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

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 $1.6 \times 10^{-19}\ \text{C}$.