If z is the symbol for an element, which pair among the following represents isotopes of the same element?
a. 51 51
z and z
25 26
b. 51 50
z and z
25 25
c. 50 50
z and z
25 27
d. 51 50
z and z
26 25

B. 51 50
Z and Z
25 25

It is because only this element have equal number of Protons (Bottom number) and are different in atomic masses (upper number)

Hope this helps!

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.