Question

There are two naturally occurring isotopes of Li, Li-6, and Li-7 and their percentage of natural abundances are Li-6: 7.5% and Li -7: 92.5%. Calculate the actual average atomic mass of Li?

Answer 1

Given:-

→ Percentage of abundance of

Li-6 = 7.5%

→ Precentage of abundance of

Li-7 = 92.5%

To find:-

→ Average atomic mass of Li.

Solution:-

From the question, we can understand that two naturally occuring isotopes are there of Lithium (Li). Mass number of one isotope is 6 [Li-6] and mass number of the other isotope is 7 [Li-7].

So, average atomic mass of Lithium :-

= [6 × 7.5/100] + [7 × 92.5/100]

= [6 × 0.075] + [7 × 0.925]

= 0.45 + 6.475

= 6.925 u

Thus, average atomic mass of Lithium (Li) is 6.925u .

Some Extra Information:-

• Isotopes are atoms of the same

element having same atomic number

but different mass numbers.

• They have the same atomic number as

because they contain same number of

protons and electrons. They have

different mass numbers beacuse they

contain different number of neutrons.