X and Y are two elements with the same mass number but different atomic numbers 27 and 30 respectively. If X has a mass number of 57, then calculate the number of neutrons in Y.

• X and Y are isobars since they have same mass number and different atomic number.
• Mass number of X is 57. Since they have same mass number, mass number of Y will also be 57.
• Atomic number of element Y is 30, so the number of protons will be 30.
• Mass number is the sum of number of protons and neutrons in that atom.
• So the number of neutrons in element Y will be Mass number of Y - Number of protons in Y.
• Number of neutrons = 57-30 = 27 

The number of neutrons in element Y will be 27.