21. The first shell of an atom can hold a maximum of:

a) 1 electron
b) 2 electrons
c) 4 electrons
d) 8 electrons
Answer: b) 2 electrons

22. Which of the following is true about ions?

a) They have an equal number of protons and electrons
b) They are neutral
c) They have gained or lost electrons
d) They have lost protons
Answer: c) They have gained or lost electrons

23. Which scientist is credited with the discovery of the neutron?

a) Niels Bohr
b) J.J. Thomson
c) James Chadwick
d) Ernest Rutherford
Answer: c) James Chadwick

24. The atomic number of an element is determined by the number of:

a) Neutrons
b) Electrons
c) Protons
d) Photons
Answer: c) Protons

25. What does the atomic mass of an element represent?

a) The mass of the electrons
b) The mass of the protons only
c) The average mass of all isotopes
d) The number of neutrons
Answer: c) The average mass of all isotopes

26. Which of the following orbitals has a spherical shape?

a) s orbital
b) p orbital
c) d orbital
d) f orbital
Answer: a) s orbital

27. The d subshell can hold a maximum of:

a) 2 electrons
b) 6 electrons
c) 10 electrons
d) 14 electrons
Answer: c) 10 electrons

28. What is the main characteristic of an element’s isotopes?

a) Different chemical properties
b) Different numbers of protons
c) Same number of neutrons
d) Same number of protons, different number of neutrons
Answer: d) Same number of protons, different number of neutrons

29. According to Bohr’s model, electrons move in:

a) Fixed paths called orbitals
b) Random paths
c) Circular paths around the nucleus
d) Helical paths
Answer: c) Circular paths around the nucleus

30. What is the main difference between an atom and a molecule?

a) An atom is charged, a molecule is not
b) An atom is a single unit, a molecule is a group of atoms
c) A molecule is smaller than an atom
d) A molecule is charged, an atom is not
Answer: b) An atom is a single unit, a molecule is a group of atoms