Answer:
The mass and atomic number
Step-by-step explanation:
There are two numbers that are written in front of the symbol. The mass number (written above) and the atomic number (written below). In mass number is equal to the sum of protons and neutrons, the atomic number is equal to the number of protons in the nucleus. The unstable nuclei are due to excess of protons or neutrons. Radioactive atoms tend to emit them intermittently and randomly.