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Answer 1 · 2016-09-04T09:51:51

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

In an atomic symbol for an isotope:

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

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

The atomic number (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"#.