An element with 18 protons and 18 neutrons is argon-36, a stable isotope of argon.
What is the name of an element with 18 protons 22 neutrons and 18 electrons?
That element is argon-40, the most abundant isotope of argon found in nature.
Eighteen protons lock it in as argon, no matter what. The 22 neutrons bump its mass number to 40 (18 + 22). Those 18 electrons keep the whole thing electrically balanced. Honestly, this is the isotope you’re mostly breathing right now—argon-40 makes up more than 99% of the argon in air. Because it won’t react with anything, engineers stuff it into light bulbs and welding torches to stop hot metal from turning into rust.
Can chlorine have 18 protons?
No, chlorine cannot have 18 protons—it always has 17 protons.
Chlorine’s atomic number is 17, so every single chlorine atom is born with exactly 17 protons. Shift that to 18 and the atom instantly becomes argon instead. Chlorine does, however, happily carry 18 neutrons in chlorine-35. Protons define the element; neutrons just come along for the ride and create isotopes. That’s why you’ll never meet a chlorine atom with 18 protons.
