Sodium has the atomic number 11. How many electrons are in a sodium ion (Na) ?

Atomic number is the number of protons of an atom. And, given that the atoms are electrically neutral and the charge of one proton is equal to the charge of one electron, the number of electrons is equal to the number of protons. So the neutral atom of sodium has 11 protons and 11 electrons. But, the sodium ion (Na+) has one positive charge, that means that it has lost one negative charge or one electron. Then, the sodium ion (Na+) has 11 - 1 = 10 electrons. Then, the answer is that there are 10 electrons in a sodium ion.