Final answer:
An atom with 20 protons and 27 neutrons is an isotope of calcium (Ca), and the correct symbol for this isotope is 47Ca or 4720Ca if including the atomic number.
Step-by-step explanation:
An atom of Element X contains 20 protons and 27 neutrons. To identify the element, one should refer to the periodic table and look for the atomic number (Z) which corresponds to the number of protons in the atom. Therefore, Element X has an atomic number of 20, which makes it calcium (Ca). Calcium's standard atomic weight is 40.078 amu, but in this particular case, we are dealing with an isotope that has additional neutrons.
To write the symbol for this isotope, we add together the numbers of protons and neutrons to get the mass number (A). For Element X, A = 20 protons + 27 neutrons = 47. Hence, the symbol for this isotope is 47Ca, with the mass number as the superscript to the left of the element symbol and the atomic number usually written as the subscript (which is often omitted since the symbol Ca already effectively conveys the atomic number of 20). Therefore, in the response, the atomic symbol for this isotope will be 47Ca or 4720Ca to include the atomic number explicitly.