A calcium atom with a mass number of 46. What is the atomic symbol? Express your answer as an isotope.

We can write is as ##" "^{46}"Ca"## or ##" "_{20}^{46}"Ca"##.

In an atomic symbol for an isotope:

The (total protons plus neutrons) goes to the upper left of the chemical symbol:

##" "_{20}^{color(blue)(46)}"Ca"##

The (protons only) goes to the lower left of the chemical symbol:

##" "_{color(blue)20}^{46}"Ca"##

We sometimes leave off the atomic number because the chemical element has a uniquely defined atomic number:

##" "_{20}^{46}"Ca = " ^{46}"Ca"##.