Sodium - A chemical element. 
Atomic Number: 11
Atomic Mass: 22.9898 amu
Sodium is a metal, but forms 
monatomic cations  Na
+ when combined with other elements, such as chlorine (as the counterion). They are important to the electrical molecular functioning of the brain. Sodium ions are used in the brain to catalyze synaptic firing in certain neurons.
. . . . . . .
Resources
https://pubchem.ncbi.nlm.nih.gov/compound/923