An atom has mass number 23 and atomic number 11. a) How many electrons are revolving around the nucleus?
b) How many electron shells are there in the atom?

A): The closest orbital to the nucleus, called the 1s orbital, can hold up to two electrons. This orbital is equivalent to the innermost electron shell of the Bohr model of the atom. It is called the 1s orbital because it is spherical around the nucleus. The 1s orbital is always filled before any other orbital.