31. The number of valence electrons in an element determines its:

a) Atomic mass
b) Chemical reactivity
c) Mass number
d) Neutron number
Answer: b) Chemical reactivity

32. In the modern periodic table, elements are arranged in:

a) Periods and groups
b) Columns and rows
c) Blocks and shells
d) Rings and sectors
Answer: a) Periods and groups

33. The concept of atomic number was introduced by:

a) John Dalton
b) Dmitri Mendeleev
c) Henry Moseley
d) Niels Bohr
Answer: c) Henry Moseley

34. An atom with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵ belongs to which period?

a) Third period
b) Fourth period
c) Fifth period
d) Sixth period
Answer: d) Sixth period

35. The principle stating that electrons occupy degenerate orbitals singly before pairing up is known as:

a) The Aufbau principle
b) Hund’s rule
c) The Pauli exclusion principle
d) The Heisenberg uncertainty principle
Answer: b) Hund’s rule

36. The energy required to remove an electron from an atom in its gaseous state is called:

a) Electronegativity
b) Ionization energy
c) Electron affinity
d) Atomic radius
Answer: b) Ionization energy

37. The ability of an atom in a molecule to attract shared electrons is known as:

a) Atomic radius
b) Electronegativity
c) Ionization energy
d) Electron affinity
Answer: b) Electronegativity

38. The atomic radius generally increases as you move:

a) From left to right across a period
b) From top to bottom in a group
c) From bottom to top in a group
d) From right to left across a period
Answer: b) From top to bottom in a group

39. The energy released when an atom gains an electron is called:

a) Ionization energy
b) Electron affinity
c) Electronegativity
d) Atomic radius
Answer: b) Electron affinity

40. The term used to describe the distance between the nucleus of an atom and its outermost electron is:

a) Atomic radius
b) Ionic radius
c) Covalent radius
d) Molecular radius
Answer: a) Atomic radius