element Chlorine has two main isotopes, Cl and Cl. Chlorine is widely used in water purification.(a) Why are the chemical properties of Cl and Cl similar? [2 marks](b) Explain why the average atomic mass of chlorine is not a simple arithmetic mean of 35 and 37. 2marks
14Assertion (A): Neutrons play a crucial role in holding the nucleus together despite the repulsion between protons.Reason (R): Neutrons, being electrically neutral, reduce the repulsive forces between protons and also contribute to the strong nuclear force.(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
15Imagine an atom of Helium. Its atomic number is 2 and its mass number is 4.(a) How many protons and electrons are present in a Helium atom? [1 mark](b) How many neutrons are present in a Helium atom? [1 mark](c) Draw a simple diagram to represent the structure of a Helium atom according to Bohr's model, showing the nucleus and electron shells. 2marks
16Situational Question: A student learns that the symbol for Iron is Fe, derived from its Latin name 'ferrum'.(a) If a new element was discovered and named 'Scientium', what would be a possible symbol for it, following IUPAC rules? [1 mark](b) Explain why using standardized symbols for elements is important for scientists worldwide. 2marks
17Case Study: India is a leader in nuclear technology for peaceful purposes. The Bhabha Atomic Research Centre (BARC) uses reactors like Dhruva for advanced research.
(a) What is a common application of an isotope of Uranium like U? [1 mark]
(b) Mention one specific area where research using neutron scattering experiments at BARC has provided key insights. [1 mark]
(c) Briefly explain why neutrons are important in nuclear reactors. [2 marks]
18Consider an atom with 18 protons and 22 neutrons.(a) What is the atomic number of this element? [1 mark](b) What is the mass number of this element? [1 mark](c) Write the electronic configuration of this element. 2marks
19Assertion (A): Atoms of the same element always have the same number of neutrons.Reason (R): Isotopes are defined as atoms of the same element with different numbers of neutrons.(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
20Application Question: Sodium (Na) has an electronic configuration of 2, 8, 1. Oxygen (O) has an electronic configuration of 2, 6.(a) What is the valency of Sodium? [1 mark](b) What is the valency of Oxygen? [1 mark](c) Predict the chemical formula of the compound formed between Sodium and Oxygen. Justify your answer based on their valencies. 2marks
Answers
1.(a) The second student's statement is correct. Most alpha particles passing through undeflected indicates a large amount of empty space within the atom.(b) The sharp deflection of some alpha particles, and a few bouncing back, indicated that the positive charge and most of the mass of the atom were concentrated in a very small central region.(c) Rutherford's model proposed that electrons revolve around the nucleus. According to classical physics, an accelerating charged particle (like an electron in circular motion) should continuously lose energy and eventually spiral into the nucleus, causing the atom to collapse. This contradicted the observed stability of atoms.
2(i) Both A and R are true, and R is the correct explanation of A.
3.
(a) The deflection of cathode rays towards a positively charged plate indicates that the particles in cathode rays are negatively charged.
(b) These particles are called electrons.
(c) No, the nature of cathode rays would not change. Cathode rays are streams of electrons, which are fundamental components of all atoms. Their properties are independent of the gas filled in the cathode ray tube or the material of the cathode.
4.
(a) Since the atom is neutral, the number of electrons equals the number of protons. If there are 15 electrons, the atomic number (Z) is 15.
(b) Mass number (A) = Number of protons (Z) + Number of neutrons. So, A = 15 + 16 = 31.
(c) The number of protons is equal to the atomic number, which is 15.
5.
(a) For X-35: Atomic number (Z) = 17. Number of protons = 17. Number of neutrons = Mass number - Atomic number = 35 - 17 = 18.
(b) For X-37: Atomic number (Z) = 17. Number of protons = 17. Number of neutrons = Mass number - Atomic number = 37 - 17 = 20.
(c) Average atomic mass = (Mass of Isotope 1 × % abundance of Isotope 1) + (Mass of Isotope 2 × % abundance of Isotope 2)
Average atomic mass = (35 × 75/100) + (37 × 25/100)
= (35 × 0.75) + (37 × 0.25)
= 26.25 + 9.25
= 35.5 u
6.
(a) The M-shell is the third shell (n=3). The maximum number of electrons is given by the formula 2n². So, 2 × 3² = 2 × 9 = 18 electrons.
(b) Electrons in the K-shell are closer to the nucleus. The energy of an electron is related to its distance from the nucleus; the closer the shell, the lower its energy. Therefore, electrons in the K-shell have less energy than those in the L-shell.
7.
(a) The atomic number is 11. The electronic configuration is determined by filling electrons in shells: K-shell (2), L-shell (8), M-shell (1). So, the configuration is 2, 8, 1.
(b) The outermost shell (M-shell) has 1 electron. So, it has 1 valence electron.
(c) Since it has 1 valence electron, it tends to lose this electron to achieve a stable octet in the L-shell. Therefore, its valency is 1.
8(iv) A is false, but R is true. Atoms with a complete octet are generally unreactive, not reactive. Atoms do strive to achieve a stable electron configuration by losing, gaining, or sharing electrons.
9.
(a) Element A has 6 protons, so its atomic number (Z) is 6.
(b) Element B has 6 protons and 8 neutrons. Its mass number (A) = protons + neutrons = 6 + 8 = 14.
(c) Elements A and B are isotopes. They have the same number of protons (atomic number = 6) but different numbers of neutrons (6 for A, 8 for B), resulting in different mass numbers (A for A = 6+6=12; A for B = 14).
10.
(a) The number of protons is equal to the atomic number, so there are 17 protons.
(b) Number of neutrons = Mass number - Atomic number = 35 - 17 = 18 neutrons.
(c) Since the atom is neutral, the number of electrons is equal to the number of protons, which is 17. The electronic configuration is 2, 8, 7.
11.
(a) An alpha particle is the nucleus of a helium atom.
(b) An alpha particle consists of 2 protons and 2 neutrons. It has a positive charge of +2.
12.
(a) For Calcium (Ca): Atomic number = 20. Number of neutrons = Mass number - Atomic number = 40 - 20 = 20 neutrons.
(b) For Argon (Ar): Atomic number = 18. Number of neutrons = Mass number - Atomic number = 40 - 18 = 22 neutrons.
(c) Calcium and Argon are isobars because they have different atomic numbers (20 and 18 respectively) but the same mass number (40).
13.
(a) The chemical properties of an atom are primarily determined by the number of electrons in its outermost shell (valence electrons). Since both isotopes of chlorine have 7 valence electrons (electronic configuration 2, 8, 7), they exhibit similar chemical properties.
(b) The average atomic mass of chlorine is not a simple arithmetic mean because the two isotopes, Cl and Cl, do not occur in nature in equal proportions. Cl is more abundant (about 75%) than Cl (about 25%). Therefore, the average atomic mass is a weighted average, taking into account their relative abundances.
14(i) Both A and R are true, and R is the correct explanation of A.
15.
(a) A Helium atom has an atomic number of 2, so it has 2 protons and 2 electrons.
(b) Mass number = protons + neutrons. So, 4 = 2 + neutrons. Therefore, there are 2 neutrons.
(c) [Diagram showing a nucleus with 2 protons and 2 neutrons, and one shell around it with 2 electrons.]
16.
(a) Possible symbols could be 'Sc' or 'Si' (if 'Scientium' implies 'Sci' and the second letter is 'ent', or if it follows the first letter and another letter like 't'). For example, 'St' or 'Sm' if the element name started with 'S'.
(b) Using standardized symbols ensures clear and universal communication among scientists worldwide, irrespective of their native language. It prevents confusion and misinterpretation of chemical information.
17.
(a) U is used as a fuel in nuclear reactors to generate electricity.
(b) Research into materials like superconductors, battery electrodes, and drug molecules has provided key insights.
(c) Neutrons are crucial in nuclear reactors because they initiate and sustain the nuclear fission chain reaction, which releases energy. They are also used for various experimental purposes in research reactors.
18.
(a) The atomic number is equal to the number of protons, so it is 18.
(b) Mass number = Number of protons + Number of neutrons = 18 + 22 = 40.
(c) The element with atomic number 18 is Argon (Ar). Its electronic configuration is 2, 8, 8.
19(iv) A is false, but R is true. Atoms of the same element can have different numbers of neutrons (isotopes). Isotopes are defined as atoms of the same element with different numbers of neutrons.
20.
(a) Sodium has 1 valence electron. It tends to lose this electron to achieve a stable configuration, so its valency is 1.
(b) Oxygen has 6 valence electrons. It tends to gain 2 electrons to achieve an octet (8 valence electrons), so its valency is 2.
(c) The chemical formula of the compound formed between Sodium (valency 1) and Oxygen (valency 2) is NaO. This is because two sodium atoms (each contributing +1 charge effectively) are needed to balance the charge of one oxygen atom (effectively -2 charge).
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