chem問題.........

monoatomic means having only one atom; diatomic means having two atoms. usually gas element bonds diatomic molecule to satisfy the octet rules (outer shell has 8 electron to become stable). so if two atoms, let say, two florine bonds together to form F2, since they have 7 electron in their outer shell, they only need 1 more atom to fulfill the octet rule, therefore, they bond covalently by sharing 1 electron from each element. that's why it is diatomic.

for argon, because it is noble gas and already has 8 electrons in its outer shell, it doesnt need to share any electron to become stable, thus, it canbe existed by itself, and thats why it is monoatomic.

In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other.

In physics and chemistry,diatomic molecule consists of two atoms of the same element, such as H2 and O2

點解argon會係monoatomic
Under Standard Temperature and Pressure (STP), all of the noble gases are monatomic. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently large temperatures.