# Atomic mass and number

- The number at the top of a periodic table entry is the mass number (the total amount of protons and neutrons added together)
- The number at the bottom is the atomic number (the amount of protons in an atom)
- The number of neutrons can be calculated by subtracting the atomic number from the mass number
- The number of electrons is the same as the atomic number
- The atomic number increases when you go down a group