The complete sentence is:
The atomic number of phosphorus is 15. It has zero charge because it has
15 electrons.
Explanation:
The atomic number is the number of protons in the nucleus. The atomic number of sodium is 11, so if the phosphorous is the fourth element to the right of sodium in the periodic table, it must have 4 protons more than sodium, therefore its atomic number is
11+4=15
For an atom, the atomic number is also equal to the number of electrons, so the atom of phosphorous has 15 electrons. Each proton has a charge of +1, while each electron has a charge of -1, therefore the total charge of the atom is
+15-15=0
