The periodic property that is used to determine whether a covalent molecule is polar or not is

Step 1: Analyze the question We need the periodic property that tells us whether a covalent bond or molecule is polar (has partial positive and negative ends).

Step 2: The concept In a covalent bond, two atoms share electrons. If both atoms attract the shared electrons equally, the bond is non-polar (e.g., Cl-Cl, H-H). If one atom attracts the shared electrons more strongly, the electrons are pulled toward it. That atom becomes partially negative (δ-) and the other partially positive (δ+). This makes the bond polar (e.g., H-Cl).

The ability of an atom to attract shared electrons in a bond is called electronegativity.

Step 3: Apply it

• The difference in electronegativity between the bonded atoms shows how polar the bond is. • Difference of 0 (or very small): non-polar covalent • Moderate difference: polar covalent • Very large difference: ionic
• For a whole molecule, you also need to consider the shape. CO₂ has polar bonds, but the molecule is non-polar because the bond dipoles cancel by symmetry. Electronegativity is still the property that decides the bond polarity in the first place.

Step 4: Check the other options

• A. Atomic radius: the size of an atom. It does not measure how strongly shared electrons are attracted.
• B. Ionization energy: the energy needed to remove an electron from a gaseous atom. It relates to forming positive ions, not to sharing electrons.
• C. Electron affinity: the energy change when an isolated gaseous atom gains an electron. It is related to electronegativity but applies to free atoms, not to atoms in a bond.
• D. Electronegativity: the tendency of an atom to attract shared electrons in a covalent bond. This is the one used to judge polarity.

Answer: D. electronegativity

Exam tip: Remember "Polar = Pull". Electronegativity is the pulling power on shared electrons. If a question mentions bond polarity or the type of bond (ionic, polar covalent, non-polar covalent), think electronegativity difference.