The atomic number of an element is equal to the number of protons in the nucleus of an atom of that element. So, boron-11 has 5 protons.
The mass number of an element is equal to the total number of protons and neutrons in the nucleus of an atom of that element. So, boron-11 has a mass number of 11, which means that it has 5 protons and 6 neutrons.
The number of electrons in an atom is equal to the number of protons. So, boron-11 has 5 electrons.
