Final answer:
The atomic number of element X is 14, which corresponds to silicon (Si). The relative atomic mass, or mass number, for the most common isotope of silicon is 28, derived from the addition of its 14 protons and 14 neutrons.
Step-by-step explanation:
The atomic number of an element X is 14, meaning it has 14 protons in its nucleus. The relative atomic mass of this element, often referred to as its mass number (A), is the sum of protons and neutrons in its nucleus. For element X with atomic number 14, which is silicon (Si), the most common isotope has a mass number of 28, indicated as 28Si. This mass number is derived from the fact that the most stable and abundant isotope of silicon has 14 neutrons in addition to its 14 protons; thus, A = N (neutrons) + Z (protons) = 14 + 14 = 28.