A monatomic species that has 18 electrons and a net charge of 2~ has

A monatomic species with 18 electrons and a net charge of 2- is an anion, specifically the sulfide ion (S²⁻). A neutral sulfur atom has 16 protons and 16 electrons [2]. By gaining two electrons to achieve a 2- charge, it reaches a total of 18 electrons [2]. This makes it isoelectronic with the noble gas argon, which also contains exactly 18 electrons in its neutral state. Option 2 is incorrect because anions have more electrons than protons. Option 3 is incorrect because species with 18 electrons (like argon or sulfide) have a stable octet with all electrons paired in their shells. Option 4 is incorrect because a species with 20 protons is calcium; a calcium ion with 18 electrons would have a 2+ charge, not a 2- charge [1].

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 25: Thunderstorm > 25.4. Lightning And Thunder > p. 348
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 47