Q13.14 Given below are densities of some solids and liquids. Give rough estimates of the size of their atoms :

Substance Atomic Mass (u) Density (103 Kg m3)
Carbon (diamond) 12.01 2.22
Gold 197 19.32
Nitrogen (liquid) 14.01 1
Lithium 6.94 0.53
Fluorine 19 1.14

Answers (1)
S Sayak

Let one mole of a  substance of atomic radius r and density \rho have molar mass M

Let us assume the atoms to be spherical

Avogadro's number is N_{A}=6.022\times 10^{23}

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3M}{4N_{A}\pi \rho } \right )^{\frac{1}{3}}

For Carbon

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3\times 12.01}{4\times 6.022\times 10^{23}\times \pi\times 2.22\times 10^{3} } \right )^{\frac{1}{3}}\\ r=1.29\AA

For gold

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3\times 197.00}{4\times 6.022\times 10^{23}\times \pi\times 19.32\times 10^{3} } \right )^{\frac{1}{3}}\\ r=1.59\AA

For Nitrogen

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3\times 14.01}{4\times 6.022\times 10^{23}\times \pi\times 1.00\times 10^{3} } \right )^{\frac{1}{3}}\\ r=1.77\AA

For Lithium

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3\times 6.94}{4\times 6.022\times 10^{23}\times \pi\times 0.53\times 10^{3} } \right )^{\frac{1}{3}}\\ r=1.73\AA

For Fluorine

\\N_{A}\frac{4}{3}\pi r^{3}\rho =M\\ r=\left ( \frac{3\times19.00}{4\times 6.022\times 10^{23}\times \pi\times 1.14\times 10^{3} } \right )^{\frac{1}{3}}\\ r=1.88\AA

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