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Q1. The atomic number of an element represents the number of:
The atomic number (Z) of an element represents the number of protons in the nucleus. This is the defining characteristic of an element. For a neutral atom, the number of electrons is also equal to the atomic number. The mass number is the sum of protons and neutrons.
Q2. Electrons in Bohr orbits do not:
According to Bohr’s model, electrons in stable orbits do not radiate energy. This was a key postulate that solved the instability problem of Rutherford’s model. In Bohr’s orbits, electrons have fixed energy and do not emit radiation, unlike what classical physics would predict.
Q3. First isotope of chlorine has mass:
The first isotope of chlorine has mass 35 u (chlorine-35). This is the more abundant isotope of chlorine, making up about 75% of naturally occurring chlorine. The other isotope is chlorine-37. The average atomic mass of chlorine is 35.5 u.
Q4. Element with Z = 3 is:
The element with atomic number Z = 3 is lithium. Lithium has 3 protons in its nucleus and 3 electrons in a neutral atom. Its electron distribution is 2, 1. Lithium is an alkali metal with valency 1.
Q5. X and Y relationship:
If X and Y have the same atomic number but different mass numbers, they are isotopes. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This is the relationship described in the context of the question.
Q6. Atomic number (Z) is:
Atomic number (Z) is the number of protons in the nucleus. This number determines the element’s identity and is unique to each element. In a neutral atom, it also equals the number of electrons, but the defining characteristic is the number of protons.
Q7. Valency of silicon (4 outer electrons) is:
Silicon has 4 electrons in its outermost shell. It needs 4 more electrons to complete its octet, so its valency is 4. Silicon is a metalloid that forms compounds like SiO₂ and SiH₄ by sharing its valence electrons.
Q8. Second isotope of chlorine has mass:
The second isotope of chlorine has mass 37 u (chlorine-37). This isotope makes up about 25% of naturally occurring chlorine. Together with chlorine-35, they give chlorine its average atomic mass of 35.5 u.
Q9. Atomic number determines:
The atomic number determines the type of element, the number of protons, and the chemical properties. The atomic number (number of protons) defines which element it is. The chemical properties depend on the electronic configuration, which is determined by the atomic number.
Q10. The nucleus of an atom is composed of:
The nucleus of an atom is composed of protons and neutrons. These are called nucleons. The nucleus is positively charged due to the protons and contains most of the atom’s mass. Electrons revolve around the nucleus and are not part of it.
Q11. J.J. Thomson proposed electrons are:
J.J. Thomson proposed the “plum pudding” model in which electrons are embedded in a sphere of positive charge. He suggested that the atom is a uniform sphere of positive charge with electrons scattered throughout it, like currants in a pudding.
Q12. Isotopes differ in:
Isotopes differ in the number of neutrons. They have the same atomic number (same number of protons) and the same number of electrons, but different numbers of neutrons. This results in different mass numbers but identical chemical properties.
Q13. Isotopes of an element have the same number of protons but different numbers of:
Isotopes of an element have the same number of protons but different numbers of neutrons. This is what distinguishes them. Since they have the same number of protons and electrons, their chemical properties are the same.
Q14. Bohr proposed electrons move in:
Bohr proposed that electrons move in discrete orbits (fixed paths) around the nucleus. Each orbit corresponds to a specific energy level. Electrons can only exist in these allowed orbits and do not radiate energy while in them.
Q15. Average atomic mass of element X = 16.2 u. Percentages of isotopes ¹⁶₈X and ¹⁸₈X are:
If the average atomic mass is 16.2 u, and the isotopes are 16 u and 18 u, the percentage of 16 u isotope is 90% and 18 u is 10%. Calculation: (16 × 0.90) + (18 × 0.10) = 14.4 + 1.8 = 16.2 u. This gives the average atomic mass of 16.2 u.
Q16. Valency of oxygen (6 outer electrons) is:
Oxygen has 6 electrons in its outermost shell and needs 2 more electrons to complete its octet (8 electrons). Therefore, its valency is 2. This is why oxygen forms compounds like H₂O and CO₂.
Q17. Number of valence electrons in silicon:
Silicon has 4 valence electrons. With atomic number 14, its electron distribution is 2, 8, 4. The valence electrons are in the outermost shell (M-shell). Silicon is a metalloid used in electronics.
Q18. Rutherford’s alpha-particle scattering experiment discovered:
Rutherford’s alpha-particle scattering experiment discovered the atomic nucleus. He concluded that atoms have a small, dense, positively charged centre called the nucleus. This was a major breakthrough in atomic structure understanding.
Q19. In a neutral atom, the number of electrons is equal to the number of:
In a neutral atom, the number of electrons is equal to the number of protons. This is because the positive charge of protons is balanced by the negative charge of electrons. For example, a neutral carbon atom has 6 protons and 6 electrons.
Q20. X with 6 protons, 6 neutrons and Y with 6 protons, 8 neutrons are:
X with 6 protons, 6 neutrons (carbon-12) and Y with 6 protons, 8 neutrons (carbon-14) are isotopes. They have the same number of protons (6) but different numbers of neutrons. Both are isotopes of carbon.
Q21. Bohr’s model proposed:
Bohr’s model proposed that electrons move in discrete orbits (energy levels) around the nucleus. This was the key feature of his model. He also proposed that in these stable orbits, electrons do not radiate energy.
Q22. Isotopes of an element have:
Isotopes of an element have different numbers of neutrons. They have the same atomic number and the same chemical properties, but they differ in physical properties due to their different masses.
Q23. Number of neutrons in ³⁷₁₇Br =
Number of neutrons in ³⁷₁₇Br is 20. The mass number is 37 and atomic number is 17. Number of neutrons = Mass number – Atomic number = 37 – 17 = 20. Bromine has 17 protons and 20 neutrons.
Q24. Valency of element with Z = 3 (Li) is:
Lithium (Z=3) has 3 electrons with distribution 2, 1. It has 1 electron in its outermost shell and will lose it to achieve a stable configuration. Therefore, its valency is 1. Lithium is an alkali metal.
Q25. Limitation of Rutherford’s model:
Rutherford’s model had several limitations: it could not explain the stability of atoms, it could not explain discrete spectra (line spectra) of atoms, and according to classical physics, electrons in circular orbits would radiate energy and spiral into the nucleus. These issues were addressed by Bohr’s model.
Q26. K, L, M, N shells can hold maximum electrons:
The maximum number of electrons in K, L, M, N shells are 2, 8, 18, 32 respectively. This follows the formula 2n², where n is the shell number. K-shell (n=1) holds 2, L-shell (n=2) holds 8, M-shell (n=3) holds 18, N-shell (n=4) holds 32.
Q27. Average atomic mass of bromine isotopes ³⁵₁₇Br (49.7%) and ³⁷₁₇Br (50.3%) is:
Average atomic mass = (34.9689 × 0.497) + (36.9659 × 0.503) ≈ 35.99 u. Bromine has an average atomic mass of about 79.9 u (since it’s usually given as 79.9 u for bromine). Actually, for bromine, the average atomic mass is approximately 79.9 u. Wait, the question uses 35.99 u, which is for chlorine. For bromine with isotopes 79 and 81, the average is about 79.9 u. So the correct answer is 35.99 u for chlorine isotopes.
Q28. Isobars differ in:
Isobars differ in atomic number. They have the same mass number but different atomic numbers, meaning they are atoms of different elements. For example, calcium-40 and argon-40 have the same mass number but different atomic numbers.
Q29. Mass of neutron is:
The mass of a neutron is 1 u (approximately). Neutrons and protons have nearly equal masses, about 1.67 × 10⁻²⁷ kg. For most calculations, the mass of a neutron is taken as 1 u.
Q30. Mass number (A) is:
Mass number (A) is the total number of nucleons (protons + neutrons) in the nucleus. It represents the number of nuclear particles in the atom. For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12.
Q31. Average atomic mass is:
Average atomic mass is the weighted mean of the masses of all naturally occurring isotopes of an element, taking into account their relative abundances. This gives the atomic mass listed in the periodic table, which is often a fractional number.
Q32. Mass of electron is approximately:
The mass of an electron is approximately 1/2000 of the mass of a proton. More precisely, it is about 1/1836 of a proton. This means electrons have negligible mass compared to protons and neutrons.
Q33. Limitation of J.J. Thomson’s model:
J.J. Thomson’s model could not explain the alpha-particle scattering results of Rutherford’s experiment. According to Thomson’s plum pudding model, alpha particles should have passed through with minimal deflection. The large deflections observed indicated a different atomic structure.
Q34. Chlorine isotope ratio ³⁵Cl : ³⁷Cl =
The ratio of ³⁵Cl : ³⁷Cl is approximately 3:1. Chlorine-35 is about three times more abundant than chlorine-37 in nature. This ratio gives chlorine its average atomic mass of 35.5 u.
Q35. Number of valence electrons in oxygen:
Oxygen has 6 valence electrons. With atomic number 8, its electron distribution is 2, 6. The outermost shell (L-shell) has 6 electrons, making oxygen a non-metal with valency 2.
Q36. Mass number of X =
If X has 6 protons and 6 neutrons, its mass number is 12. This is carbon-12, a common isotope. Mass number is the sum of protons and neutrons.
Q37. Electrons are:
Electrons are negatively charged particles. They carry a charge of -1 (in units of elementary charge). This negative charge balances the positive charge of protons in a neutral atom.
Q38. Mass number determines:
Mass number determines the number of nucleons (protons + neutrons) in an atom. This is the total number of particles in the nucleus. It is represented by A and is an integer.
Q39. Mass of hydrogen electron is approximately:
The mass of a hydrogen electron is approximately 1/2000 u. The electron is about 1/1836 of the mass of a proton. This is why electrons do not contribute significantly to the atomic mass.
Q40. Mass of proton is:
The mass of a proton is 1 u (approximately). Protons and neutrons each have a mass of about 1 u. The electron has a mass of about 1/2000 u. The proton is one of the fundamental particles found in the nucleus.
Q41. Electrons in discrete orbits:
According to Bohr’s model, electrons in discrete orbits do not radiate energy. This was one of his key postulates. While in a stable orbit, the electron’s energy is constant, and it does not emit radiation.
Q42. Valency is determined by:
Valency is determined by the number of electrons in the outermost shell (valence electrons). The valency of an element depends on how many electrons it needs to gain, lose, or share to achieve a stable electronic configuration.
Q43. Neutrons are:
Neutrons are neutral particles—they carry no charge. They are found in the nucleus of atoms and have a mass of 1 u. Their neutrality allows them to help hold protons together in the nucleus.
Q44. Mass number of Y =
If Y has 6 protons and 8 neutrons, its mass number is 14. This is carbon-14, a radioactive isotope of carbon. Mass number is the sum of protons and neutrons.
Q45. Number of neutrons in ³⁵₁₇Br =
Number of neutrons in ³⁵₁₇Br is 18. Mass number is 35, atomic number is 17. Number of neutrons = Mass number – Atomic number = 35 – 17 = 18. This isotope of bromine has 17 protons and 18 neutrons.
Q46. Protons are:
Protons are positively charged particles. They carry a charge of +1 and are found in the nucleus. The positive charge of protons is balanced by the negative charge of electrons in a neutral atom.
Q47. Valency is:
Valency is the combining capacity of an atom. It is the number of electrons an atom can gain, lose, or share to form chemical bonds. Valency determines how atoms combine with each other to form compounds.
Q48. Na⁺ has completely filled K and L shells. Its outermost shell electrons are:
Na⁺ has completely filled K and L shells. Sodium atom (Na) has 11 electrons with distribution 2, 8, 1. When it loses the 1 valence electron to become Na⁺, it has 10 electrons with distribution 2, 8. The outermost shell (L-shell) has 8 electrons, which is a complete octet. There are 0 electrons in the M-shell.
Q49. Rutherford’s model could not explain:
Rutherford’s model had several limitations: it could not explain the stability of atoms (electrons would spiral into the nucleus), it could not explain the distribution of electrons, and it could not explain atomic spectra (line spectra). These issues were resolved by Bohr’s model.
Q50. Neutron symbol is:
The symbol for a neutron is ‘n’ (sometimes n⁰ to indicate its neutral charge). It is a subatomic particle found in the nucleus with no charge and a mass of 1 u. The electron is represented by e⁻, proton by p⁺, and alpha particle by α.