Periodic trends in atomic number and reactivity

Steps:

• Identify atomic numbers: Potassium (K) is 19, argon (Ar) is 18.
• Compare protons: Atomic number equals number of protons in neutral atoms.
• Evaluate trends: Periods show varying properties; groups show similar properties and increasing reactivity down Group 1.
• Assess bonding: Non-metals form both covalent and ionic bonds.

Why A is correct:

• By definition, atomic number is the number of protons; K (19) has one more proton than Ar (18).

Why the others are wrong:

• B: Elements in the same period have different chemical properties due to varying electron configurations.
• C: Non-metals can form ionic bonds, such as in salts with metals.
• D: Group 1 metals become more reactive from Li to Cs due to easier electron loss with larger atomic size.

Final answer: A