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Q1. Number of valence electrons in oxygen atom =
Oxygen atom has 6 valence electrons. Oxygen has atomic number 8 with electronic configuration 2, 6. The outermost shell (L-shell) contains 6 electrons, which are the valence electrons. Oxygen needs 2 more electrons to complete its octet, giving it a valency of 2.
Q2. Number of electrons in ²³Na⁺ =
²³Na⁺ has 10 electrons. Sodium atom (²³Na) has 11 electrons (atomic number 11). When it loses one electron to form Na⁺, it has 10 electrons. The mass number 23 is not needed here—the number of electrons depends on the charge (+1 means loss of one electron).
Q3. Number of electrons in ²⁴Mg²⁺ =
²⁴Mg²⁺ has 10 electrons. Magnesium atom has 12 electrons (atomic number 12). Mg²⁺ has lost 2 electrons (since the charge is +2), so it has 12 – 2 = 10 electrons. The mass number 24 indicates the number of nucleons, not electrons.
Q4. Mass number =
Mass number is the total number of protons and neutrons in the nucleus. It is denoted by A. The mass number represents the total nucleons in the atom. For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12.
Q5. Electronic configuration of Al =
Aluminium (Al) has atomic number 13, so its electronic configuration is 2, 8, 3. It has 2 electrons in K-shell, 8 in L-shell, and 3 in M-shell. The 3 valence electrons give aluminium a valency of 3.
Q6. Number of valence electrons in ²⁰Ne =
Neon (²⁰Ne) has 8 valence electrons. Neon has atomic number 10 with electronic configuration 2, 8. The outermost shell (L-shell) has 8 electrons. Neon is a noble gas with a complete octet and valency 0.
Q7. Number of valence electrons in Na⁺ ion =
Na⁺ ion has 8 valence electrons. Sodium atom has electronic configuration 2, 8, 1. After losing 1 electron to become Na⁺, it has configuration 2, 8. The outermost shell (L-shell) has 8 electrons. Na⁺ has a complete octet like neon.
Q8. Number of neutrons in ³⁷Cl =
³⁷Cl has 20 neutrons. The mass number is 37 and atomic number is 17. Number of neutrons = Mass number – Atomic number = 37 – 17 = 20. This isotope of chlorine has 17 protons and 20 neutrons.
Q9. Number of protons in Cl⁻ ion =
Cl⁻ ion has 17 protons. The number of protons in an ion is the same as in the neutral atom. Chlorine has atomic number 17, so it has 17 protons. The charge of -1 means it has gained one electron, but the number of protons remains unchanged.
Q10. Cl⁻ has the same electronic configuration as:
Cl⁻ has the same electronic configuration as argon. Chlorine (atomic number 17) has configuration 2, 8, 7. Cl⁻ has gained one electron to become 2, 8, 8. This is the same configuration as argon (atomic number 18, configuration 2, 8, 8). Both have a complete octet.
Q11. Cation is formed when:
A cation is formed when an atom loses electrons. Since electrons are negatively charged, losing them results in a net positive charge. For example, Na loses one electron to form Na⁺. Cations are positively charged ions.
Q12. Atomic species with 11 protons, 12 neutrons =
Atomic species with 11 protons and 12 neutrons has mass number = 11 + 12 = 23. The element with atomic number 11 is sodium (Na). Therefore, the species is ²³Na. Sodium has 11 protons, 11 electrons, and 12 neutrons.
Q13. Atomic species with 17 protons and 20 neutrons =
Mass number = protons + neutrons = 17 + 20 = 37. The element with 17 protons is chlorine (Cl). Therefore, the species is ³⁷Cl. This is one of the stable isotopes of chlorine.
Q14. Number of valence electrons in Cl⁻ ion is:
Cl⁻ ion has 8 valence electrons. Chlorine atom has configuration 2, 8, 7. When it gains one electron to become Cl⁻, it has configuration 2, 8, 8. The outermost shell has 8 electrons, giving Cl⁻ a complete octet.
Q15. Atomic species with 17 protons and 18 neutrons =
Mass number = protons + neutrons = 17 + 18 = 35. The element with 17 protons is chlorine (Cl). Therefore, the species is ³⁵Cl. This is the most common isotope of chlorine.
Q16. Atomic number of sulphur (S) = 16, mass number = 32, number of neutrons = ?
Number of neutrons = Mass number – Atomic number = 32 – 16 = 16. Sulphur-32 has 16 protons and 16 neutrons. Sulphur is an element with atomic number 16 and has stable isotopes.
Q17. Na⁺ has the same electronic configuration as:
Na⁺ has the same electronic configuration as neon (Ne). Sodium atom (Na) has configuration 2, 8, 1. Na⁺ loses one electron to become 2, 8, which is the same as neon (atomic number 10, configuration 2, 8). Both have a complete octet.
Q18. Which of the following has 17 electrons?
Cl atom has 17 electrons. Chlorine has atomic number 17, so a neutral chlorine atom has 17 electrons. Cl⁻ has 18 electrons, Na⁺ has 10 electrons, and K⁺ has 18 electrons. Only Cl atom has 17 electrons.
Q19. Number of neutrons in ¹⁶O =
¹⁶O has 8 neutrons. Oxygen has atomic number 8 (8 protons). Mass number is 16. Number of neutrons = Mass number – Atomic number = 16 – 8 = 8. This is the most common isotope of oxygen.
Q20. Number of valence electrons in Al =
Aluminium (Al) has 3 valence electrons. With atomic number 13, its configuration is 2, 8, 3. The outermost shell (M-shell) has 3 electrons, giving aluminium a valency of 3.
Q21. Number of valence electrons in neutral F atom =
Fluorine atom has 7 valence electrons. Fluorine has atomic number 9 with configuration 2, 7. The outermost shell has 7 electrons, and fluorine needs 1 more electron to complete its octet. This gives fluorine a valency of 1.
Q22. Atomic number = number of:
Atomic number is the number of protons in the nucleus. It is denoted by Z and is the defining characteristic of an element. In a neutral atom, the number of electrons is also equal to the atomic number, but atomic number specifically refers to protons.
Q23. Which ion has completely filled K and L shells?
All of the mentioned ions have completely filled K and L shells. Na⁺ has configuration 2, 8 (K and L shells are full). Cl⁻ has configuration 2, 8, 8 (K, L, M shells full). K⁺ has configuration 2, 8, 8 (K, L, M shells full). All have at least the K and L shells completely filled.
Q24. Electronic configuration of oxygen (O) =
Oxygen has electronic configuration 2, 6. Oxygen has atomic number 8: 2 electrons in K-shell and 6 in L-shell. The L-shell is the outermost shell, containing 6 valence electrons.
Q25. Total electrons in ³⁷Cl⁻ =
³⁷Cl⁻ has 18 electrons. Chlorine atom has 17 electrons (atomic number 17). Cl⁻ has gained one electron, so it has 17 + 1 = 18 electrons. The mass number 37 indicates the number of nucleons, not electrons.
Q26. Sodium loses how many electrons to form Na⁺?
Sodium loses 1 electron to form Na⁺. Sodium atom has 11 electrons (configuration 2, 8, 1). It loses the single electron in its outermost shell to achieve a stable configuration like neon. Na⁺ has 10 electrons with configuration 2, 8.
Q27. Number of valence electrons in Cl atom =
Chlorine atom has 7 valence electrons. Chlorine has atomic number 17 with configuration 2, 8, 7. The outermost shell contains 7 electrons, so chlorine needs 1 more electron to complete its octet.
Q28. Atomic species with 1 proton and mass number 2 is:
Atomic species with 1 proton and mass number 2 is deuterium. Deuterium has 1 proton and 1 neutron (mass number 2). It is an isotope of hydrogen. Tritium has 1 proton and 2 neutrons (mass number 3), and protium has 1 proton and 0 neutrons (mass number 1).
Q29. Number of neutrons in H⁺ ion =
H⁺ ion has 0 neutrons. Hydrogen has atomic number 1 (1 proton). The most common isotope of hydrogen (protium) has 0 neutrons. The charge (⁺) means it has lost its single electron, but the number of neutrons remains unchanged at 0.
Q30. Correct electronic configuration of sodium (Na) is:
Sodium has electronic configuration 2, 8, 1. Sodium has atomic number 11: 2 electrons in K-shell, 8 in L-shell, and 1 in M-shell. This is the correct order, with shells filled from the inside out.
Q31. Total number of electrons in ²⁰Ne =
²⁰Ne has 10 electrons. Neon has atomic number 10, so a neutral neon atom has 10 electrons. The mass number 20 indicates 10 protons and 10 neutrons. The number of electrons is always equal to the atomic number in a neutral atom.
Q32. Number of neutrons in ³²S =
³²S has 16 neutrons. Sulphur has atomic number 16 (16 protons). Mass number is 32. Number of neutrons = 32 – 16 = 16. This is the most common isotope of sulphur.
Q33. Number of valence electrons in ²⁴Mg =
²⁴Mg has 2 valence electrons. Magnesium has atomic number 12 with configuration 2, 8, 2. The outermost shell (M-shell) has 2 valence electrons. Magnesium loses these 2 electrons to achieve a stable configuration.
Q34. Atomic species with 1 proton, 0 neutrons, and 0 electrons is:
Atomic species with 1 proton, 0 neutrons, and 0 electrons is a hydrogen ion (H⁺). It is essentially just a proton. The most common hydrogen isotope (protium) has 1 proton, 0 neutrons, and 1 electron. When it loses its electron, it becomes H⁺.
Q35. Element with electronic configuration 2,8,2 =
Electronic configuration 2, 8, 2 corresponds to magnesium (Mg). Atomic number = 2 + 8 + 2 = 12, which is magnesium. Calcium has 20 electrons (2, 8, 8, 2), sodium has 11 electrons (2, 8, 1), and neon has 10 electrons (2, 8).
Q36. Number of electrons in Mg²⁺ =
Mg²⁺ has 10 electrons. Magnesium atom has 12 electrons (atomic number 12). Mg²⁺ has lost 2 electrons (since the charge is +2), so it has 12 – 2 = 10 electrons. Its configuration is 2, 8, which is like neon.
Q37. Number of neutrons in ²³Na =
²³Na has 12 neutrons. Sodium has atomic number 11 (11 protons). Mass number is 23. Number of neutrons = 23 – 11 = 12. This is the only stable isotope of sodium.
Q38. Anion is formed when:
An anion is formed when an atom gains electrons. Since electrons are negatively charged, gaining them gives the atom a net negative charge. For example, chlorine gains one electron to become Cl⁻. Anions are negatively charged ions.
Q39. Number of electrons in Na atom =
Na atom has 11 electrons. Sodium has atomic number 11, so a neutral sodium atom has 11 electrons. Its electronic configuration is 2, 8, 1. The valence electron is in the outermost shell.
Q40. Atomic number of fluorine (F) = 9, mass number = 19, number of neutrons = ?
Number of neutrons = Mass number – Atomic number = 19 – 9 = 10. Fluorine-19 has 9 protons and 10 neutrons. This is the only stable isotope of fluorine.
Q41. Number of electrons in H⁺ ion =
H⁺ ion has 0 electrons. Hydrogen atom has 1 proton and 1 electron. When it loses its only electron to become H⁺, it has no electrons. H⁺ is simply a proton.
Q42. Atomic species with 12 protons and 12 neutrons =
Atomic species with 12 protons and 12 neutrons has mass number = 12 + 12 = 24. The element with atomic number 12 is magnesium (Mg). Therefore, the species is magnesium-24 (²⁴Mg). This is the most common isotope of magnesium.
Q43. Atomic species with 8 protons and 10 neutrons =
Atomic species with 8 protons and 10 neutrons has mass number = 8 + 10 = 18. The element with atomic number 8 is oxygen (O). Therefore, the species is ¹⁸O. This is a stable isotope of oxygen.
Q44. Atomic species with 12 protons and mass number 24 is:
The element with 12 protons is magnesium (atomic number 12). With mass number 24, this is magnesium-24 (²⁴Mg). Silicon has 14 protons, sodium has 11 protons, and aluminium has 13 protons.
Q45. Number of protons in H⁺ ion =
H⁺ ion has 1 proton. Hydrogen has atomic number 1, so it has 1 proton. The ion has lost its electron, but the number of protons remains the same (1). H⁺ is a positively charged ion (a proton).
Q46. Valence electrons of K atom =
Potassium (K) atom has 1 valence electron. Potassium has atomic number 19 with electronic configuration 2, 8, 8, 1. The outermost shell (N-shell) has 1 electron. Potassium loses this electron to form K⁺.
Q47. Electronic configuration of F⁻ ion =
F⁻ ion has electronic configuration 2, 8. Fluorine atom has configuration 2, 7. When it gains one electron to become F⁻, it has 8 electrons in the outermost shell, giving configuration 2, 8. This is the same as neon, a noble gas.
Q48. Electronic configuration of ²³Na⁺ =
²³Na⁺ has electronic configuration 2, 8. Sodium atom has configuration 2, 8, 1. Na⁺ has lost one electron, so it has configuration 2, 8. This is the same as neon (noble gas configuration). The mass number 23 is not relevant to the electronic configuration.
Q49. Atomic species with 1 proton and 1 neutron =
Atomic species with 1 proton and 1 neutron is deuterium. It is an isotope of hydrogen with mass number 2. Deuterium has 1 proton, 1 neutron, and 1 electron. It is also called heavy hydrogen. Tritium has 1 proton and 2 neutrons.
Q50. Number of electrons in K⁺ ion =
K⁺ ion has 18 electrons. Potassium atom has 19 electrons (atomic number 19). K⁺ has lost one electron (since it has a +1 charge), so it has 19 – 1 = 18 electrons. Its electronic configuration is 2, 8, 8, which is the same as argon.