Question

What is the exact atomic weight of arsenic?
(A) 74.94
(B) 24.98
(C) 80.00
(D) 182.47

Answer 1

Hint: The exact atomic weight of any element depends on the total number of protons and total number of neutrons. But if the element has isotopes then the average atomic weight is needed to be calculated. This is done by the specific formula which represents the average atomic mass calculated as a product of the atomic weight of a specific element and its abundance in nature.

Complete step by step answer:
Let us solve this illustration by discussing the element i.e. arsenic in detail;
Arsenic-
It is a naturally occurring metalloid trace element with atomic number as 33 and atomic symbol as As. It is a micronutrient mainly found in water, air, food and soil.
It is usually used in industrial purposes and in pharmaceutical companies, but it is highly toxic. Exposure of the element by inhalation and ingestion can cause health disorders.
As is present in period 4 and group 15 in the modern periodic table. Thus, it is a p-block element and is solid at room temperature.
Arsenic is known with 33 isotopes with at least 10 isomers. But have only 3 main types of isotopes out of which only one is stable with 100% of abundance i.e. $^{75}As$. Thus, it is a monoisotopic element and its atomic weight is hence determined solely by $^{75}As$.
Therefore, the exact atomic weight of arsenic is 74.922 as it has 33 protons and nearly 42 neutrons.

Hence, option (A) is correct.

Note: Do note that this element is monoisotopic instead of having a large number of isotopes due to sole abundance of $^{75}As$. This is a non-essential element which is now replaced by other productive elements for industrial and commercial purposes due to its toxicity.