Atomic Structure and Isotopes: Question 5

Worked solution

Part (a): Protons and neutrons in lithium-7

The proton number of lithium is $3$, so every lithium atom, regardless of isotope, has $3$ protons.

For lithium-7, the nucleon (mass) number is $7$, so:

$\text{neutrons} = 7 - 3 = 4$

Part (b): Relative atomic mass from isotopic abundance

The relative atomic mass is a weighted mean of the isotopes’ mass numbers, weighted by their percentage abundance:

$A_r = \frac{(\text{mass}_1 \times \text{abundance}_1) + (\text{mass}_2 \times \text{abundance}_2)}{100}$

Substituting the values for lithium:

$A_r = \frac{(6 \times 7.5) + (7 \times 92.5)}{100} = \frac{45 + 647.5}{100} = \frac{692.5}{100} = 6.925$

Giving the answer to 3 significant figures:

$A_r = 6.93$

Part (c): Why the isotopes count as the same element

Lithium-6 and lithium-7 both have proton number $3$, so a neutral atom of each isotope has $3$ electrons, arranged in the same electronic configuration ($2,1$). The extra neutron in lithium-7 adds mass but has no effect on the arrangement of electrons.

Because chemical properties are determined by electron arrangement rather than by the number of neutrons, both isotopes react identically. This means the recycling plant does not need to separate them, they can be extracted and used together as a single element, lithium, even though their atoms differ in mass.

Final answers

• (a) Protons $= 3$, neutrons $= 4$.
• (b) $A_r = \boxed{6.93}$ (3 s.f.).
• (c) Both isotopes have 3 electrons and identical electronic configuration, so they have identical chemical properties.