Question

The element thallium is 70% thallium-205 and 30% thallium-203. Calculate its relative atomic mass to 1 decimal place

Answer 1

You have to get product of mass number and  percentage abundance and then obtain the sum of the two values to get the relative atomic mass ($M_r$), then divide everything by 100 to remove percentage

where m₁= mass number of thallium 205, a₁= abundance of thallium 205

and m₂= mass of thallium 203 and a₂= abundance of thallium 203

Thus

$M_r= \frac{(m_1\times a_1) +(m_2\times a_2)}{100}$

$M_r= \frac{(205\times70) +(203\times30)}{100}\\M_r=204.4$

Answer 2

You have to get product of mass number and  percentage abundance and then obtain the sum of the two values to get the relative atomic mass (), then divide everything by 100 to remove percentage

where m₁= mass number of thallium 205, a₁= abundance of thallium 205

and m₂= mass of thallium 203 and a₂= abundance of thallium 203

Thus