1.- The mass number of an atom equals the number of neutrons plus the number of protons. Then, for this atom the number of protons equals 24 less 13 = 11. The number of protons is the atomic number, so the answer is that the atomic number is 11.
2.- A = 38, means that the isotope of argon has mass number of 38. Given that the atomic number of argon is 18, this isotope has 18 protons and 38 less 18 = 20 neutrons.Then, the nuclear symbol is Ar precedeed by the numbers 38 (upper left corner) and 18 (lower left corner).
