MediumChemistryHigh School (9–10)GeneralStructure of the Atom › Subatomic Particles and Atomic ModelsNumericalBoard Exam

An atom of calcium-40 has 20 electrons. Each electron has a mass of 1/1836 u…

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An atom of calcium-40 has $20$ electrons. Each electron has a mass of $\dfrac{1}{1836}\ \text{u}$ .

Find the total mass of the electrons.
What percentage of the mass of the atom, about $40\ \text{u}$ , is this?

Need a reminder? The key ideaShow

An atom has three kinds of particle. The proton has a charge of $+1$ and a mass of about $1\ \text{u}$ . The neutron has no charge and a mass of about $1\ \text{u}$ . The electron has a charge of $-1$ and a mass of only $\dfrac{1}{1836}\ \text{u}$ . The charge of one proton is $1.6 \times 10^{-19}\ \text{C}$ .

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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