2. Give short answer.
(i) Distinguish between shell and sub-shell.
Answer:
Shell: A shell is the main energy level around the nucleus, denoted by the principal quantum number (n = 1, 2, 3, ...). It represents the average distance of electrons from the nucleus.
Sub-shell: A sub-shell is a subdivision of a shell, denoted by the azimuthal quantum number (l = 0, 1, 2, ...). Each shell
contains one or more sub-shells (s, p, d, f) with slightly different energies. For example, the M shell (n=3) contains 3s, 3p, and 3d sub-shells.
(ii) Why an atom is electrically neutral?
Answer: An atom is electrically neutral because the number of positively charged protons in the nucleus equals the number of negatively charged electrons surrounding the nucleus. The positive charge of the protons is exactly balanced by the negative charge of the electrons. For example, a carbon atom has 6 protons (+6) and 6 electrons (-6), resulting in a net charge of zero. Neutrons have no charge and do not affect the neutrality.
(iii) How many sub-shells are there in N shell.
Answer: The N shell corresponds to the principal quantum number n = 4. The number of sub-shells in a shell is equal to n. Therefore, the N shell has 4 sub-shells: 4s, 4p, 4d, and 4f.
(iv) Give notation for sub-shells of M shell.
Answer: The M shell corresponds to n = 3. Its sub-shells are:
3s (l = 0)
3p (l = 1)
3d (l = 2)
(v) List the sub-shells of M shell in order of increasing energy.
Answer: According to the (n + l) rule, the order of increasing energy for the M shell sub-shells is: 3s < 3p < 3d
This is because the (n + l) values are: 3s (3+0=3), 3p (3+1=4), 3d (3+2=5).
(vi) Can you identify an atom without knowing number of neutrons in it?
Answer: Yes, an atom can be identified without knowing the number of neutrons. The identity of an atom is determined by its atomic number (number of protons), which is unique to each element. For example, any atom with 6 protons is carbon, regardless of whether it has 6, 7, or 8 neutrons. The number of neutrons only determines the isotope, not the element itself.
3. The electronic configurations listed are incorrect. Explain what mistake have been made in each and write correct electronic configurations.
x = 1s², 2s², 2p⁶, 3s³
Answer: The mistake is in the 3s sub-shell: it cannot hold 3 electrons. The maximum capacity of an s sub-shell is 2 electrons. Correct configuration: 1s², 2s², 2p⁶, 3s² (This would be an incorrect configuration for any element; the correct configuration for 12 electrons is 1s², 2s², 2p⁶, 3s²).
y = 1s², 2s², 2p⁷
Answer: The mistake is in the 2p sub-shell: it cannot hold 7 electrons. The maximum capacity of a p sub-shell is 6 electrons. Correct configuration: 1s², 2s², 2p⁶ (for Neon).
z = 1s², 2s², 2p⁶, 3s², 3p⁶, 3s²
Answer: The mistake is that the 3s sub-shell appears twice. After filling 3s², 3p⁶, the next sub-shell should be 4s, not 3s again. Correct configuration: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s² (for Calcium).
4. Which orbital in each of the following pairs is lower in energy?
(a) 2s, 2p
Answer: 2s is lower in energy. According to the (n + l) rule, for the same n, the orbital with lower l has lower energy (l=0 for s, l=1 for p).
(b) 3p, 3d
Answer: 3p is lower in energy. (n + l) for 3p = 3+1=4; for 3d = 3+2=5. Lower (n+l) means lower energy.
(c) 3s, 4s
Answer: 3s is lower in energy. For 3s, n=3; for 4s, n=4. Lower n means lower energy.
5. Draw Bohr's Model for the following atoms indicating the location for electron, protons and neutrons:
(a) Potassium (Atomic No 19, Mass No. 39)
Answer: (Diagram Description)
Nucleus: Contains 19 protons and 20 neutrons (39 - 19 = 20). Shells:
K shell (n=1): 2 electrons.
L shell (n=2): 8 electrons.
M shell (n=3): 8 electrons.
N shell (n=4): 1 electron.
Electronic Configuration: 2, 8, 8, 1.
(b) Silicon (Atomic No 14, Mass No. 28)
Answer: (Diagram Description)
Nucleus: Contains 14 protons and 14 neutrons (28 - 14 = 14).
Shells:
K shell (n=1): 2 electrons.
L shell (n=2): 8 electrons.
M shell (n=3): 4 electrons.
Electronic Configuration: 2, 8, 4.
(c) Argon (Atomic No. 18, Mass No. 39)
Answer: (Diagram Description)
Nucleus: Contains 18 protons and 21 neutrons (39 - 18 = 21). Shells:
K shell (n=1): 2 electrons.
L shell (n=2): 8 electrons.
M shell (n=3): 8 electrons.
Electronic Configuration: 2, 8, 8.
6. Write electronic configuration for the following elements:
(a) ₁₉K
Answer: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹
(b) ₁₂Mg
Answer: 1s², 2s², 2p⁶, 3s²
(c) ₁₃Al
Answer: 1s², 2s², 2p⁶, 3s², 3p¹
(d) ₁₈Ar
Answer: 1s², 2s², 2p⁶, 3s², 3p⁶
11. State the importance and uses of isotopes in various fields of life.
Answer: Isotopes have numerous important uses in various fields:
Medicine:
Cobalt-60 and Iodine-131 are used in radiotherapy to treat cancer. Technetium-99m is used in diagnostic imaging. Iodine-123 is used for thyroid function tests.
Agriculture:
Radioisotopes are used to study plant metabolism and fertilizer uptake. They are used to preserve food by killing bacteria (food irradiation).
Archaeology:
Carbon-14 dating is used to determine the age of ancient artifacts and fossils.
Energy:
Uranium-235 is used as fuel in nuclear reactors to generate electricity.
Industry:
Radioisotopes are used to detect leaks in pipelines and to measure the thickness of materials.
Research:
Isotopes are used as tracers in chemical and biological research.
12. The atomic number of an element is 23 and its mass number is 56.
a. How many protons and electrons does an atom of this element have?
Answer: The atomic number is 23, so the atom has 23 protons. In a neutral atom, the number of electrons equals the number of protons, so it has 23 electrons.
b. How many neutrons does this atom have?
Answer: Number of neutrons = Mass number - Atomic number = 56 - 23 = 33 neutrons.
13. The atomic symbol of aluminium is written as ₁₃Al²⁷. What information do you get from it?
Answer: From the symbol ₁₃Al²⁷, we get the following information:
Al: The chemical symbol for aluminium.
13 (subscript): The atomic number, which means aluminium has 13 protons and 13 electrons. 27 (superscript): The mass number, which means the total number of protons and neutrons is 27.
Number of Neutrons: Mass number - Atomic number = 27 - 13 = 14 neutrons.
