Atomic theory problems and solutions

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  1. Question: What is the atomic number of an element that has 16 protons? Solution: The atomic number is 16.
  2. Question: How many neutrons does an atom of Carbon-14 (¹⁴C) have? Solution: The atom has 8 neutrons (14 – 6 = 8).
  3. Question: Identify the isotope with 17 protons and 18 neutrons. Solution: This isotope is Chlorine-35 (³⁵Cl).
  4. Question: Which subatomic particle determines the chemical behavior of an atom? Solution: The electrons determine the chemical behavior of an atom.
  5. Question: Which rule states that electrons fill lower energy orbitals first? Solution: This is the Aufbau principle.
  6. Question: An atom of ²³⁸U undergoes α-decay. What is the atomic number and mass number of the resulting atom? Solution: The resulting atom is ²³⁴Th with an atomic number of 90 (92 – 2 = 90) and a mass number of 234 (238 – 4 = 234).
  7. Question: How many valence electrons does an atom of sulfur (S) have? Solution: Sulfur has 6 valence electrons.
  8. Question: What is the maximum number of electrons that can be held in the second energy level (n=2)? Solution: The second energy level can hold a maximum of 8 electrons.
  9. Question: What does A stand for in the isotope notation AᵢZ? Solution: A stands for the mass number (the total number of protons and neutrons).
  10. Question: Which atomic model introduced the concept of quantum energy levels? Solution: The Bohr model introduced this concept.
  11. Question: Name the scientist who discovered the electron. Solution: J.J. Thomson discovered the electron.
  12. Question: What is the charge of a neutron? Solution: The neutron has no charge; it is neutral.
  13. Question: How many electrons can a single orbital hold? Solution: A single orbital can hold a maximum of 2 electrons.
  14. Question: What is the atomic mass of an atom with 20 protons, 20 electrons, and 21 neutrons? Solution: The atomic mass is 41 amu (20 protons + 21 neutrons = 41).
  15. Question: What are isotopes? Solution: Isotopes are atoms of the same element with different numbers of neutrons, and hence different mass numbers.
  16. Question: Which principle states that it’s impossible to simultaneously measure both the position and momentum of an electron with absolute precision? Solution: This is the Heisenberg Uncertainty Principle.
  17. Question: An atom of ²⁴Mg²⁺ loses two electrons. What is the atomic number of the resulting ion? Solution: The atomic number is 12, which doesn’t change with the loss or gain of electrons.
  18. Question: Which subatomic particle contributes to the mass of the atom but not to its charge? Solution: The neutron contributes to the mass of the atom but not to its charge.
  19. Question: Which theory proposed that atoms of the same element can have different masses? Solution: Dalton’s Atomic Theory was the first to propose this, later leading to the concept of isotopes.
  20. Question: If the electron configuration of an atom is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹, which element is it? Solution: The atom is Potassium (K).
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