Question

Argon has three naturally occurring isotopes, with mass numbers $36,38,$and $40$.

a. Write the atomic symbol for each of these atoms.

b. How are these isotopes alike?

c. How are they different?

d. Why is the atomic mass of argon listed on the periodic table, not a whole number?

e. Which isotope is the most abundant in a sample of argon?

Short answer

(a) The required atomic symbols are, ${}_{18}{}^{36}Ar,{}_{18}{}^{38}Ar,{}_{18}{}^{40}Ar$

(b) All of them have same no. of protons and electrons.

(c) They have different no. of neutrons

(d) Due to existence of various isotopes

(e) The isotope which is most abundant is, ${}_{18}{}^{40}Ar$

Step-by-step solution

Part (a) Step 1: Given information

We need to find out atomic symbols of isotopes of argon having mass no. $36,38,40$

Part (a) Step 2: Simplify

We know that,

Atomic no. of argon is, $Z=18$

And standard representation of an element is,${}_Z{}^{A}X$

Therefore,

Atomic symbol of argon having mass no. $A=36$is, ${}_{18}{}^{36}Ar$

And Atomic symbol of argon having mass no. $A=38$is, ${}_{18}{}^{38}Ar$

And Atomic symbol of argon having mass no.$A=40$is,${}_{18}{}^{40}Ar$

Part (b) Step 1: Given information

We need to find the similarity between the isotopes

Part (b) Step 2: Explanation

We know that,

Atomic no. is equal to no. of protons or no. of electrons.

Now, from part (a) it is clear that all isotopes have same atomic no.

Therefore, similarity between the isotopes is that they all have same no. of protons and electrons.

Part (c) Step 1: Given information

We need to find the difference between the isotopes.

Part (c) Step 2: Explanation 

We know that,

Mass no. is equal to sum of no. of protons and no. of neutrons

From part (a) it is clear that all isotopes have different mass no. but from part (b) it is clear that no. of protons are equal in every isotope

So, the above case is possible only if the no. of neutrons are different.

Therefore, difference between isotopes is that they have different no. of neutrons.

Part (d) Step 1: Given information

We need to find that why the atomic mass of argon is not a whole no. in periodic table.

Part (d) Step 2: Explanation

The periodic table's atomic mass is a weighted average of all the element's isotopes. In the periodic table, the atomic mass of argon is a weighted average of $36,38,40$. As a result, atomic mass is not a whole number in the periodic table.

Part (e) Step 1: Given information

We need to find the most abundant isotope sample of argon listed in part (a)

Part (e) Step 2: Explanation

In the periodic table, the atomic mass of argon is $39.95$, which is the closest to $40$. The most common isotope has an atomic mass of $40$ and its atomic symbol is, ${}_{18}{}^{40}Ar$.