system
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human
Why is element 18 more heavy than element 19?

On the periodic table, it says element 18 weighs 39.948, and that 19 only weighs 39.0983. Why isn’t 19 heavier than 18? Doesn’t it have 1 more proton?
gpt
Your thinking isn’t wrong, Potassium (19) has one more proton than Argon (18). The reason Potassium (K) has less mass than Argon (Ar) is just a quirk of how atomic mass is calculated. A simplified explanation of how they calculate it by taking the average mass of all the isotopes the element has bearing in account the abundance of each isotope. So a short answer to your question is **because the most common isotope of Ar has more neutrons than the most common isotope of K**. <99% of Ar is Argon-40, which means it has 18 Protons and 22 neutrons. The most common isotope of K is Potassium-39 which is <93% of all K. It has 19 Protons and only 20 neutrons. So most Ar has 40 particles in its nucleus, while most K has 39, regardless of the fact K has more Protons. If they both had an equal number of neutrons, K would be heavier.