Isotopes of an element have the same atomic number but different mass numbers (The mass number represents the sum of the number of protons and neutrons of an element.).
Therefore, The correct option is (c) different number of neutrons.
In Fig. 9.26, A, B, C and D are four points on a circle. AC and BD intersect at a point E such that ∠ BEC = 130° and ∠ ECD = 20°. Find ∠ BAC.