The correct answer is: 2) "argon" ; or, " Ar " ;
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→ {which is the corresponding symbol for "argon" in the "Periodic Table of Elements"}.
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Explanation:
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Note that among the answer choices provided;
Answer choice: " 3) "hydrogen sulfide" {sic} is misspelled and is supposed to be: " 3) hydrogen sulfide " .
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Note that "argon" {" Ar " } is one single element. "Argon" is one of the 6 (six) "noble gases" , which are all "monatomic" as a gas; and are:
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1) helium (He), 2) neon (Ne), 3) argon (Ar), 4) krypton (Kr), 5) xenon, and: 6) radon (Rn) .
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Let us review the other answer choice provided to see if they are incorrect:
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Choice 1) "oxygen" ; is INCORRECT. "Oxygen" exists in gaseous form as either: " O₂ " —"oxygen gas"—which is "diatomic" [has 2 (two) atoms of oxygen per molecule—and as such, NOT "monatomic" ;
or: " O₃ " — "ozone gas" —which is "triatomic" [has 3 (three) atoms of oxygen per molecule—and as such, NOT "monatomic".
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Choice 3) "hydrogen sulfide" {Note the correct spelling: "sulfide"} — is INCORRECT. "Hydrogen sulfide" ; that is: " H₂ S " — is a gaseous compound that has 2 (two) atoms of hydrogen per molecule AND 1 (one) atom of sufur per molecule—with a total of 3 (three) atoms per molecule. Thus, "hydrogen sulfide" is "triatomic" —and as such, NOT "monatomic".
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Choice 4) "Ammonia" is INCORRECT. "Ammonia" ; that is: "NH ₃ " — exists as a gaseous compound that has 1 (one) atom of nitrogen per molecule AND 3 (three) atoms of hydrogen per molecule—with a total of 4 (four) atoms per molecule. Thus, "ammonia" is "polyatomic" —more specficially, "tetratomic"—and such, NOT "monoatomic".
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Shortcut tip: We can actually rule out Choices 3 and 4 really easily because they are "compounds" (consist of more than one element), which means, at the minimum, they would be "diatomic or having an even greater number of atoms"; and as such: NOT "monatomic" .
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Hope this answer—and explanation—are of help to you!
Best wishes!
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