i can think of 3 subatomic particles found in a nucleus protons, neutrons, and electrons
18. the blank should be "
"
19. the question seems to be answered.
20. 
Answer: D.atomic mass.
Explanation: The general representation of an element is given by: 
X = chemical symbol of element
Z= atomic number= which represents the number of electrons.
A= mass number or atomic mass =which represents the number of electrons + number of neutrons
Thus mass number is greater than number of electrons and number of neutrons as it is the sum of both.