Here's why:

* Sodium (Na) has an atomic number of 11, meaning it has 11 protons.

* Neutral sodium also has 11 electrons to balance the positive charge of the protons.

* Sodium with 10 electrons has lost one electron, making it a positively charged ion.

* The positive charge is indicated by the superscript + following the symbol.

Therefore, the symbol for sodium with 11 protons and 10 electrons is Na⁺, which represents a sodium ion with a +1 charge.