If an atom of sodium had an atomic number of 11, and lost and electron, what would you have?

The atomic number of an element tells you how many protons each atom has. Losing an electron won't affect the number of protons. It will, however, change the charge on the atom. Protons are positive and electrons are negative. If there's more positive than negative then the atom has a positive charge. So the charge for the sodium atom would be + 1